BACKGROUNDOF THE INVENTION
Silk fiber and secreted proteins from the domesticated silkworm Bombyx mori have
been used for centuries in the textile industry. The secreted proteins have more recently been
used as a biomaterial for biomedical applications, including as a structural component and as
a protein solution. Natively, silkworm proteins exist as an amalgam of the silk proteins
fibroin and sericin, in which sericin serves as a glue-like substance that binds with fibroin and
maintains the shape of the cocoon. Removal of sericin, such as through detergent- mediated
extraction, or in high-heat and high-alkaline washing, results in sericin-free fibroin fibers that
include heavy and light chain fibroin proteins associated through a single disulfide linkage.
Conversion of these fibrils into water-soluble silk fibroin protein requires the addition of a
concentrated heavy salt (e.g., 8-10M lithium bromide), which interferes with inter- and intra
molecular ionic and hydrogen bonding that would otherwise render the fibroin protein waterinsoluble.
Applications of silk fibroin proteins typically require the removal of the high LiBr salt
concentrations, such as through the use of dialysis, so that the salts do not interfere with
proper material function in a given environment. Without these salts to compete with ionic
and hydrogen bonding of the solubilized silk fibroin, silk fibroin protein solutions are
relatively unstable, are vulnerable to protein aggregation, and often precipitate out of aqueous
solutions. The aggregation is thought to occur through interactions between fibroin proteins,
and then subsequent material gelation driven through beta-sheet secondary protein structure
formation between the hydrophobic amino acid motifs of the fibroin heavy chains. Upon
formation of these structures, the transition from soluble fibroin solution to insoluble fibroin
gel is rapid and is largely irreversible, thereby limiting application of the solution for aqueous
solution-based applications because of limited material shelf-life.
To combat the gelling propensity of aqueous fibroin, attempts have been made to
minimize protein aggregation and subsequent beta-sheet formation. Lowering the fibroin
concentration in solution is a colligative approach aimed to attenuate the protein-protein
interactions, which precede the formation of these structures, yet may result in a fibroin
solution that is too dilute for relevant protein applications. Alternatively, modifications to the
aqueous solution that would impede protein aggregation and/or beta-sheet formation (e.g.,
solution pH, addition of stabilizing excipients) may forestall these events. However, these
modifications and chemical additions can limit downstream applications by increasing
biological toxicity or by introducing incompatible agents in the solution. Accordingly, what
is needed is a silk-derived protein (SDP) material that is resistant to aggregation and that has
a shelf-life stability profile useful across various industries.
A novel strategy to avoid the aforementioned vulnerabilities of aqueous silk fibroin is
to modify the biochemical structure and qualities of the silk fibroin protein itself rather than
the aqueous solution environment. Toward this end, modifications to the silk fiber extraction
process and/or the conditions involved in the production of aqueous silk fibroin can impact
the primary sequence of amino acids, and therefore, the chemistry responsible for protein
aggregation and beta-sheet formation. As such, the development of a process for modifying
silk fibroin materials could dramatically extend the stability and shelf-life of a silk solution
product.
SUMMARY
The invention provides a protein composition derived from silk fibroin. The
composition intrinsically possesses enhanced solubility and stability in aqueous solutions. In
one embodiment, the invention provides a protein composition prepared by a process
comprising heating an aqueous fibroin solution at an elevated pressure. The aqueous fibroin
solution includes lithium bromide at a concentration of at least 8M. The aqueous fibroin
solution is heated to at least about 105 °C (221 °F) under a pressure of at least about 10 PSI
for at least about 20 minutes, to provide the protein composition. The polypeptides of the
protein composition comprise less than 8.5% serine amino acid residues, and the protein
composition has an aqueous viscosity of less than 5 cP as a 10% w/w solution in water.
In other embodiments, the invention provides a protein composition prepared by a
process comprising heating an aqueous fibroin solution at an elevated pressure, wherein the
aqueous fibroin solution comprises lithium bromide at a concentration of 9-10M, and wherein
the aqueous fibroin solution is heated to a temperature in the range of about 115 °C (239 °F)
to about 125 °C (257 °F), under a pressure of about 14 PSI to about 20 PSI for at least about
20 minutes; to provide the protein composition. The protein composition can include less
than 6.5% serine amino acid residues and the protein composition can have an aqueous
viscosity of less than 10 cP as a 15% w/w solution in water.
The invention also provides a fibroin-derived protein composition that possesses
enhanced stability in aqueous solution, wherein: the primary amino acid sequences of the
fibroin-derived protein composition differs from native fibroin by at least by at least 4% with
respect to the combined difference in serine, glycine, and alanine content; cysteine disulfide
bonds between the fibroin heavy and fibroin light protein chains are reduced or eliminated;
the composition has a serine content that is reduced by greater than 25% compared to native
fibroin protein; and wherein the average molecular weight of the silk derived protein is less
than about 100 kDa.
In another embodiment, the invention provides a fibroin-derived protein composition
that possesses enhanced stability in aqueous solution, wherein: the primary amino acid
sequences of the fibroin-derived protein composition differs from native fibroin by at least by
at least 6% with respect to the combined difference in serine, glycine, and alanine content;
cysteine disulfide bonds between the fibroin heavy and fibroin light protein chains are
reduced or eliminated; the composition has a serine content that is reduced by greater than
40% compared to native fibroin protein; and wherein the average molecular weight of the silk
derived protein is less than about 96 kDa.
The invention further provides a fibroin-derived protein composition that possesses
enhanced stability in aqueous solutions, wherein: the primary amino acid sequences of the
fibroin-derived protein composition is modified from native silk fibroin; cysteine disulfide
bonds between the fibroin heavy and fibroin light protein chains are reduced or eliminated;
the average molecular weight of the silk derived protein is less than about 100 kDa; and the
fibroin-derived protein composition maintains an optical absorbance at 550 nm of less than
0.25 for at least two hours after five seconds of ultrasonication.
In another embodiment, the invention provides a fibroin-derived protein composition
that possesses enhanced stability in aqueous solutions, wherein: the primary amino acid
sequences of the fibroin-derived protein composition is modified from native silk fibroin such
that they differ from native fibroin by at least by at least 5% with respect to the combined
difference in serine, glycine, and alanine content; cysteine disulfide bonds between the
fibroin heavy and fibroin light protein chains are reduced or eliminated; the average
molecular weight of the silk derived protein is less than about 96 kDa; and the fibroin-derived
protein composition maintains an optical absorbance at 550 nm of less than 0.2 for at least
two hours after five seconds of ultrasonication.
The fibroin-derived protein composition can be isolated and/or purified as a dry
powder or film, for example, by dialysis and/or filtration. Alternatively, the fibroin-derived
protein composition can be isolated and/or purified as a stable aqueous solution, which can be
modified for use as a food or beverage composition, or as a therapeutic formulation, such as
an ophthalmic formulation.
BRIEF DESCRIPTION OF THE DRAWINGS
The following drawings form part of the specification and are included to further
demonstrate certain embodiments or various aspects of the invention. In some instances,
embodiments of the invention can be best understood by referring to the accompanying
drawings in combination with the detailed description presented herein. The description and
accompanying drawings may highlight a certain specific example, or a certain aspect of the
invention. However, one skilled in the art will understand that portions of the example or
aspect may be used in combination with other examples or aspects of the invention.
Figure 1. Flowchart illustrating key processing steps for the generation of both SDP
solution and prior art silk fibroin solution. The SDP Production Process contains an
additional step (italicized in center) to enhance solution stability over time, which is not
performed during the prior art silk fibroin solution production process.
Figure 2. Picture showing results of the Lawrence Stability Test for a stable SDP
solution (Sample 1, on left, produced by the process described in Example 1), and a prior art
silk fibroin solution (Sample 2, on right, produced by standard hydrolysis conditions). Visual
inspection reveals that Sample 1 is a stable aqueous solution that has not gelled, while
Sample 2 has gelled, and therefore is not a stable aqueous solution.
Figure 3. Picture of a gel showing process-mediated modification of aqueous silk
fibroin protein to SDP solution. The picture shows the molecular weight (MW) distribution
of an SDP Solution (Lane 3, autoclaved) versus a prior art silk fibroin solution (Lane 2, nonautoclaved).
A protein standard ladder (Lane 1) and associated weights (numbers to the left
of Lane 1) are provided as a reference of MW. A prominent MW band at 23-26 kDa in Lane
2 is noteworthy and is entirely absent following the autoclaving process, indicating that
degradation of the fibroin light chain contributes to the enhanced stability of the SDP protein
material. Also a clear shift is observed in MW range of fibroin protein following autoclaving
(Lane 3), indicating modification of the natural silk fibroin protein to the SDP material
composition.
Figure 4A-B. Images demonstrating that (A) SDP Solution material does not gel,
while (B) Prior Art Silk Fibroin solution material gelled within 2 hours following
ultrasonication.
Figure 5. Impact of the fibroin processing as described herein on protein solution
stability and viscosity. Summary graph illustrating solution viscosity as a function of protein
concentration in Prior Art Silk Fibroin (PASF), PASF heated to 225 °F for 30 minutes
(PASF-225 °F), and SDP. PASF and PASF-225 °F demonstrated a sharp increase in viscosity
in solutions of >75 mg/g (7.5% w/w), and could not be concentrated higher than 200 mg/g
without proteins falling out of solution. In contrast, SDP maintained a low viscosity
throughout all concentrations, and was able to be concentrated to levels exceeding 240 mg/g.
Figure 6A-B. Heat treatment of silk fibroin protein generates pyruvate, indicating
modification of the fibroin primary structure. (A) Summary graph showing pyruvate
concentrations in silk fibroin protein solutions (50 mg/mL) exposed to no heat, or upon
heating to 65 °C (-150 °F), 90 °C (-200 °F), and 99 °C (-210 °F). (B) The duration of heat
treatment further enhances pyruvate formation from silk fibroin. A 30-minute exposure of
aqueous silk proteins to 99 °C (-210 °F) causes a nearly twofold increase in pyruvate levels
relative to pyruvate levels at the time this temperature was initially achieved, and over
fourfold that of non-heated samples.
Figure 7A-B. Heat treatment alters the amino acid composition of native silk protein.
(A) Summary graph showing serine composition as a percentage of total amino acids in
fibroin protein in non-heated (i.e., 'no heat' / prior art) silk protein solution (left column, 10%
serine) and SDP solution previously subject to processing as described in Example 1 (e.g.,
-121 °C, 17 psi) for 30 minutes ('heat'; right column, 5.7% serine). Heat treatment reduced
serine composition by over 40% in SDP solution samples when compared to prior art silk
fibroin solution samples. (B) Summary graph depicting percent concentrations of glycine
and alanine in a prior art silk fibroin protein solution (left 'no heat' columns) and a heat and
pressure processed (-121 °C, 17 psi) SDP solution (right 'heat' columns). Heat and pressure
processing facilitates an increase in levels of glycine and alanine relative to prior art silk
fibroin solution controls.
Figure 8. Various sample formulations were placed on a hydrophobic wax surface:
phosphate buffered saline (PBS), TheraTears (TT), Blink, Systane Balance (SB), and a 5%
w/v SDP formulation (shown on the left side of the figure). Formulation solution spreading
was imaged and the spreading area was then measured at time points before and after
mechanical spreading (data shown on the right side of the figure). After mechanical
spreading, the SDP formulation showed significantly enhanced spreading (by over threefold)
compared to all other sample formulations.
Figure 9A-C. Amino acid transformation in SDP impairs secondary protein
structures and permits dissolution. (A) FTIR spectra of both pre-processed and postprocessed
water-annealed samples of PASF Solution and SDP Solution. The prior art
samples show significant beta-sheet signature peaks post water-annealing around 1624 cm 1
and 1510 cm 1, while the spectrum of the SDP solution does not indicate formation of betasheet
peaks and instead indicates significantly reduced beta-sheet content post-processing.
(B) Representative images of SDP and PASF silk solutions desiccated to form films. (C)
Subsequent dissolution of SDP films in water was complete, indicating that no beta-sheet
secondary structures had formed; however, PASF films were unable to dissolve entirely,
rendering a mixture of partially dissolved PASF and undissolved beta-sheet-containing
protein aggregates.
Figure 10. Enzymatic cleavage of silk fibroin with trypsin enhances instability and
accelerates gel formation but does not affect SDP. Summary graph depicting absorbance
(550 nm) of ultrasonicated PASF previously treated with trypsin for indicated time points.
Increasing absorbance indicates fibroin beta-sheet formation which culminates in gel
formation, demonstrating solution instability.
Figure 11. Summary graph depicting longitudinal optical absorbance (550 nm) of
PASF solutions treated with 0 (control), 10, or 100 mM dithiothreitol (DTT) 30 minutes prior
to ultrasonication. The data provide an indicator of secondary structure formation in PASF or
SDP solutions over time. Accumulating beta-sheet formation, shown as an increasing
absorbance, occurs immediately after sonication of PASF solutions (e.g., within 30 minutes
and subsequently increasing). Conversely, SDP exhibits no tendency towards secondary
structure formation. Reduction of disulfide bridges with DTT retards beta-sheet and
subsequent gel formation compared to control PASF, but the material ultimately forms
significant amounts of beta-sheets. In contrast, SDP showed no tendency to form secondary
structures and remained stable. Thus, reduction of disulfide bridges in native fibroin
improves stability but does not prevent gel formation, and longitudinal instability of PASF is
abolished in SDP.
Figure 12. Heating PASF in the absence of lithium bromide (LiBr) impairs gelation.
Summary graph depicting optical absorbance (550 nm) from PASF solutions not heated
(control) or heated at -200 °F for indicated durations prior to ultrasonication. Solution
heating caused an increase in basal absorbance which increased with heating duration relative
to non-heated PASF. All PASF solutions demonstrated an increase in absorbance over time,
indicating change in protein properties. In contrast, SDP exhibited no change in absorbance
following ultrasonication throughout the duration of the experiment.
DETAILED DESCRIPTION
The invention provides a protein composition derived from silk fibroin. The protein
composition possesses enhanced solubility and stability in aqueous solutions. The primary
amino acid sequence of native fibroin is modified in the fibroin-derived protein composition
such that cysteine disulfide bonds between the fibroin heavy and fibroin light protein chains
are reduced or eliminated. Additionally, the composition can have a serine content that is
reduced by greater than 40% compared to native fibroin protein, and the average molecular
weight of the proteins in the composition is less than about 100 kDa.
Definitions
The following definitions are included to provide a clear and consistent understanding
of the specification and claims. As used herein, the recited terms have the following
meanings. All other terms and phrases used in this specification have their ordinary
meanings as one of skill in the art would understand. Such ordinary meanings may be
obtained by reference to technical dictionaries, such as Hawley 's Condensed Chemical
Dictionary 14th Edition, by R.J. Lewis, John Wiley & Sons, New York, N.Y., 2001.
References in the specification to "one embodiment", "an embodiment", etc., indicate
that the embodiment described may include a particular aspect, feature, structure, moiety, or
characteristic, but not every embodiment necessarily includes that aspect, feature, structure,
moiety, or characteristic. Moreover, such phrases may, but do not necessarily, refer to the
same embodiment referred to in other portions of the specification. Further, when a
particular aspect, feature, structure, moiety, or characteristic is described in connection with
an embodiment, it is within the knowledge of one skilled in the art to affect or connect such
aspect, feature, structure, moiety, or characteristic with other embodiments, whether or not
explicitly described.
The singular forms "a," "an," and "the" include plural reference unless the context
clearly dictates otherwise. Thus, for example, a reference to "a component" includes a
plurality of such components, so that a component X includes a plurality of components X. It
is further noted that the claims may be drafted to exclude an optional element. As such, this
statement is intended to serve as antecedent basis for the use of exclusive terminology, such
as "solely," "only," "other than", and the like, in connection with any element described
herein, and/or the recitation of claim elements or use of "negative" limitations.
The term "and/or" means any one of the items, any combination of the items, or all of
the items with which this term is associated. The phrases "one or more" and "at least one" are
readily understood by one of skill in the art, particularly when read in context of its usage.
For example, the phrase can mean one, two, three, four, five, six, ten, 100, or any upper limit
approximately 10, 100, or 1000 times higher than a recited lower limit.
The term "about" can refer to a variation of ± 5%, ± 10%, ± 20%, or ± 25% of the
value specified. For example, "about 50" percent can in some embodiments carry a variation
from 45 to 55 percent. For integer ranges, the term "about" can include one or two integers
greater than and/or less than a recited integer at each end of the range. Unless indicated
otherwise herein, the term "about" is intended to include values, e.g., weight percentages,
proximate to the recited range that are equivalent in terms of the functionality of the
individual ingredient, element, the composition, or the embodiment. The term about can also
modify the end-points of a recited range as discuss above in this paragraph.
As will be understood by the skilled artisan, all numbers, including those expressing
quantities of ingredients, properties such as molecular weight, reaction conditions, and so
forth, are approximations and are understood as being optionally modified in all instances by
the term "about." These values can vary depending upon the desired properties sought to be
obtained by those skilled in the art utilizing the teachings of the descriptions herein. It is also
understood that such values inherently contain variability necessarily resulting from the
standard deviations found in their respective testing measurements.
As will be understood by one skilled in the art, for any and all purposes, particularly
in terms of providing a written description, all ranges recited herein also encompass any and
all possible sub-ranges and combinations of sub-ranges thereof, as well as the individual
values making up the range, particularly integer values. A recited range (e.g., weight
percentages or carbon groups) includes each specific value, integer, decimal, or identity
within the range. Any listed range can be easily recognized as sufficiently describing and
enabling the same range being broken down into at least equal halves, thirds, quarters, fifths,
or tenths. As a non-limiting example, each range discussed herein can be readily broken
down into a lower third, middle third and upper third, etc. As will also be understood by one
skilled in the art, all language such as "up to", "at least", "greater than", "less than", "more
than", "or more", and the like, include the number recited and such terms refer to ranges that
can be subsequently broken down into sub-ranges as discussed above. In the same manner,
all ratios recited herein also include all sub-ratios falling within the broader ratio.
Accordingly, specific values recited for radicals, substituents, and ranges, are for illustration
only; they do not exclude other defined values or other values within defined ranges for
radicals and substituents.
One skilled in the art will also readily recognize that where members are grouped
together in a common manner, such as in a Markush group, the invention encompasses not
only the entire group listed as a whole, but each member of the group individually and all
possible subgroups of the main group. Additionally, for all purposes, the invention
encompasses not only the main group, but also the main group absent one or more of the
group members. The invention therefore envisages the explicit exclusion of any one or more
of members of a recited group. Accordingly, provisos may apply to any of the disclosed
categories or embodiments whereby any one or more of the recited elements, species, or
embodiments, may be excluded from such categories or embodiments, for example, for use in
an explicit negative limitation.
The term "contacting" refers to the act of touching, making contact, or of bringing to
immediate or close proximity, including at the cellular or molecular level, for example, to
bring about a physiological reaction, a chemical reaction, or a physical change, e.g., in a
solution, in a reaction mixture, in vitro, or in vivo.
For a therapeutic application, an "effective amount" refers to an amount effective to
treat a disease, disorder, and/or condition, or to bring about a recited effect. For example, an
effective amount can be an amount effective to reduce the progression or severity of the
condition or symptoms being treated. Determination of a therapeutically effective amount is
within the capacity of persons skilled in the art. The term "effective amount" is intended to
include an amount of a composition described herein, or an amount of a combination of
peptides described herein, e.g., that is effective to treat or prevent a disease or disorder, or to
treat the symptoms of the disease or disorder, in a host. Thus, an "effective amount"
generally means an amount that provides the desired effect.
For process and preparation applications, an "effective amount" refers to an amount
effective to bring about a recited effect, such as an amount necessary to form products in a
reaction mixture. Determination of an effective amount is typically within the capacity of
persons skilled in the art, especially in light of the detailed disclosure provided herein. The
term "effective amount" is intended to include an amount of a compound or reagent described
herein, or an amount of a combination of compounds or reagents described herein, or
conditions related to a process described herein, e.g., that is effective to form the desired
products in a reaction mixture. Thus, an "effective amount" generally means an amount that
provides the recited desired effect.
Fibroin is derived from the Bombyx mori silkworm cocoon. The protein fibroin
includes a heavy chain that is about 350-400 kDa in molecular weight and a light chain that is
about 25 kDa in molecular weight, wherein the heavy and light chains are linked together by
a disulfide bond. The primary sequences of the heavy and light chains are known in the art.
The fibroin protein chains possess hydrophilic N and C terminal domains, and alternating
blocks of hydrophobic/hydrophilic amino acid sequences allowing for a mixture of steric and
electrostatic interactions with surrounding molecules in solution. At low concentration
dilutions (1% or less) the fibroin protein molecule is known to take on an extended protein
chain form and not immediately aggregate in solution. The fibroin protein is highly miscible
with hydrating molecules such as HA, PEG, glycerin, and CMC, has been found to be highly
biocompatible, and integrates or degrades naturally within the body through enzymatic
action. Native fibroin, or prior art silk fibroin (PASF), is known in the art and has been
described by, for example, Daithankar et al. (Indian J. Biotechnol. 2005, 4, 115-121).
The terms "silk-derived protein" (SDP) and "fibroin-derived protein" are used
interchangeably herein. These materials are prepared by the processes described herein
involving heat, pressure, and a high concentration of a heavy salt solution. Therefore 'silkderived'
and 'fibroin-derived' refers to the starting material of the process that modifies the
silk fibroin protein to arrive at a protein composition with the structural, chemical and
physical properties described herein.
Fibroin-Derived Protein Composition Preparation
The fibroin-derived protein composition described herein possesses enhanced stability
compared to native fibroin in aqueous solutions. The enhanced stability achieved by the
fibroin-derived protein composition, also referred herein as a silk-derived protein (SDP),
allows the material to remain in solution significantly longer than the native / prior art silk
fibroin proteins (referred to herein as PASF). The enhanced stability of the SDP material also
allows for the preparation of SDP solutions of high concentration without aggregation,
precipitation, or gelation. In commercial applications such as with food, beverage, eye drops,
or applications requiring protein to be soluble in solution, the enhanced stability provides
suitably lengthy shelf-life and increased quality of the product by reducing protein
aggregation. Potential aggregation of protein in solution negatively impacts a product's
desired performance for a particular application. The ability to concentrate the SDP to high
constitutions in solution (over 50% w/v or > 500 mg/mL) is significantly advantageous for
inventorying a useful working solution that can be used as-is or diluted for any number of
applications. Examples of such applications are the use of SDP as an ingredient in food,
beverage, or ophthalmic formulations as a protein supplement or additive.
The enhanced stability in aqueous solutions is derived from transforming the primary
amino acid sequences of the native fibroin protein into the SDP material. The changes in the
primary sequence decreases the susceptibility of the molecules to aggregate. Aggregation
eventually leads to gel formation. In the transformation of the native fibroin, both serine and
cysteine amino acids are cleaved in the presence of high heat and dehydrating conditions.
Similarly, Patchornik et al. (J. Am. Chem. Soc. 1964, 86, 1206) demonstrated that a
dehydroalanine (DHA) intermediate is formed from serine and cysteine in solution. The
amino acid degradation is further driven when in the presence of a strong dehydrating solvent
system, such as the 50-55% w/v LiBr solution as described herein, in which a hydride shift
takes place to induce removal of water. The degradation reaction can take place in the
presence of hydroxide ions (e.g., pH 7.5 to pH 11), which further drives cleavage of the DHA
intermediate. This cleavage forms an amide, a pyruvoyl peptide, and LiBr. One viable
chemical mechanism is outlined in Scheme 1 for a serine amino acid, which scheme is also
applicable for cysteine amino acids. Chemical alteration of the serine and cysteine amino
acids of the PASF protein into a DHA intermediate with further hydrolytic cleavage leads to
enhanced solution stability of the SDP products.
Scheme 1. Schematic detailing an underlying chemical reaction for serine and cysteine
degradation. Degradation is driven by the production of a DHA intermediate that is formed
from a hydride shift reaction in the presence of a dehydrating high salt concentration
environment. Degradation of DHA is then accomplished through an SN2 reaction within the
basic solvent environment.
The cleavage reaction discussed above significantly affect macromolecular properties
of the resulting peptides, which results in an aqueous solution of stabilized SDP material.
The initial protein aggregation of fibroin is believed to be instigated by interactions of the
native fibroin heavy and light chains at the cysteine amino acids as described by Greving et
al. (Biomacromolecules 2012, 13(3): 676-682). The cysteine amino acids within the fibroin
light and heavy protein chains interact with one another through disulfide linkages. These
disulfide bridges participate in fibroin protein aggregation and gel network flocculation.
Without the native fibroin light chain present, the proteins are significantly less susceptible to
aggregation. Therefore, the process described herein effectively reduces the native fibroin
light chain's ability to form disulfide bonds by reducing cysteine content and thus reducing or
eliminating disulfide bond-forming capability. Through this mechanism, the transformative
process described herein functionally stabilizes the resulting SDP in solution by reducing or
eliminating the ability to form cysteine-derived aggregations.
In addition to aggregation-inducing disulfide bridges, the susceptibility of the silk
fibroin to further aggregate into flocculated structure is also driven by the protein's amino
acid chemistry as described by Mayen et al. (Biophysical Chemistry 2015, 197:10-17).
Molecular modeling of silk fibroin serine, alanine, and glycine amino acid sequences have
shown that the presence of serine enhances initial protein-to-protein interaction through a
greater propensity to create hydrogen bonding between adjacent fibroin protein chain
moieties. The models demonstrate that reduced serine and increased alanine and glycine
decrease the initial propensity for protein aggregation. The molecular modeling observations
indicate that by altering the native amino acid chemistry of the fibroin protein a material
could be generated that would have higher stability in aqueous solution.
One strategy to accomplish enhanced stability is to eliminate charged functional
groups, such as hydroxyls, from the protein. Due to the relatively high electronegativity of
hydroxyl groups, this chemistry can drive both hydrogen bonding with available hydrogen
atoms and non-specific charge interactions with positively charged amino acid groups.
Almost 12% of the native fibroin protein's content is composed of serine, which bears a
hydroxyl functional group. Therefore, by reducing the availability of hydroxyl groups that
facilitate hydrogen bonding, the overall protein stability in solution may be enhanced. The
process described herein effectively reduces the amount of serine content and increases the
relative alanine and glycine content, which eliminates the number of available hydroxyl
groups available to create hydrogen bonds. Through this mechanism the process described
herein functionally stabilizes the resulting SDP in solution extended periods of time (e.g., at
least several [6-8] months, and/or for more than 1.5 years; extended studies are ongoing,
indicating that stability may be maintained for more than 2 years, or more than 3 years).
In addition to the reduction of cysteine and serine moieties, solvent charge interaction
is important for stabilizing a protein solution. After initial protein flocculation, the gelation
process is believe to continue to drive closer associations among the native fibroin heavy
chains, which leads to both intra- and inter-molecular beta-sheet formation among
hydrophobic blocks of the heavy chains. Once significant beta-sheet formation occurs, the
fibroin solution transitions to a gel. As the solution transitions to a gel, and the fibroin
becomes insoluble and is no longer useful as a solution-based product. To prevent gelation,
the pH of the SDP solution can be raised to high alkalinity to enhance stability, for example
over a pH of 7.5. As a result, the increased pH produces additional free hydroxyl groups that
form a valence shield around the SDP molecules in solution. The formed valence shield acts
to produce a zeta potential that stabilizes the protein by reducing protein-protein interactions
derived from hydrogen bonding or non-specific charged and/or hydrophobic interactions.
The fibroin-transformation process functionally stabilizes processed SDP in solution through
this mechanism and others.
SDP material can be prepared by the following process.
1. Silk cocoons are prepared by removing pupae material and pre-rinsing in warm water.
2. Native fibroin protein fibers are extracted from the gum-like sericin proteins by
washing the cocoons in water at high water temperature, typically 95 °C or more, at an
alkaline pH.
3. The extracted fibroin fibers are dried and then dissolved using a solvent system that
neutralizes hydrogen bonding between the beta-sheets; a 54% LiBr aqueous solution of
20% w/v silk fibroin protein is effective for this neutralization step.
4. The dissolved fibroin protein in LiBr solution is processed in an autoclave environment
(-121 °C [-250 °F], at -15-17 PSI pressure, for approximately 30 minutes at
temperature).
5. The heat processed fibroin protein and LiBr solution are then dialyzed to remove
lithium and bromide ions from solution. At this point in the process the material has
been chemically transformed to SDP.
6. The dialyzed SDP is then filtered to remove any non-dissolved aggregates and
contaminating bioburden.
The SDP solution is produced using a distinctly different process than the process
used for current silk fibroin solution production, as schematically illustrated in Figure 1.
Notably, the autoclaving of the silk fibroin protein while it is combined with LiBr in solution
initiates chemical transitions to produce the stabilized SDP material. The fibroin protein is
dissolved in LiBr solution, which neutralizes hydrogen bonding and electrostatic interactions
of the solubilized native fibroin protein. This leaves the protein without specific secondary
structure confirmations in solution. As a result, the thermodynamic energy required to
hydrolyze covalent bonding within the fibroin protein chain is at its lowest energy
requirements to initiate hydrolytic cleavage.
In one embodiment the temperature is set to 121 °C for 30 minutes at 15-17 PSI
autoclave conditions. However, in various embodiments, the processing conditions may be
modified to stabilize the SDP material to varying degrees. In other embodiments, additional
protein solubilization agents can be used in the process, including other or additional halide
salts such as calcium chloride and sodium thiocyanate, organic agents such as urea, guanidine
hydrochloride, and 1,1,1,3,3,3-hexafluoroisopropanol, additional strong ionic liquid solution
additives such as calcium nitrate and l-butyl-3-methylimidazolium chloride, or a
combination thereof.
Fibroin-Derived Protein Compositions
The invention provides a protein composition derived from silk fibroin, which
composition possesses enhanced solubility and stability in aqueous solutions. In one
embodiment, the invention provides a protein composition prepared by a process comprising
heating an aqueous fibroin solution at an elevated pressure. The aqueous fibroin solution
includes lithium bromide at a concentration of at least 8M. The aqueous fibroin solution is
heated to at least about 105 °C (221 °F) under a pressure of at least about 10 PSI for at least
about 20 minutes, to provide the protein composition. The polypeptides of the protein
composition comprise less than 8.5% serine amino acid residues, and the protein composition
has an aqueous viscosity of less than 5 cP as a 10% w/w solution in water.
In other embodiments, the invention provides a protein composition prepared by a
process comprising heating an aqueous fibroin solution at an elevated pressure, wherein the
aqueous fibroin solution comprises lithium bromide at a concentration of 9-10M, and wherein
the aqueous fibroin solution is heated to a temperature in the range of about 115 °C (239 °F)
to about 125 °C (257 °F), under a pressure of about 15 PSI to about 20 PSI for at least about
20 minutes; to provide the protein composition. The protein composition can include less
than 6.5% serine amino acid residues and the protein composition can have an aqueous
viscosity of less than 10 cP as a 15% w/w solution in water.
The invention also provides a fibroin-derived protein composition that possesses
enhanced stability in aqueous solution, wherein: the primary amino acid sequences of the
fibroin-derived protein composition differs from native fibroin by at least by at least 4% with
respect to the combined difference in serine, glycine, and alanine content (fibroin-derived vs.
PASF); cysteine disulfide bonds between the fibroin heavy and fibroin light protein chains
are reduced or eliminated; and the composition has a serine content that is reduced by greater
than 25% compared to native fibroin protein. The average molecular weight of the fibroinderived
protein composition can be less than about 100 kDa and greater than about 25 kDa.
In another embodiment, the invention provides a fibroin-derived protein composition
that possesses enhanced stability in aqueous solution, wherein: the primary amino acid
sequences of the fibroin-derived protein composition differs from native fibroin by at least by
at least 6% with respect to the combined difference in serine, glycine, and alanine content;
cysteine disulfide bonds between the fibroin heavy and fibroin light protein chains are
reduced or eliminated; and the composition has a serine content that is reduced by greater
than 40% compared to native fibroin protein. The average molecular weight of the fibroinderived
protein composition can be less than about 96 kDa and greater than about 25 kDa.
The invention further provides a fibroin-derived protein composition that possesses
enhanced stability in aqueous solutions, wherein: the primary amino acid sequences of the
fibroin-derived protein composition is modified from native silk fibroin; cysteine disulfide
bonds between the fibroin heavy and fibroin light protein chains are reduced or eliminated;
the average molecular weight of the fibroin-derived protein composition is less than about
100 kDa and greater than about 25 kDa; and the fibroin-derived protein composition
maintains an optical absorbance at 550 nm of less than 0.25 for at least two hours after five
seconds of ultrasonication. For example, a 5% w/w solution of the protein composition can
maintain an optical absorbance of less than 0.1 at 550 nm after five seconds of ultrasonication
at 10 Hz and 20% amplitude, which are the standard conditions used for ultrasonication
described herein.
In another embodiment, the invention provides a fibroin-derived protein composition
that possesses enhanced stability in aqueous solutions, wherein: the primary amino acid
sequences of the fibroin-derived protein composition is modified from native silk fibroin such
that they differ from native fibroin by at least by at least 5% with respect to the combined
difference in serine, glycine, and alanine content; cysteine disulfide bonds between the
fibroin heavy and fibroin light protein chains are reduced or eliminated; the average
molecular weight of the fibroin-derived protein composition is less than about 96 kDa and
greater than about 25 kDa; and the fibroin-derived protein composition maintains an optical
absorbance at 550 nm of less than 0.2 for at least two hours after five seconds of
ultrasonication.
In various embodiments, the fibroin-derived protein composition can be isolated
and/or purified as a dry powder or film, for example, by dialysis and/or filtration.
Alternatively, the fibroin-derived protein composition can be isolated and/or purified as a
stable aqueous solution, which can be modified for use as a food or beverage composition, or
as a therapeutic formulation, such as an ophthalmic formulation. The invention therefore also
provides a food or beverage composition that includes a protein composition described herein
and a food or beverage component. Food components can include one or more of simple
sugars, disaccharides, carbohydrates, fats, oils, vitamins, minerals, and water. Beverage
components can include one or more of water, a coloring agent (e.g., a synthetic colorant, or
a natural colorant such as saffron), vitamins, and minerals.
In various embodiments, the amino acid composition of the fibroin-derived protein
differs from the amino acid composition of native fibroin by at least by at least 4%, by at
least 4.5%, by at least 5%, or by at least 5.5%, or by at least 6%, with respect to the content
of serine, glycine, and alanine combined.
The composition can have a serine content that is reduced by greater than 25%, by
greater than 30%, by greater than 35%, by greater than 40%, or by greater than 45%,
compared to the serine content of native fibroin protein.
The average molecular weight of the fibroin-derived protein composition can be less
than about 100 kDa, less than about 98 kDa, less than about 96 kDa, less than about 95 kDa,
less than about 90 kDa, less than about 85 kDa, less than about 80 kDa, less than about 75
kDa, or less than about 70 kDa. In various embodiments, the average molecular weight of
the fibroin-derived protein composition can be greater than about 30 kDa, greater than about
35 kDa, greater than about 40 kDa, greater than about 50 kDa, greater than about 60 kDa, or
greater than about 70 kDa. Accordingly, the (weight average) average molecular weight of
the fibroin-derived protein composition can be about 30 kDa to about 100 kDa, about 30 kDa
to about 96 kDa, about 30 kDa to about 90 kDa, about 35 kDa to about 80 kDa, about 35 kDa
to about 70 kDa, about 40 kDa to about 60 kDa. In various embodiments, the average
molecular weight of the fibroin-derived protein composition is about 60 kDa to about 80 kDa,
about 50 kDa to about 70 kDa, about 40 kDa to about 60 kDa, about 30 kDa to about 50 kDa,
about 35 kDa to about 45 kDa, or about 40 kDa to about 43 kDa.
In various embodiments, the protein composition has an aqueous viscosity of less than
4 cP as a 10% w/w solution in water. In additional embodiments, the protein composition has
an aqueous viscosity of less than 10 cP as a 24% w/w solution in water.
In some embodiments, the protein composition is soluble in water at 40% w/w
without any precipitation observable by ocular inspection.
In various embodiments, the protein composition does not gel upon ultrasonication of
an aqueous solution of the protein composition at concentrations of up to 10% w/w. In
additional embodiments, the protein composition does not gel upon ultrasonication of an
aqueous solution of the protein composition at concentrations of up to 15% w/w, up to 20%
w/w, up to 25% w/w, up to 30% w/w, up to 35% w/w, or up to 40% w/w.
In some embodiments, the protein composition comprises less than 8% serine amino
acid residues. In other embodiments, the protein composition comprises less than 7.5%
serine amino acid residues, less than 7% serine amino acid residues, less than 6.5% serine
amino acid residues, or less than 6% serine amino acid residues.
In some embodiments, the protein composition comprises greater than 46.5% glycine
amino acids, relative to the total amino acid content of the protein composition. In other
embodiments, the protein composition comprises greater than 47% glycine amino acids,
greater than 47.5% glycine amino acids, or greater than 48% glycine amino acids.
In some embodiments, the protein composition comprises greater than 30% alanine
amino acids, relative to the total amino acid content of the protein composition. In other
embodiments, the protein composition comprises greater than 30.5% alanine, greater than
31% alanine, or greater than 31.5% alanine.
In some embodiments, the protein composition completely re-dissolves after being
dried to a thin film. In various embodiments, the protein composition lacks beta-sheet protein
structure in aqueous solution. In certain embodiments, the protein composition maintains an
optical absorbance in aqueous solution of less than 0.25 at 550 nm after at least five seconds
of ultrasonication.
In some embodiments, protein composition is in combination with water. The protein
composition can completely dissolve in water at a concentration of 10% w/w, or even greater
concentrations such as 15% w/w, 20% w/w, 25% w/w, 30% w/w, 35% w/w, or 40% w/w. In
some embodiments, the protein composition can be isolated and purified, for example, by
dialysis, filtration, or a combination thereof.
In various embodiments, the protein composition enhances the spreading of an
aqueous solution comprising the protein composition and ophthalmic formulation
components, for example, compared to the spreading of a corresponding composition that
does not include the protein composition. The enhanced spreading can result in an increase
in surface area of the aqueous solution by greater than twofold, or greater than threefold.
In some embodiments, the protein composition does not form a gel at concentrations
up to 20% w/v, up to 30% w/v, or up to 40% w/v. The protein composition can remain in
solution up to a viscosity of at least 9.8 cP.
In some embodiments, the fibroin-derived protein composition can have glycinealanine-
glycine-alanine (GAGA) segments of amino acids that comprise at least about 47.5%
of the amino acids of the fibroin-derived protein composition. The fibroin-derived protein
composition can also have glycine-alanine-glycine-alanine (GAGA) segments of amino acids
that comprise at least about 48%, at least about 48.5%, at least about 49%, at least about
49.5%, or at least about 50%, of the amino acids of the protein composition.
In various embodiments, the fibroin-derived protein composition can have glycinealanine
(GA) segments of amino acids that comprise at least about 59% of the amino acids of
the fibroin-derived protein composition. The fibroin-derived protein composition can also
have glycine-alanine (GA) segments of amino acids that comprise at least about 59.5%, at
least about 60%, at least about 60.5%, at least about 61%, or at least about 61.5%, of the
amino acids of the protein composition.
In another embodiment, the primary amino acid sequences of the fibroin-derived
protein composition differs from native fibroin by at least by at least 6% with respect to the
combined difference in serine, glycine, and alanine content; the average molecular weight of
the fibroin-derived protein composition is less than about 100 kDa; and the fibroin-derived
protein composition maintains an optical absorbance at 550 nm of less than 0.25 for at least
two hours after five seconds of ultrasonication. Thus, in one specific embodiment, the
invention provides a fibroin-derived protein composition that possesses enhanced stability in
aqueous solution, wherein: the primary amino acid sequences of the fibroin-derived protein
composition differs from native fibroin by at least by at least 6% with respect to the
combined difference in serine, glycine, and alanine content; cysteine disulfide bonds between
the fibroin heavy and fibroin light protein chains are reduced or eliminated; the composition
has a serine content that is reduced by greater than 40% compared to native fibroin protein;
and wherein the average molecular weight of the fibroin-derived protein composition is less
than about 96 kDa.
In another embodiment, the invention provides a fibroin-derived protein composition
that possesses enhanced stability in aqueous solutions, wherein: the primary amino acid
sequences of the fibroin-derived protein composition is modified from native silk fibroin;
cysteine disulfide bonds between the fibroin heavy and fibroin light protein chains are
reduced or eliminated; the average molecular weight of the fibroin-derived protein
composition is less than about 100 kDa; and the fibroin-derived protein composition
maintains an optical absorbance at 550 nm of less than 0.25 for at least two hours after five
seconds of ultrasonication. In one specific embodiment, the primary amino acid sequences of
the fibroin-derived protein composition is modified from native silk fibroin such that they
differ from native fibroin by at least by at least 5% with respect to the combined difference in
serine, glycine, and alanine content; the average molecular weight of the fibroin-derived
protein composition is less than about 96 kDa; and the fibroin-derived protein composition
maintains an optical absorbance at 550 nm of less than 0.2 for at least two hours after five
seconds of ultrasonication.
Thus, in one specific embodiment, the invention provides a fibroin-derived protein
composition that possesses enhanced stability in aqueous solutions, wherein: the primary
amino acid sequences of the fibroin-derived protein composition is modified from native silk
fibroin such that they differ from native fibroin by at least by at least 5% with respect to the
combined difference in serine, glycine, and alanine content; cysteine disulfide bonds between
the fibroin heavy and fibroin light protein chains are reduced or eliminated; the average
molecular weight of the fibroin-derived protein composition is less than about 96 kDa; and
the fibroin-derived protein composition maintains an optical absorbance at 550 nm of less
than 0.2 for at least two hours after five seconds of ultrasonication.
The invention also provides a protein composition prepared by a process comprising
heating an aqueous fibroin solution at an elevated pressure, wherein the aqueous fibroin
solution comprises lithium bromide at a concentration of at least 8M, and wherein the
aqueous fibroin solution is heated to at least about 105 °C (221 °F) under a pressure of at least
about 10 PSI for at least about 20 minutes; to provide the protein composition, wherein the
protein composition comprises less than 8.5% serine amino acid residues and the protein
composition has an aqueous viscosity of less than 5 cP as a 10% w/w solution in water.
Therefore, the invention provides a method of preparing a fibroin-derived protein
composition comprising one or more of the process steps described herein.
In one embodiment, the concentration of lithium bromide is about 8.5M to about
11M. In some embodiments, the concentration of lithium bromide is about 9M to about 10M,
or about 9.5M to about 10M.
In some embodiments, the aqueous fibroin solution that contains lithium bromide is
heated to at least about 107 °C (225 °F), at least about 110 °C (230 °F), at least about 113 °C
(235 °F), at least about 115 °C (239 °F), or at least about 120 °C (248 °F).
In some embodiments, the aqueous fibroin solution that contains lithium bromide is
heated under a pressure of at least about 12 PSI, at least about 14 PSI, at least about 15 PSI,
or at least about 16 PSI, up to about 18 PSI, or up to about 20 PSI.
In some embodiments, the aqueous fibroin solution that contains lithium bromide is
heated for at least about 20 minutes, at least about 30 minutes, at least about 45 minutes, or at
least about 1 hour, up to several (e.g., 12-24) hours.
In some embodiments, the protein composition has an aqueous viscosity of less than 4
cP as a 10% w/w solution in water. In various embodiments, the protein composition has an
aqueous viscosity of less than 10 cP as a 24% w/w solution in water.
In some embodiments, the protein composition can be dissolved in water at 40% w/w
without observable precipitation.
In some embodiments, the fibroin has been separated from sericin.
In some embodiments, lithium bromide has been removed from the protein
composition to provide a purified protein composition. In various embodiments, the protein
composition has been isolated and purified, for example, by dialysis, filtration, or a
combination thereof.
In various embodiments, the protein composition does not gel upon ultrasonication of
an aqueous solution of the composition at concentrations of up to 10% w/w, up to 15% w/w,
up to 20% w/w, up to 25% w/w, up to 30% w/w, up to 35% w/w, or up to 40% w/w.
In additional embodiments, the protein composition has properties as described above,
and amino acid compositions as described above regarding serine, glycine, and alanine
content.
In various embodiments, the protein composition re-dissolves after drying as a thin
film. The protein composition can lack beta-sheet protein structure in solution. The protein
composition can maintain an optical absorbance in solution of less than 0.25 at 550 nm after
at least five seconds of ultrasonication.
In one specific embodiment, the invention provides a protein composition prepared by
a process comprising heating an aqueous fibroin solution at an elevated pressure, wherein the
aqueous fibroin solution comprises lithium bromide at a concentration of 9-10M, and wherein
the aqueous fibroin solution is heated to a temperature in the range of about 115 °C (239 °F)
to about 125 °C (257 °F), under a pressure of about 15 PSI to about 20 PSI for at least about
30 minutes; to provide the protein composition, wherein the protein composition comprises
less than 6.5% serine amino acid residues and the protein composition has an aqueous
viscosity of less than 10 cP as a 15% w/w solution in water.
Further embodiments of the invention are described herein below.
The invention provides a novel silk-derived protein (SDP) composition that is
chemically distinct from native silk fibroin protein. The SDP has enhanced stability in
aqueous solution. The SDP can be used in a method for forming a food composition, a
beverage, or an ophthalmic formulation comprising combining food, beverage, or ophthalmic
ingredients with a protein composition described herein, for example, a protein composition
aqueous solution. The solution can include about 0.01% to about 92% w/v SDP. The
solution can be about 8% to about 99.9% w/v water.
In one embodiment, the SDP material with enhanced solution stability can be used as
an ingredient in a beverage for human or animal consumption, such as an ingredient or
additive in a sports drink, nutrient drink, soft drink, or in bottled water. In another
embodiment, the SDP material with enhanced solution stability can be used as an ingredient
in a food product, such as in dairy products, cereal, or processed foods. In yet another
embodiment, the SDP material with enhanced solution stability can be used as an ingredient
in an eye drop formulation, such as in artificial tears, ocular lubricants, lid scrubs, or
therapeutic formulations.
In one aspect, the invention provides a process that induces hydrolysis, amino acid
degradation, or a combination thereof, of fibroin protein such that the average molecular
weight of the protein is reduced from about 100-200 kDa for silk fibroin produced using prior
art methods to about 30-90 kDa, or about 30-50 kDa, for the SDP material described herein.
The resulting polypeptides can be a random assortment of peptides of various molecular
weights averaging to the ranges recited herein. In addition, the amino acid chemistry can be
altered by reducing cysteine content to non-detectable levels by standard assay procedures.
For example, the serine content can be reduced by over 50% from the levels found in the
native fibroin, which can result in increases of overall alanine and glycine content by 5%
(relative amino acid content), as determined by standard assay procedures. The process can
provide a protein composition where the fibroin light chain protein is not discernable after
processing, as well when the sample is run using standard SDS-PAGE electrophoresis
methods. Furthermore, the resulting SDP material forms minimal to no beta-sheet protein
secondary structure post-processing, while silk fibroin solution produced using prior art
methods forms significant amounts of beta-sheet secondary structure. In one embodiment,
the SDP material can be prepared by processing silk fibroin fibers under autoclave or
autoclave-like conditions (i.e., approximately 120 °C and 14-18 PSI) in the presence of a 40-
60% w/v lithium bromide (LiBr) solution.
In some embodiments, the invention provides a food or beverage product that
includes the SDP as an ingredient. The SDP can serve to provide additional protein content,
resulting in improved nutritional value, health benefits, and/or therapeutic advantages to the
human or animal that consumes the food or beverage. In one embodiment, the SDP is
included in a beverage such as water, a sport drink, an energy drink, or a carbonated drink. In
another embodiment, the SDP is included in food products such as yogurt, energy bars,
cereal, bread, or pasta.
The food or beverage product can include an effective amount of SDP, such as about
0.01% by weight to about 92% by weight of SDP. In various embodiments, the SDP can be
present in about 0.1% by weight to about 30% by weight, about 0.5% by weight to about 20%
by weight, or about 1% by weight to about 10% by weight. In certain specific embodiments,
the SDP can be derived from Bombyx mori silkworm fibroin.
In another embodiment, the invention provides an ophthalmic composition for the
treatment of dry eye syndrome in a human or mammal. The composition can be an aqueous
solution that includes an amount of SDP effective for treating dry eye syndrome. For
example, the aqueous solution can include about 0.01% by weight to about 80% by weight
SDP. In other embodiments, the aqueous solution can include SDP at about 0.1% by weight
to about 10% by weight, or about 0.5% by weight to about 2% by weight. In certain specific
embodiments, the ophthalmic composition can include about 0.05% w/v SDP, about 0.1%
w/v SDP, about 0.2% w/v SDP, about 0.25% w/v SDP, about 0.5% w/v SDP, about 0.75%
w/v SDP, about 1% w/v SDP, about 1.5% w/v SDP, about 2% w/v SDP, about 2.5% w/v
SDP, about 5% w/v SDP, about 8% w/v SDP, or about 10% w/v SDP. The SDP can be
derived from Bombyx mori silkworm fibroin.
In various embodiments, the ophthalmic formulation can include additional
components in the aqueous solution, such as a demulcent agent, a buffering agent, and/or a
stabilizing agent. The demulcent agent can be, for example, hyaluronic acid (HA),
hydroxyethyl cellulose, hydroxypropyl methylcellulose, dextran, gelatin, a polyol,
carboxymethyl cellulose (CMC), polyethylene glycol, propylene glycol (PG), hypromellose,
glycerin, polysorbate 80, polyvinyl alcohol, or povidone. The demulcent agent can be
present, for example, at about 0.01% by weight to about 10% by weight, or at about 0.2% by
weight to about 2% by weight. In one specific embodiment, the demulcent agent is HA. In
various embodiments, the HA can be present at about 0.2% by weight of the formulation.
The buffering or stabilizing agent of an ophthalmic formulation can be phosphate
buffered saline, borate buffered saline, citrate buffer saline, sodium chloride, calcium
chloride, magnesium chloride, potassium chloride, sodium bicarbonate, zinc chloride,
hydrochloric acid, sodium hydroxide, edetate disodium, or a combination thereof.
An ophthalmic formulation can further include an effective amount of an
antimicrobial preservative. The antimicrobial preservative can be, for example, sodium
perborate, polyquaterium- 1 (e.g., Polyquad® preservative), benzalkonium (BAK) chloride,
sodium chlorite, brimonidine, brimonidine purite, polexitonium, or a combination thereof.
An ophthalmic formulation can also include an effective amount of a vasoconstrictor,
an anti-histamine, or a combination thereof. The vasoconstrictor or antihistamine can be
naphazoline hydrochloride, ephedrine hydrochloride, phenylephrine hydrochloride,
tetrahydrozoline hydrochloride, pheniramine maleate, or a combination thereof.
In one embodiment, an ophthalmic formulation can include an effective amount of
fibroin-derived protein as described herein in combination with water and one or more
ophthalmic components. The ophthalmic components can be, for example, a) polyvinyl
alcohol; b) PEG-400 and hyaluronic acid; c) PEG-400 and propylene glycol, d) CMC and
glycerin; e) propylene glycol and glycerin; f ) glycerin, hypromellose, and PEG-400; or a
combination of any one or more of the preceding components. The ophthalmic formulation
can include one or more inactive ingredients such as HP-guar, borate, calcium chloride,
magnesium chloride, potassium chloride, zinc chloride, and the like. The ophthalmic
formulation can also include one or more ophthalmic preservatives such as sodium chlorite
(Purite® preservative (NaClCh), polyquad, BAK, EDTA, sorbic acid, benzyl alcohol, and the
like. Ophthalmic components, inactive ingredients, and preservatives can be included at
about 0.1% to about 5% w/v, such as about 0.25%, 0.3%, 0.4%, 0.5%, 1%, 2%, 2.5%, or 5%,
or a range in between any two of the aforementioned values.
Accordingly, the invention provides a silk derived protein (SDP) composition that
possesses enhanced stability in aqueous solutions in which the primary amino acid sequence
of native fibroin is modified from native silk fibroin, wherein cysteine disulfide bonds
between the fibroin heavy and fibroin light protein chains reduced or eliminated; wherein the
composition has a serine content that is reduced by greater than 40% compared to native
fibroin protein; and wherein the average molecular weight of the silk derived protein is less
than about 96 kDa.
The invention also provides an ophthalmic formulation for the treatment of
ophthalmic disorders in a human or mammal, wherein the ophthalmic formulation comprises
water and an effective amount of the SDP as described above. The invention further provides
an ophthalmic composition for use as an eye treatment in a human or mammal, wherein the
ophthalmic composition comprises water, one or more of a buffering agent and stabilizing
agent, and an effective amount of the SDP as described above.
The SDP is highly stable in water, where shelf life solution stability is more than
twice that of native silk fibroin in solution. For example, the SDP is highly stable in water,
where shelf life solution stability is more than 10 times greater compared to native silk
fibroin in solution. The SDP material, when in an aqueous solution, does not gel upon
sonication of the solution at a 5% (50 mg/mL) concentration. In other embodiments, the SDP
material, when in an aqueous solution, does not gel upon sonication of the solution at a 10%
(100 mg/mL) concentration.
The SDP material can have the fibroin light chain over 50% removed when compared
to native silk fibroin protein. The SDP material can have a serine amino acid content of less
than about 8% relative amino acid content, or a serine amino acid content of less than about
6% relative amino acid content.
The SDP material can have a glycine amino acid content above about 46.5%. The
SDP material can have an alanine amino acid content above about 30% or above about
30.5%. The SDP material can have no detectable cysteine amino acid content, for example,
as determined by HPLC analysis of the hydrolyzed polypeptide of the protein composition.
The SDP material can form 90% less, 95% less, or 98% less beta-sheet secondary
protein structures as compared to native silk fibroin protein, for example, as determined by
the FTIR analysis described in Example 8 below.
The invention additionally provides an ophthalmic composition for use as an eye
treatment in a human or mammal, the composition comprising an aqueous solution including
an effective amount of SDP material as described above, and a buffering or stabilizing agent.
The invention yet further provides an ophthalmic formulation for the treatment of
ophthalmic disorders in human or mammal, the composition comprising an aqueous solution
including an effective amount of SDP material with enhanced stability as described herein.
The invention also provides a method for forming a beverage mixture, a food
composition, or an ophthalmic composition, with silk protein comprising combining a food,
beverage, or ophthalmic components with the fibroin-derived protein composition described
herein.
The following Examples are intended to illustrate the above invention and should not
be construed as to narrow its scope. One skilled in the art will readily recognize that the
Examples suggest many other ways in which the invention could be practiced. It should be
understood that numerous variations and modifications may be made while remaining within
the scope of the invention.
Example 1. Silk-Derived Protein Preparation and the Lawrence Stability Test
Materials. Silkworm cocoons were obtained from Tajima Shoji Co., Ltd., Japan.
Lithium bromide (LiBr) was obtained from FMC Lithium, Inc., NC. An autoclave was
obtained from Tuttnauer Ltd., NY. The 3,500 Da molecular- weight cutoff (MWCO) dialysis
membranes were obtained from ThermoScientific, Inc., MA. An Oakton Bromide (Br)
double-junction ion-selective electrode was obtained from ISE, Oakton Instruments, IL.
Processing. Two samples, SDP and prior art silk fibroin, were prepared as illustrated
in Figure 1. Briefly, SDP was produced by submerging pupae-free, cut silkworm cocoons
(3-5 cuts/cocoon) into 95 °C heated, deionized water (di O) containing 0.3 wt% NaC03 at
233 mL water/gram of cocoons. Cocoons were agitated in this solution for 75 minutes to
dissolve sericin, thereby separating it from the silk fibers. The cocoons were subsequently
washed four times in like dilutions of di O for 20 minutes per rinse to remove residual
sericin from the washed silk fibers. The fibers were then dried in a convection oven at 60 °C
for 2 hours, weighed, and dissolved in 54 wt% LiBr in water at a ratio of 4 LiBr volume per
gram of extracted fiber. This solution was covered and then warmed in a convection oven at
60 °C for 2 hours to expedite extracted fiber dissolution. The solution was then placed in an
autoclave and exposed to sterilization conditions (121 °C, 17 PSI, 90-100% humidity) for 30
minutes to facilitate fibroin transformation. The resulting solution was allowed to cool to
room temperature, then diluted to 5% fibroin with di O and dialyzed to remove LiBr salts
using a 3,500 Da MWCO membrane. Multiple exchanges were performed in di O until Br
ions were less than 1-ppm as determined in the hydrolyzed fibroin solution read on an Oakton
Bromide (Br) double-junction ion-selective electrode. The solution was then further filtered
using a 1-5 porosity filter followed by filtration through a 0.2 polishing filter. This
product is referred to as 'SDP Solution' in Figure 2.
A 'control' silk fibroin solution was prepared as illustrated in Figure 1 to provide the
'Prior Art Silk Fibroin Solution' shown in Figure 2. Except the autoclave step, the same
process was performed as described above. A sampling volume (5 mL) from each sample
was placed in separate 20 mL glass beakers and the beakers were sealed with foil. The
samples were then subjected to the Lawrence Stability Test.
The Lawrence Stability Test is performed by placing the aqueous protein test solution
(5% w/v, 50 mg/mL) within the autoclave chamber. The autoclave is then activated for a
cycle at 121 °C, 17 PSI, for 30 minutes, at 97-100% humidity. After completion of the cycle,
the solution is allowed to cool and is then removed from the autoclave chamber. The solution
is then shaken to observe solution gelation behavior. If the solution has gelled upon shaking
for ~10 seconds, the sample fails the Lawrence Stability Test. Failing the test indicates that
the material is inherently unstable as a protein solution.
The Lawrence Stability Test was performed on both the SDP Solution and the Prior
Art Silk Fibroin Solution. The Prior Art Silk Fibroin Solution sample gelled immediately and
therefore failed the Lawrence Stability Test. Conversely, the SDP Solution sample remained
in solution indefinitely and therefore passed the Lawrence Stability Test. The lack of
gelation can be attributed to the fact that SDP Solution production incorporated the
autoclave-processing step as indicated in Figure 1 above. An image of the different test
results (not-gelled vs. gelled) is shown in Figure 2.
Example 2. Silk-Derived Protein Molecular Weight Characterization
To evaluate the effect of processing on the molecular weight distribution of
solubilized protein, SDP Solution and Prior Art Silk Fibroin Solution were subjected to
polyacrylamide gel electrophoresis (PAGE), which separates proteins by molecular weight.
Specifically, 15g of each sample was mixed with running buffer containing sodium dodecyl
sulfate and dithiothreitol (Biorad Inc., CA) to remove any secondary folding structures and
disulfide bonds, respectively. The mixtures were then heated to 70 °C for 5 minutes. The
mixtures were loaded along with a 2.5-200 kDa molecular weight ladder (Life Technologies,
CA) onto pre-cast, 4-12% polyacrylamide gradient gels containing Bis-Tris buffer salts (Life
Technologies, CA), and then exposed to 120V electric field for 90 minutes on a BioRad
PowerPac Power supply (BioRad Inc., CA). The gels were then removed and placed in
Coomassie Blue stain for 12 hours to stain proteins, followed by 6 hours of washing in
di O. The gels were then scanned on a Biorad GS-800 Calibrated Desitometer (BioRad
Inc., CA).
The resulting gel is shown in Figure 3. The results show that the processing
employed to prepare the SDP solution significantly shifts the average molecular weight from
150-200 kDa to less than 80 kDa (Figure 3). The shift in molecular weight clearly indicates
a transformation of the primary and/or secondary structure of the original native fibroin. In
addition, the fibroin light chain of fibroin is not present in the SDP after the autoclaving
process (visible at 23-26 kDa in Lane 2 for the prior art fibroin), which indicates that the
fibroin light chain portion of the protein has been degraded or removed by the processing.
These results demonstrate that the autoclave processing transforms the native fibroin protein
to a new material that has smaller peptide fragments than the native fibroin protein. The
process further degrades/modifies the fibroin light chain. These transformations result in an
SDP material that possesses enhanced solution stability as a result of these chemical changes.
Example 3. Silk-Derived Protein Stability Study
To further determine the functional impact of the autoclave process on the stability of
the resulting SDP compared to the stability of prior art fibroin, the samples were analyzed
using the methods of Wang et al. (Biomaterials 2008, 29(8): 1054-1064) to mimic a wellcharacterized
model of silk fibroin protein gelation. Volumes of both samples (0.5 mL, SDP
and PASF) were added to 1.7 mL clear centrifuge tubes and were subjected to ultrasonication
(20% amplitude, 10 Hz, 15 seconds). The clear tubes containing the solutions were then
visually monitored for gel formation as a screen for gelation.
The SDP Solution samples failed to form gels, as shown in Figure 4A. Even 3
months post-sonication, the SDP samples remained in solution and lacked protein
aggregation as determined by visual inspection. The Prior Art Silk Fibroin Solution sample
gelled rapidly (within 2 hours) following sonication. The resulting gelled Prior Art Silk
Fibroin is shown in Figure 4B. These results further indicate that the autoclave process
transforms the prior art fibroin to a new material and induces stability to the resulting SDP
material.
Example 4. Impact of Heating on the Viscosity Profile of Aqueous Silk Solutions
The physicochemical properties of PASF and SDP were investigated, with particular
attention paid to the impact of protein concentration on solution viscosity. It has been shown
by Zafar et al. (Biomacromolecules 2015, 16(2):606-6 14) that silk fibroin heavy and light
chain proteins are distinct in their rheological properties, and therefore, differential
degradation rates of these constituents in PASF would imbue unpredictable changes to the
viscosity of a given solution over time. Furthermore, the impact of total fibroin protein
concentration on viscosity is non-linear, also shown by Zafar et al, and escalates rapidly as
purified fibroin solutions exceed 100 mg/mL, thus restricting the useable concentration of the
protein solution for a particular application.
To determine whether these limitations could be overcome through amino acid
transformations that culminate in SDP, 80-100 mL of PASF or autoclave-treated SDP were
generated at 50-80 mg/mL. To assess the impact of heating PASF to a level below
autoclaving conditions, PASF was heated to 225 °F for 30 minutes in a jacketed reaction
vessel as described above. Purified solutions were placed in 140 mm shallow plastic weigh
boats in a laminar flow hood (Baker Sterilgard 56400, Class II) at ~22 °C to facilitate
evaporation. At periodic intervals, concentrating samples were collected to measure protein
content (calculated in % w/w) and assess viscosity using a viscometer (Brookfield LVDV-E,
spindle 00). Measurements were made at spindle revolutions per minute (about 1-100 rpm)
that permitted a torque range that would permit accurate viscosity measurements, measured at
25 °C, on 16 mL sample volumes.
As summarized in Figure 5, the viscosity of PASF rose precipitously when solutions
exceeded 75 mg/g. Furthermore, PASF could not be concentrated to >200 mg/g, at which
point fibroin protein began to become insoluble. PASF solutions heated to 225 °F prior to
dialysis demonstrated the impact of heat on solution viscosity. In particular, heated PASF
exhibited decreased viscosities at any given concentration relative to non-heated PASF. In
contrast, SDP exhibited minimal changes in solution viscosity at concentrations at or below
140 mg/g (Figure 5). Furthermore, SDP viscosity remained below -lOcP at protein
concentrations where PASF could no longer stay in solution (e.g., at 240 mg/g). Importantly,
SDP was capable of remaining homogeneous at concentrations exceeding 400 mg/g, where
viscosities stayed below 150 cP.
An aqueous solution of SDP thus exhibits lower viscosity when compared to PASF at
all concentrations above 4% w/w. Additionally, gelation begins to occur at about 20% w/w
for the PASF solution at which point accurate viscosity measurements where not possible,
while the SDP material increased in concentration without exhibiting gelation, aggregating
behavior, or significant increases in viscosity through 25% w/w solutions.
Taken together, these results clearly demonstrate that the process-related protein
transformations described herein for the preparation of SDP are needed for the production of
a highly-concentrated, low viscosity protein solution.
Example 5. Formation of Pyruvoyl Peptides
The chemical reaction illustrated in Scheme 1 described above results in the
production of a pyruvoyl peptide. The pyruvoyl peptide degrades into pyruvate, which
readily detectable by a standard pyruvate assay. To demonstrate that the application of heat
and pressure (e.g., the environment in an autoclave) could facilitate pyruvoyl generation in
silk fibroin protein processing, aqueous silk fibroin solution (5% w/v in water) was produced
using the prior art method described above. The material was then heated in a thermally
jacketed beaker (ChemGlass, NJ) at defined temperatures up to 210 °F, or just below the
boiling point of the protein solution. Specifically, Prior Art Silk Fibroin protein solutions
were heated to -65 °C (-150 °F), -90 °C (-200 °F), or -99 °C (-210 °F), and then sampled
upon reaching these temperatures to measure pyruvate concentrations via colorimetric assay
(Pyruvate Assay Kit, MAK071, Sigma-Aldrich).
The production of pyruvate increased in both 90 °C and 99 °C heated samples.
Pyruvate increased by 50% at 99 °C (Figure 6A). To determine whether pyruvate conversion
was further enhanced over time, the samples were heated to 99 °C and maintained at this
temperature for 30 minutes (Figure 6B). Sustained heating caused a robust increase in
pyruvate formation, generating more than a fourfold increase in pyruvate relative to nonheated
samples. These results indicate that upon heating the silk fibroin protein, there is a
chemical conversion to pyruvoyl containing material as detected by pyruvate assay. From this
data it can be concluded that within a more extreme heating environment, such as in an
autoclave process, the silk fibroin protein will be stimulated to produce pyruvate to an even
greater extent. This provides further evidence that the final SDP product is a chemically
distinct entity from the Prior Art Silk Fibroin.
Example 6. Amino Acid Profile Analysis
The impact of heating silk fibroin fibers dissolved in 9.3M LiBr solution on amino
acid profile was investigated using ion-exchange chromatography (AOAC Official Method
994. 12, Amino Acids.com, MN). Samples were produced using the processes described in
Example 1 for both the SDP Solution and the Prior Art Silk Fibroin Solution. The solutions
were then submitted in like-concentrations for evaluation by chromatography. Particular
attention was paid to the amino acids serine, glycine, and alanine, given their prominent
constitution in the silk fibroin protein primary sequence and their key roles in secondary
structure formation.
SDP solution samples were found to contain 40% less serine relative to Prior Art Silk
Fibroin Solution samples (Figure 7A), and a corresponding increase in the levels of glycine
and alanine (Figure 7B). These results indicate a significant change in amino acid content,
which changes result in different (and enhanced) chemical and physical properties as a result
of the autoclave process. These results also corroborate with findings in the literature by
Mayen et al. (Biophysical Chemistry 2015. 197:10-17) where increased serine content was
shown to increase initial aggregation of silk fibroin proteins, while silk fibroin protein with
increasing glycine and alanine content required greater energy thresholds to initially
aggregate. Therefore, by reducing serine and increasing both alanine and glycine content, the
propensity (and/or possibility) for aggregation is reduced or eliminated, leading to the greater
solution stability of SDP.
Example 7. Artificial Tear Formulations
The SDP solution was used to formulate an artificial tear for use in treating
ophthalmic conditions and disorders. The artificial tears can be specifically formulated and
used for the treatment of the disorder 'dry eye'. The artificial tears can also be formulated and
used for treatment of an ocular wound created by either accidental or surgical insult.
Incorporation of SDP into artificial tear formulations is especially advantageous
because it increases the spreadability of the formulation. SDP-containing artificial tears also
have an extremely long shelf-life due to their solution stability. The block co-polymer
arrangement of hydrophilic and hydrophobic amino acid groups located in the back-bone of
the SDP protein allows the molecules to interact with both water-soluble and water-insoluble
chemistries within the tear film. When included as an ingredient in an artificial tear eye drop
formulation, the SDP ingredient acts to enhance the spreadability of the artificial tear, which
provides additional comfort to the patient and prolonged efficacy to the product. The
aggregating groups of prior art silk fibroin solution are not required to enable this spreading
property, so it is advantageous to remove these regions to enhance protein product stability in
solution over time.
The enhanced spreading ability of the artificial tear was demonstrated by comparing
leading brand artificial tear products with an artificial tear formulated using the SDP
ingredient. A test protocol was used to evaluate the effect of mechanical spreading on the
wetting ability of various eye drop products. Phosphate buffered saline (PBS), TheraTears®
(TT) artificial tears by AVR, Blink® Tears eye drops by AMO, Systane Balance® (SB) eye
drops by Alcon, and a formulation containing SDP were compared in the experiment. For
reference, PBS contained 100 mmol PBS salts in water, TT contains 0.25% wt.
carboxymethyl cellulose (CMC) as the active ingredient with additional buffering salts in
water, Blink contains 0.2% wt. hyaluronic acid (HA) as the active ingredient with buffering
salts in water, SB contains 0.6% wt. propylene glycol (PG) as the active ingredient with HPguar
and mineral oil as enhancing excipients with buffering salts, and the SDP solution
contained 0.25% wt. CMC with 1% wt./vol. SDP and buffering salts (i.e., 0.01M phosphate
buffer containing 137 mM NaCl).
This group of compared products included multiple demulcents and additional active
ingredient-enhancing excipients. Figure 8 shows that inclusion of the SDP ingredient
enhances mechanical spreading ability by nearly fourfold over the leading brand
formulations. This post-spreading enhancement can be compared to the eye-lid wiping
across the ocular surface. Thus the SDP ingredient allows for better comfort to the patient
while enhancing efficacy and stability of the product. Furthermore, the addition of SDP over
prior art silk fibroin solution as an ingredient is advantageous because the prior art silk
fibroin solution would gel and aggregate during the product shelf life. These aggregates
would be unacceptable in an ophthalmic formulation based on current United States
Pharmacopeia (USP) requirements (Particulate Matter in Injections: USP <788-789>).
Example 8. Analysis of Protein Secondary Structure
The impact of autoclave processing on secondary structure formation was assessed.
A 5%> w/v SDP Solution and a 5% w/v Prior Art Silk Fibroin Solution (100 L each) were
cast on 14 mm diameter silicone rubber surfaces (n=6) and allowed to air dry into solid films
over several hours. The films were then assessed by ATR-FTIR (Nicolet iSlO, Thermo
Scientific, MA) at a 4 nm resolution of 16 scans each.
The films were also processed for 5 hours within a water-annealing chamber, which is
a vacuum container with water filled in the basin to create a 100% RH environment. The
water vapor induces secondary structure formation of fibroin protein films, most notably beta
sheet structures as shown by Jin et al. (Advanced Functional Materials 2005, 15:1241-1247).
The film samples were then reanalyzed with the FTIR as described above. Spectral analysis
revealed that SDP and Prior Art Silk Fibroin films produced similar IR signatures before
material processing, but the SDP material lacked the ability to form beta sheet secondary
structures post-processing as noted by the absence of well-known beta sheet absorption peaks
in the Amide I and II regions of the IR spectrum at 1624 cm 1 and 1510 cm 1, respectively
(Figure 9A). This finding represents a significant difference in material composition of the
two samples, which is a direct indication that the amino acid chemistry is inherently different
in the SDP and PASF samples.
To represent the impact of secondary structure functionally, 150 mL samples of the
solutions were both dried within a convective clean air environment at room temperature for
48 hours. This resulted in the formation of solid protein material that demonstrated
significant differences in appearance between the two solutions (Figure 9B). Most notably,
the autoclave-processed SDP material demonstrated a darker yellow translucency that
indicates chemical changes to aromatic amino acids, when compared to the transparent and
more pellucid PASF material. In addition, the SDP material formed a dried skin that
prevented the lower region of the volume from completely dehydrating and thus partially
remained liquid. This was not the case for the Prior Art Silk Fibroin material, which was
completely dried and physically distorted into a wavy material. These results indicate
significant changes to the material's mechanical properties, and thus chemical interactions, as
a result of the autoclave processing to form the SDP material.
To assess solubility as a function of the autoclave processing, samples of both dried
materials were weighed and reconstituted in deionized water (di O). For the SDP material
the tough outer skin later was peeled off and weighed, while for the Prior Art Silk Fibroin a
portion of the material was broken off and weighed. For both samples, 500 mg of material
was added to 25 mL of di O (2% w/v solution) and then vortexed at high speed setting for
10 minutes. Interestingly, the SDP material completely dissolved in the di O volume, while
the Prior Art Silk Fibroin material dissolved only very minimally (Figure 9C). These results
indicate the material solubility and solubility chemistry was distinctly changed between the
SDP and Prior Art Silk Fibroin materials due to the autoclave processing.
Example 9. Impact of Enzymatic Fibroin Cleavage on Solution Stability
To identify whether the increased stability of SDP is a direct consequence of amino
acid transformation or merely due to the production of smaller fibroin proteins generated by
hydrolysis, Prior Art Silk Fibroin (PASF) was treated with the serine protease trypsin to
enzymatically break down fibroin as has been performed by Shaw (Biochem. J. 1964, 93(1),
45-54). In brief, 0.5-1.0 mg/mL of trypsin isolated from bovine pancreas (Sigma-Aldrich,
T1426, MO) was added to PASF (78 mg/mL) solution containing HEPES buffer salts, mixed,
and then incubated at 37 °C for 1, 2, 4, or 6 hours. Reactions were stopped with the addition
of 2 mM phenylmethylsulfonyl fluoride (PMSF), and the extent of fibroin fractionation was
measured by ID-PAGE and densitometry as described in Example 3.
Table 1. Average molecular weight of prior art silk fibroin enzymatically cleaved with
trypsin over increasing durations.
As shown in Table 1, trypsin treatment proved effective to progressively reduce the
average molecular weight of PASF until 4 hours. These materials were then subjected to
ultrasonication to initiate beta-sheet formation and gelation as performed and described by
Wang et al. (Biomaterials 2008. 29(8): 1054-1064). The fractionation of PASF with trypsin
caused a dramatic acceleration in the kinetics of gelation, however, which is summarized in
Figure 10. Specifically, 1 hour trypsin treatment of PASF induced gel formation by ~40
minutes following sonication, which was slowed to -60 minutes in PASF exposed to trypsin
for 4 hours. Control PASF (in the presence of deactivated trypsin) exhibited increasing
instability with time which reached maximal beta-sheet formation (indicated by absorbance at
550 nm, Figure 10) by approximately 1300 minutes (data not shown). In contrast, silkderived
proteins (SDP) showed no tendency toward instability during this time frame,
evidenced by a minimal and unchanging absorbance at 550 nm (Figure 10). These results
indicate that fractionation of PASF by enzymatic cleavage of select peptide bonds, without
amino acid transformation, are ineffective and in fact counter-productive to forestall betasheet
formation, instability, and gel formation.
Example 10. Impact of Disulfide Bonds on Fibroin Stability
The association between the fibroin heavy and light chain dimers exists through a
single covalent disulfide bond, as elucidated by Tanaka et al. (Biochim Biophys Acta. 1999,
1432(1):92-103). Dimer separation can instigate fibroin peptide-peptide interactions, which
culminate in insoluble protein aggregation (Shulha et al, Polymer 2006, 47:5821-5830) that
precedes beta-sheet formation and gelation (Nagarkar et al, Phys. Chem. Chem. Phys. 2010,
12:3834-3844). Therefore, to determine whether disruption of the fibroin heavy-light chain
dimer stimulates protein aggregation and therefore instability, PASF was treated with the
disulfide bond reducing agent dithiothreitol (DTT, Sigma-Aldrich, MO) at 0 (control), 10, or
100 mM, and was then subjected to ultrasonication to instigate beta-sheet and gel formation.
As shown in Figure 11, the reduction of the disulfide bond in PASF with 10 mM DTT
decelerated, but did not inhibit instability, indicated by increasing 550 nm absorbance over
time relative to control samples. These effects were further pronounced with 100 mM DTT,
but still ineffective at forestalling instability. In contrast, SDP exhibited no tendency toward
instability following ultrasonication, indicated by an unchanging baseline absorbance, which
was unaffected by the addition of DTT (Figure 11). Collectively, these results demonstrate
that the fibroin disulfide bridge participates in the mechanisms underlying PASF instability,
but their reduction is ineffective to prohibit beta-sheet formation and gelation.
Example 11. Fibroin Stability Requires Heat in the Presence of Lithium Bromide
To determine whether the heat-mediated transformation of amino acids in PASF
requires lithium bromide, studies were undertaken to identify if similar stability could be
achieved when PASF was heated in the final aqueous solution (lacking lithium bromide). To
this end, PASF was prepared (without additional heating) at 50 mg/mL concentration and
then heated in a jacketed reaction beaker (Chemglass, CG-1 103-01, NJ) connected to a
heater/chiller (Neslab, RTE-7, ThermoScientific, MA) actively circulating silicone oil heat
exchange fluid (AceGlass, 141 15-05, NJ). The circulator was set to -200 °F, the temperature
just below the boiling point of PASF lacking lithium bromide salts, and allowed to stabilize
for 15 ± 1 minutes. PASF (25 mL) was incubated in the reaction beaker with a PTFE-coated
stir bar and placed on a stir plate (IKA C-MAG HS7, NC) to ensure solution temperature
homogeneity. PASF temperature was actively monitored throughout the heating period using
an external thermocouple (Omega HH-603, Omega Engineering, CT), and samples (3 mL)
were removed at the following timepoints:
Drawn samples were then subjected to ultrasonication to instigate beta-sheet
formation and gelation, as has been described previously by Wang et al. (Biomaterials 2008,
29(8): 1054-1064). To compare these results with PASF that had been heated in the presence
of lithium bromide, SDP was also ultrasonicated separately, and absorbance at 550 nm
monitored longitudinally to compare the kinetics of fibroin instability. As shown in Figure
12, the application of heat to PASF actually increased baseline absorbance and hence
instability relative to non-heated control samples. Furthermore, the duration of heat exposure
(from 30 to 120 minutes) to PASF was inversely proportional to basal absorbance, indicating
that the presence of lithium bromide during production of SDP is needed to achieve the
minimal absorbance observed in this latter solution (Figure 12). Furthermore, 550 nm
absorbance continued to escalate in all of the heated PASF solutions over time, but did not
change from baseline in ultrasonicated SDP solutions, thus clearly demonstrating that heattreated
samples were undergoing beta-sheet formation and therefore becoming unstable.
Collectively, these results indicate that heat mediated hydrolysis of PASF in the absence of
lithium bromide is insufficient to mediate the amino acid transformations that facilitate
protein stability to the same degree as for the SDP described herein.
Embodiments of the invention include the following enumerated embodiments.
1. A fibroin-derived protein composition that possesses enhanced stability in an aqueous
solution, wherein: the primary amino acid sequences of the fibroin-derived protein
composition differ from native fibroin by at least by at least 4% with respect to the combined
amino acid content of serine, glycine, and alanine; cysteine disulfide bonds between the
fibroin heavy and fibroin light protein chains of fibroin are reduced or eliminated; the
composition has a serine content that is reduced by greater than 25% compared to native
fibroin protein; and wherein the average molecular weight of the fibroin-derived protein
composition is less than about 100 kDa and optionally greater than 25 kDa.
2. The protein composition of embodiment 1 wherein the protein composition has an
aqueous viscosity of less than 4 cP as a 10% w/w solution in water.
3. The protein composition of embodiment 1 or 2 wherein the protein composition has
an aqueous viscosity of less than 10 cP as a 24% w/w solution in water.
4. The protein composition of any one of embodiments 1-3 wherein the protein
composition is soluble in water at 40% w/w without precipitation.
5. The protein composition of any one of embodiments 1-4 wherein the protein
composition does not gel upon ultrasonication of an aqueous solution of the protein
composition at concentrations of up to 10% w/w.
6. The protein composition of any one of embodiments 1-5 wherein the protein
composition comprises less than 8% serine amino acid residues.
7. The protein composition of any one of embodiments 1-6 wherein the protein
composition comprises less than 6% serine amino acid residues.
8. The protein composition of any one of embodiments 1-7 wherein the protein
composition comprises greater than 46.5% glycine amino acids.
9. The protein composition of any one of embodiments 1-8 wherein the protein
composition comprises greater than 48% glycine amino acids.
10. The protein composition of any one of embodiments 1-9 wherein the protein
composition comprises greater than 30% alanine amino acids.
11. The protein composition of any one of embodiments 1-10 wherein the protein
composition comprises greater than 31.5% alanine amino acids.
12. The protein composition of any one of embodiments 1-11 wherein the protein
composition completely re-dissolves in water after being dried to a thin film.
13. The protein composition of any one of embodiments 1-12 wherein the protein
composition lacks beta-sheet protein structure in aqueous solution.
14. The protein composition of any one of embodiments 1-13 wherein the protein
composition maintains an optical absorbance in aqueous solution of less than 0.25 at 550 nm
after at least five seconds of ultrasonication.
15. The protein composition of any one of embodiments 1-14 in combination with water,
wherein the protein composition completely dissolves in the water at a concentration of 40%
w/w.
16. The fibroin-derived protein composition of any one of embodiments 1-15 wherein the
primary amino acid sequences of the fibroin-derived protein composition differ from native
fibroin by at least by at least 6% with respect to the combined difference in serine, glycine,
and alanine content; the average molecular weight of the fibroin-derived protein is less than
about 100 kDa and greater than about 30 kDa; and the fibroin-derived protein composition
maintains an optical absorbance at 550 nm of less than 0.25 for at least two hours after five
seconds of ultrasonication.
17. A fibroin-derived protein composition that possesses enhanced stability in aqueous
solutions, wherein: the primary amino acid sequences of the fibroin-derived protein
composition is modified from native silk fibroin; cysteine disulfide bonds between the fibroin
heavy and fibroin light protein chains are reduced or eliminated; the average molecular
weight of the fibroin-derived protein is less than about 100 kDa and greater than 25 kDa; and
the fibroin-derived protein composition maintains an optical absorbance at 550 nm of less
than 0.25 for at least two hours after five seconds of ultrasonication.
17A. The fibroin-derived protein composition of embodiment 17 that includes one or more
of the elements of any one of embodiments 1-15.
18. The fibroin-derived protein composition of embodiment 17 or 17A wherein the
primary amino acid sequences of the fibroin-derived protein composition is modified from
native silk fibroin such that they differ from native fibroin by at least by at least 5% with
respect to the combined difference in serine, glycine, and alanine content; the average
molecular weight of the fibroin-derived protein composition is less than about 70 kDa; and
the fibroin-derived protein composition maintains an optical absorbance at 550 nm of less
than 0.2 for at least two hours after five seconds of ultrasonication.
19. A protein composition prepared by a process comprising heating an aqueous fibroin
solution at an elevated pressure, wherein the aqueous fibroin solution comprises lithium
bromide at a concentration of at least 8M, and wherein the aqueous fibroin solution is heated
to at least about 105 °C (221 °F) under a pressure of at least about 10 PSI for at least about 20
minutes; to provide the protein composition, wherein the protein composition comprises less
than 8.5% serine amino acid residues and the protein composition has an aqueous viscosity of
less than 5 cP as a 10% w/w solution in water.
19A. The protein composition of embodiment 19 that includes one or more of the elements
of any one of embodiments 1-15.
20. A food or beverage composition comprising the protein composition of any one of
embodiments 1-19 and a food or beverage component.
While specific embodiments have been described above with reference to the
disclosed embodiments and examples, such embodiments are only illustrative and do not
limit the scope of the invention. Changes and modifications can be made in accordance with
ordinary skill in the art without departing from the invention in its broader aspects as defined
in the following claims.
All publications, patents, and patent documents cited herein are incorporated by
reference herein, as though individually incorporated by reference. No limitations
inconsistent with this disclosure are to be understood therefrom. The invention has been
described with reference to various specific and preferred embodiments and techniques.
However, it should be understood that many variations and modifications may be made while
remaining within the spirit and scope of the invention.
What is claimed is:
1. A fibroin-derived protein composition that possesses enhanced stability in an aqueous
solution, wherein:
the primary amino acid sequences of the fibroin-derived protein composition differ
from native fibroin by at least by at least 4% with respect to the combined amino acid content
of serine, glycine, and alanine;
cysteine disulfide bonds between the fibroin heavy and fibroin light protein chains of
fibroin are reduced or eliminated;
the composition has a serine content that is reduced by greater than 25% compared to
native fibroin protein; and
wherein the average molecular weight of the fibroin-derived protein composition is
less than about 100 kDa.
2. The protein composition of claim 1wherein the protein composition has an aqueous
viscosity of less than 4 cP as a 10% w/w solution in water.
3. The protein composition of claim 1wherein the protein composition has an aqueous
viscosity of less than 10 cP as a 24% w/w solution in water.
4. The protein composition of claim 1 wherein the protein composition is soluble in
water at 40% w/w without precipitation.
5. The protein composition of claim 1wherein the protein composition does not gel
upon ultrasonication of an aqueous solution of the protein composition at concentrations of
up to 10% w/w.
6. The protein composition of any one of claims 1-5 wherein the protein composition
comprises less than 8% serine amino acid residues.
7. The protein composition of claim 6 wherein the protein composition comprises less
than 6% serine amino acid residues.
8. The protein composition of any one of claims 1-5 wherein the protein composition
comprises greater than 46.5% glycine amino acids.
9. The protein composition of claim 8 wherein the protein composition comprises
greater than 48% glycine amino acids.
10. The protein composition of any one of claims 1-5 wherein the protein composition
comprises greater than 30% alanine amino acids.
11. The protein composition of claim 10 wherein the protein composition comprises
greater than 31.5% alanine amino acids.
12. The protein composition of any one of claims 1-5 wherein the protein composition
completely re-dissolves in water after being dried to a thin film.
13. The protein composition of any one of claims 1-5 wherein the protein composition
lacks beta-sheet protein structure in aqueous solution.
14. The protein composition of any one of claims 1-5 wherein the protein composition
maintains an optical absorbance in aqueous solution of less than 0.25 at 550 nm after at least
five seconds of ultrasonication.
15. The protein composition of any one of claims 1-5 in combination with water, wherein
the protein composition completely dissolves in the water at a concentration of 40% w/w.
16. The fibroin-derived protein composition of any one of claims 1-5 wherein the primary
amino acid sequences of the fibroin-derived protein composition differ from native fibroin by
at least by at least 6% with respect to the combined difference in serine, glycine, and alanine
content; the average molecular weight of the fibroin-derived protein is less than about 100
kDa and greater than about 30 kDa; and the fibroin-derived protein composition maintains an
optical absorbance at 550 nm of less than 0.25 for at least two hours after five seconds of
ultrasonication.
17. A fibroin-derived protein composition that possesses enhanced stability in aqueous
solutions, wherein:
the primary amino acid sequences of the fibroin-derived protein composition is
modified from native silk fibroin;
cysteine disulfide bonds between the fibroin heavy and fibroin light protein chains are
reduced or eliminated;
the average molecular weight of the fibroin-derived protein is less than about 100 kDa
and greater than 25 kDa; and
the fibroin-derived protein composition maintains an optical absorbance at 550 nm of
less than 0.25 for at least two hours after five seconds of ultrasonication.
18. The fibroin-derived protein composition of claim 17 wherein the primary amino acid
sequences of the fibroin-derived protein composition is modified from native silk fibroin such
that they differ from native fibroin by at least by at least 5% with respect to the combined
difference in serine, glycine, and alanine content; the average molecular weight of the
fibroin-derived protein composition is less than about 70 kDa; and the fibroin-derived protein
composition maintains an optical absorbance at 550 nm of less than 0.2 for at least two hours
after five seconds of ultrasonication.
19. A protein composition prepared by a process comprising heating an aqueous fibroin
solution at an elevated pressure,
wherein the aqueous fibroin solution comprises lithium bromide at a concentration of
at least 8M, and
wherein the aqueous fibroin solution is heated to at least about 105 °C (221 °F) under
a pressure of at least about 10 PSI for at least about 20 minutes;
to provide the protein composition, wherein the protein composition comprises less
than 8.5% serine amino acid residues and the protein composition has an aqueous viscosity of
less than 5 cP as a 10% w/w solution in water.
20. A food or beverage composition comprising the protein composition of any one of
claims 1, 17, or 19 and a food or beverage component.