Optical rotatory dispersion and circular dichroism of the beta-form of silk fibroin in solution

Proteins in Ionic Liquids: Reactions, Applications, and Futures

Biopolymer processing and handling is greatly facilitated by the use of ionic liquids, given the increased solubility, and in some cases, structural stability imparted to these molecules. Focussing on proteins, we highlight here not just the key drivers behind protein-ionic liquid interactions that facilitate these functionalities, but address relevant current and potential applications of protein-ionic liquid interactions, including areas of future interest.

Experimental Methods for Characterizing the Secondary Structure and Thermal Properties of Silk Proteins

Silk proteins are biopolymers produced by spinning organisms that have been studied extensively for applications in materials engineering, regenerative medicine, and devices due to their high tensile strength and extensibility. This remarkable combination of mechanical properties arises from their unique semi-crystalline secondary structure and block copolymer features. The secondary structure of silks is highly sensitive to processing, and can be manipulated to achieve a wide array of material profiles. Studying the secondary structure of silks is therefore critical to understanding the relationship between structure and function, the strength and stability of silk-based materials, and the natural fiber synthesis process employed by spinning organisms. However, silks present unique challenges to structural characterization due to high-molecular-weight protein chains, repetitive sequences, and heterogeneity in intra- and interchain domain sizes. Here, experimental techniques used to study the secondary structure of silks, the information attainable from these techniques, and the limitations associated with them are reviewed. Ultimately, the appropriate utilization of a suite of techniques discussed here will enable detailed characterization of silk-based materials, from studying fundamental processing-structure-function relationships to developing commercially useful quality control assessments.

Cooperative Assembly of a Peptide Gelator and Silk Fibroin Afford an Injectable Hydrogel for Tissue Engineering

Silk fibroin (SF) from Bombyx mori has received increasing interest in biomedical fields, because of its slow biodegradability, good biocompatibility, and low immunogenicity. Although SF-based hydrogels have been studied intensively as a potential matrix for tissue engineering, weak gelation performance and low mechanical strength are major limitations that hamper their widespread applicability. Therefore, searching for new strategies to improve the SF gelation property is highly desirable in tissue engineering research. Herein, we report a facile approach to induce rapid gelation of SF by a small peptide gelator (e.g., NapFF). Following the simple mixing of SF and NapFF in water, a stable hydrogel of SF was obtained in a short time period at physiological pH, and the minimum gelation concentration of SF can reach as low as 0.1%. In this process of gelation, NapFF not only can behave itself as a gelator for supramolecular self-assembly, but also can trigger the conformational transition of the SF molecule from random coil to β-sheet structure via hydrophobic and hydrogen-bonding interactions. More importantly, for the generation of a scaffold with favorable cell-surface interactions, a new peptide gelator (NapFFRGD) with Arg-Gly-Asp (RGD) domain was applied to functionalize SF hydrogel with improved bioactivity for cell adhesion and growth. Following encapsulating the vascular endothelial growth factor (VEGF), the SF gel was subcutaneously injected in mice, and served as an effective matrix to trigger the generation of new blood capillaries in vivo.

Chemoenzymatic synthesis of polypeptides containing the unnatural amino acid 2-aminoisobutyric acid

Polypeptides containing 2-aminoisobutyric acid (Aib) units as an unnatural amino acid residue were synthesized by papain-catalyzed chemoenzymatic polymerization of a tripeptide ethyl ester l-Ala-Aib-l-Ala-OEt in an aqueous medium. The Aib-containing polypeptide adopted an α-helix conformation in both the solid and solution phases, which was induced by the periodic Aib residue.

Phase Behaviour and Miscibility Studies of Collagen/Silk Fibroin Macromolecular System in Dilute Solutions and Solid State

Miscibility is an important issue in biopolymer blends for analysis of the behavior of polymer pairs through the detection of phase separation and improvement of the mechanical and physical properties of the blend. This study presents the formulation of a stable and one-phase mixture of collagen and regenerated silk fibroin (RSF), with the highest miscibility ratio between these two macromolecules, through inducing electrostatic interactions, using salt ions. For this aim, a ternary phase diagram was experimentally built for the mixtures, based on observations of phase behavior of blend solutions with various ratios. The miscibility behavior of the blend solutions in the miscible zones of the phase diagram was confirmed quantitatively by viscosimetric measurements. Assessing the effects of biopolymer mixing ratio and salt ions, before and after dialysis of blend solutions, revealed the importance of ion-specific interactions in the formation of coacervate-based materials containing collagen and RSF blends that can be used in pharmaceutical, drug delivery, and biomedical applications. Moreover, the conformational change of silk fibroin from random coil to beta sheet, in solution and in the final solid films, was detected by circular dichroism (CD) and Fourier transform infrared spectroscopy (FTIR), respectively. Scanning electron microscopy (SEM) exhibited alterations of surface morphology for the biocomposite films with different ratios. Surface contact angle measurement illustrated different hydrophobic properties for the blended film surfaces. Differential scanning calorimetry (DSC) showed that the formation of the beta sheet structure of silk fibroin enhances the thermal stability of the final blend films. Therefore, the novel method presented in this study resulted in the formation of biocomposite films whose physico-chemical properties can be tuned by silk fibroin conformational changes by applying different component mixing ratios.

Lithium-free processing of silk fibroin

Silk fibroin protein was purified from Bombyx mori silkworm cocoons using a novel dialysis strategy to avoid fibroin aggregation and pre-mature formation of β-sheets. The degummed silk fibers were dissolved in Ajisawa's reagent, a mixture of CaCl2-EtOH-H2O, that is less expensive than lithium bromide. The dissolved solutions were dialyzed against either water or urea solution with a stepwise decrease in concentration. When the steps of 4 M-2 M-1 M-0 M urea (referred to as silk-TS-4210) were adopted, the purified silk fibroin had smaller aggregates (<10 nm), similar average molecular weight (225 kDa) and a lower content of β-sheet (∼15%) compared to the sample processing methods (silk-TS-210, 10, 0) studied here. This outcome was close to the fibroin purified by the lithium bromide (silk-Li-0) method. Polyvinyl alcohol-emulsified silk microspheres generated using the purified solution had a similar size distribution and morphology when compared to lithium bromide dissolved solutions, while glycerol-blended silk films showed different mechanical properties. The silk-Li-0 generated films with the highest breaking strength (5.7 MPa ± 0.3) while the silk-TS-4210 had the highest extension at break (215.1% ± 12.5). The films prepared from silk-TS-4210 were cytocompatible to support the adhesion and proliferation of human mesenchymal stem cells, with improvements compared to the other samples likely due to the porous morphology of these films.

Silk-Its Mysteries, How It Is Made, and How It Is Used

This article reviews fundamental and applied aspects of silk-one of Nature's most intriguing materials in terms of its strength, toughness, and biological role-in its various forms, from protein molecules to webs and cocoons, in the context of mechanical and biological properties. A central question that will be explored is how the bridging of scales and the emergence of hierarchical structures are critical elements in achieving novel material properties, and how this knowledge can be explored in the design of synthetic materials. We review how the function of a material system at the macroscale can be derived from the interplay of fundamental molecular building blocks. Moreover, guidelines and approaches to current experimental and computational designs in the field of synthetic silklike materials are provided to assist the materials science community in engineering customized finetuned biomaterials for biomedical applications.

Silk protein aggregation kinetics revealed by Rheo-IR

The remarkable mechanical properties of silk fibres stem from a multi-scale hierarchical structure created when an aqueous protein "melt" is converted to an insoluble solid via flow. To directly relate a silk protein's structure and function in response to flow, we present the first application of a Rheo-IR platform, which couples cone and plate rheology with attenuated total reflectance infrared spectroscopy. This technique provides a new window into silk processing by linking shear thinning to an increase in molecular alignment, with shear thickening affecting changes in the silk protein's secondary structure. Additionally, compared to other static characterization methods for silk, Rheo-IR proved particularly useful at revealing the intrinsic difference between natural (native) and reconstituted silk feedstocks. Hence Rheo-IR offers important novel insights into natural silk processing. This has intrinsic academic merit, but it might also be useful when designing reconstituted silk analogues alongside other polymeric systems, whether natural or synthetic.

NMR study of the structures of repeated sequences, GAGXGA (X = S, Y, V), in Bombyx mori liquid silk

The silk fibroin stored in the silk gland of the Bombyx mori silkworm, called "liquid silk", is spun out and converted into the silk fiber with extremely high strength and high toughness. Therefore it is important to determine the silk structure before spinning called Silk I at an atomic level to clarify the fiber formation mechanism. We proposed the repeated type II β-turn structure as Silk I in the solid state with the model peptide (AG)15 and several solid state NMR techniques previously. In this paper, the solution structure of native "liquid silk" was determined with solution NMR, especially for tandem repeated sequences with (GAGXGA)n (X = S, Y, V) and GAASGA motifs in the B. mori silk fibroin. The assignment of the (13)C, (15)N, and (1)H solution NMR spectra for the repetitive sequence motifs was achieved, and the chemical shifts were obtained. The program, TALOS-N, to predict the backbone torsion angles from the chemical shifts of proteins was applied to these motifs with (13)Cα, (13)Cβ, (13)CO, (1)Hα, (1)HN, and (15)N chemical shifts. The twenty-five best matches of torsion angles (ϕ, φ) were well populated and mainly fell into the regions for typical type II β-turn structures in the (ϕ, φ) map for the GAGXGA (X = S, Y, V) motifs. In contrast, (ϕ, φ) plots for motif GAASGA were scattered, indicating that the motif is in a disordered structure. Furthermore, inter-residue HN-Hα NOE cross peaks between i-th and (i+2)th residues in GAGXGA (X = S, Y, V) motifs were observed, supporting the repeated type II β-turn structure. Thus, we could show the presence of the repeated type II β-turn structure in "liquid silk".

Sodium dodecyl sulfate-induced rapid gelation of silk fibroin

The in situ formation of injectable silk fibroin (SF) hydrogels have potential advantages over various other biomaterials due to the minimal invasiveness during application. Biomaterials need to gel rapidly under physiological conditions after injection. In the current paper, a novel way to accelerate SF gelation using an anionic surfactant, sodium dodecyl sulfate (SDS), as a gelling agent is reported. The mechanism of SDS-induced rapid gelation was determined. At low surfactant concentrations, hydrophobic interactions among the SF chains played a dominant role in the association, leading to decreased gelation time. At higher concentrations of surfactant, electrostatic repulsive forces among micellar aggregates gradually became dominant and gelation was hindered. Gel formation involves the connection of clusters formed by the accumulation of nanoparticles. This process is accompanied by the rapid formation of β-sheet structures due to hydrophobic and electrostatic interactions. It is expected that the silk hydrogel with short gelation time will be used as an injectable hydrogel in drug delivery or cartilage tissue engineering.

Conformational change from antiparallel beta-sheet to alpha-helix in a series of depsipeptide, -(Leu-Leu-Lac)(n)-: syntheses, spectroscopic studies, and crystal structures of Boc-Leu-Lac-OEt and Boc-(Leu-Leu-Lac)(n)-OEt (n = 1, 2)

The depsipeptides Boc-Leu-Lac-OEt (1) and Boc-(Leu-Leu-Lac)(n)-OEt (n = 1, 2) (2 and 3, respectively) (Boc = tert-butyloxycarbonyl, Lac = L-lactic acid residue) has been synthesized and studied by crystallographic, CD spectroscopic, and ESI-MS analyses. In the packing cells, those three compounds adopt beta-strand conformations. Each molecule is linked into a dimer (1) or an infinite assembly (2 and 3) by tight hydrogen bonds of the type NH...O==C. Interestingly, the hexamer, 3 shows the first example of antiparallel pleated beta-sheet crystal structure for a depsipeptide molecule. In the packing cells, especially for 3, the ester groups O--C==O are perpendicularly oriented to the amide groups NH--C==O and beta-sheet planes to avoid the interaction between --O--(ester) and O==C. Therefore, when the chain length become longer, the O...O==C repulsion interaction works as a beta-sheet breaker and hence promotes an alpha-helical structure as observed for Boc-(Leu-Leu-Lac)(3)-Leu-Leu-OEt (4) (Oku et al. Biopolymers 2004, 75, 242-254) and Boc-(Leu-Leu-Lac)(n)-OEt (n = 4-6) (5-7) (Katakai et al., Biopolymers 1996, 38, 285-290), in which the O...O==C repulsion does not cause significant structural changes in alpha-helical main chains. Therefore from the structural and spectroscopic analyses, we have found governing factors for the specificity in the beta-sheet and alpha-helix decision in this series of depsipeptides, -(Leu-Leu-Lac)(n)-.

Construction of bioactive chimeric MHC class I tetramer by expression and purification of human–murine chimeric MHC heavy chain and β2m as a fusion protein in Escherichia coli

Major histocompatibility (MHC) class I tetramers are used in the quantitative analysis of epitope peptide-specific CD8+ T-cells. An MHC class I tetramer was composed of 4 MHC class I complexes and a fluorescently labeled streptavidin (SA) molecule. Each MHC class I complex consists of an MHC heavy chain, a beta(2)-microglobulin (beta(2)m) molecule and a synthetic epitope peptide. In most previous studies, an MHC class I complex was formed in the refolding buffer with an expressed MHC heavy chain molecule and beta(2)m, respectively. This procedure inevitably resulted in the disadvantages of forming unwanted multimers and self-refolding products, and the purification of each kind of monomer was time-consuming. In the present study, the genes of a human/murine chimeric MHC heavy chain (HLA-A2 alpha1, HLA-A2 alpha2 and MHC-H2D alpha3) and beta(2)m were tandem-cloned into plasmid pET17b and expressed as a fusion protein. The recombinant fusion protein was refolded with each of the three HLA-A2 restricted peptides (HBc18-27 FLPSDFFPSI, HBx52-60 HLSLRGLPV, and HBx92-100 VLHKRTLGL) and thus three chimeric MHC class I complexes were obtained. Biotinylation was performed, and its level of efficiency was observed via a band-shift assay in non-reducing polyacrylamide gel electrophoresis (PAGE). Such chimeric MHC class I tetramers showed a sensitive binding activity in monitoring HLA/A2 restrictive cytotoxic T lymphocytes (CTLs) in immunized HLA/A*0201 transgenic mice.

β‐Silks: Enhancing and Controlling Aggregation

It appears that fiber-forming proteins are not an exclusive group but that, with appropriate conditions, many proteins can potentially aggregate and form fibrils; though only certain proteins, for example, amyloids and silks, do so under normal physiological conditions. Even so, this suggests a ubiquitous aggregation mechanism in which the protein environment is at least as important as the sequence. An ideal model system in which forced and natural aggregation has been observed is silk. Silks have evolved specifically to readily form insoluble ordered structures with a wide range of structural functionality. The animal, be it silkworm or spider, will produce, store, and transport high molecular weight proteins in a complex environment to eventually allow formation of silk fibers with a variety of mechanical properties. Here we review fiber formation and its prerequisites, and discuss the mechanism by which the animal facilitates and modulates silk assembly to achieve controlled protein aggregation.

Structural Conformation of Spidroin in Solution: A Synchrotron Radiation Circular Dichroism Study

Spider silk is made and spun in a complex process that tightly controls the conversion from soluble protein to insoluble fiber. The mechanical properties of the silk fiber are modulated to suit the needs of the spider by various factors in the animal's spinning process. In the major ampullate (MA) gland, the silk proteins are secreted and stored in the lumen of the ampulla. A particular structural fold and functional activity is determined by the spidroins' amino acid sequences as well as the gland's environment. The transition from this liquid stage to the solid fiber is thought to involve the conversion of a predominantly unordered structure to a structure rich in beta-sheet as well as the extraction of water. Circular dichroism provides a quick and versatile method for examining the secondary structure of silk solutions and studying the effects of various conditions. Here we present the relatively novel technique of synchrotron radiation based circular dichroism as a tool for investigating biomolecular structures. Specifically we analyze, in a series of example studies on structural transitions induced in liquid silk, the type of information accessible from this technique and any artifacts that might arise in studying self-assembling systems.

Spectroscopic investigation on gel-forming ?-sheet assemblage of peptide derivatives

The conformational studies of peptide derivatives A and B in a gel state were studied by using circular dichroism (CD), Fourier transformed infrared (FTIR), and fluorescence spectroscopic techniques. Birefringence and electron microscopic studies were carried out to characterize the morphological aspects of the fibrils in the gel. The FTIR spectra of the peptides show the absence of free NH in the gel state, implying that the intermolecular hydrogen-bond formation is the driving force for the aggregation. The CD spectrum of the peptide gels shows the presence of antiparallel and parallel beta-sheet conformation for peptide derivatives A and B, respectively. Electron microscopic studies (EM) of the peptide derivatives A and B reveal that peptide A formed rigid, rod-like structures without cross-linking and peptide B formed loose fibrils organized into highly noncovalently cross-linked mesh-like structural aggregates. Peptide A was much more soluble in alcoholic solvents than peptide B, and no birefringence was observed with Congo red (CR) staining in the temperature range of 0-80 degrees C. The spectroscopic studies indicate that peptide B consists of domains having a significant amount of beta-sheet structure and exhibiting golden yellow birefringence between 53 and 56 degrees C when stained with Congo red. On the other hand, peptide A gives no evidence of birefringence under polarized light. Fluorescence probe binding studies with pyrene in gel state with peptides A and B indicates the polarity in the interior of the aggregates. The data presented in the present work indicate that peptide B forms fibrils, which is similar to amyloid aggregates that are present in biological systems.

The natural silk spinning process. A nucleation-dependent aggregation mechanism?

The spinning mechanism of natural silk has been an open issue. In this study, both the conformation transition from random coil to beta sheet and the beta sheet aggregation growth of silk fibroin are identified in the B. mori regenerated silk fibroin aqueous solution by circular dichroism (CD) spectroscopy. A nucleation-dependent aggregation mechanism, similar to that found in prion protein, amyloid beta (Abeta) protein, and alpha-synuclein protein with the conformation transition from a soluble protein to a neurotoxic, insoluble beta sheet containing aggregate, is a novel suggestion for the silk spinning process. We present evidence that two steps are involved in this mechanism: (a) nucleation, a rate-limiting step involving the conversion of the soluble random coil to insoluble beta sheet and subsequently a series of thermodynamically unfavorable association of beta sheet unit, i.e. the formation of a nucleus or seed; (b) once the nucleus forms, further growth of the beta sheet unit becomes thermodynamically favorable, resulting a rapid extension of beta sheet aggregation. The aggregation growth follows a first order kinetic process with respect to the random coil fibroin concentration. The increase of temperature accelerates the beta sheet aggregation growth if the beta sheet seed is introduced into the random coil fibroin solution. This work enhances our understanding of the natural silk spinning process in vivo.

Quantitative IR spectrophotometry of peptide compounds in water (H2O) solutions. II. Amide absorption bands of polypeptides and fibrous proteins in alpha-, beta-, and random coil conformations

Infrared spectra of poly(D,L-alanine), poly(L-glutamic acid), poly(L-lysine), silk fibroin, and tropomyosin have been registered for various conformations of the polypeptide chain. Assuming additivity of the main- and side-chain absorption, spectral parameters of amide I and II absorption bands corresponding to alpha-, beta-, and random coil conformations have been derived. The amide I band parameters for H2O and D2O have been compared.

CD and small-angle x-ray scattering of silk fibroin in solution

We investigated the structure of silk fibroin dissolved in water and in water-organic solvent mixtures by CD and small-angle x-ray scattering (SAXS). CD spectra indicated a disordered secondary structure in water and a beta-sheet conformation in aqueous organic solvents, such as methanol, dioxane, and trifluoroethanol (in trifluoroethanol a transient form evolving toward beta-sheet conformation was seen just after dissolution). The SAXS technique indicated the presence of fibroin particles of lamellar shape. The molecular weight was 188,000 daltons in water and 302,000 daltons in aqueous methanol.

Beta-structure of polypeptides in non-aqueous solutions. I. Spectral characteristics of the polypeptide backbone

The optical rotatory features of the beta-structure of the polypeptides in non-aqueous solutions and films cast from these solutions have been investigated. The beta-structure of poly-S-benzyl-L-cysteine, poly-S-carbobenzoxy-L-cysteine and poly-S-benzyl-L-cysteine, poly-S-carbobenzoxy-L-cysteine and poly-O-carbo-bands of their films. The optical rotatory dispersion (ORD) and circular dichroism (CD) spectra of these polypeptides are found to be very similar in both film and solution. In solvents promoting the beta-structure, the polypeptides are characterized by CD troughs in the n-pi* transition region of the peptide chromophore. The ORD spectra are found to be positive in sign throughout the visible and accessible ultraviolet regions and are interpreted in terms of the possible existence of a relatively much larger positive pi-pi* CD bands as compared with the negative n-pi* band. The rotatory data obtained in the non-aqueous solution are compared with those obtained for other poly peptides in aqueous solutions, with respect to the type and extent of beta-structure present.

Identification of beta,beta-turns and unordered conformations in polypeptide chains by vacuum ultraviolet circular dichroism

Different conformations of polypeptides were characterized by measurements of the circular dichroism (CD) extended into the vacuum ultraviolet region. (i) The linear beta-pleated sheet structure was characterized in a broad ultraviolet region down to 165 nm by examination of copolypeptides composed of alternating hydrophobic and hydrophilic amino-acid residues, e.g., poly(Lys-Leu-Lys-Leu). A short-wavelength intense band was found at about 169 nm, which is characteristic of beta-pleated sheet conformation. (ii) The beta-turns were experimentally measured using poly(Ala(2)-Gly(2)) in a broad spectral region down to 165 nm with accuracy. The observed CD spectrum is in excellent qualitative agreement with the theoretical curve calculated by Woody for the beta-turns of type II and/or I of Venkatachalam. The similarity in shape between the theoretical curve and the observed CD spectra suggests a dominance of beta-turn segments in the poly(Ala(2)-Gly(2)) structure. The presence of beta-turns in poly(Ala(2)-Gly(2)) is also in agreement with the characterization of this polypeptide by solid state methods (electron microscopy and x-ray diffraction). The CD spectrum of beta-turns is characterized by a very intense band at 207.5 nm and strong negative bands at 191 and 169 nm. Copolypeptides such as poly(Ala(2)-Gly(3)) and poly(Ala(3)-Gly(3)) yielded a similar type of CD spectrum, analysis of which indicates that a large fraction of their residues is contained in beta-turn regions. (iii) The CD spectrum of the unordered chain of these alternating copolypeptides in salt-free solution is observed in the vacuum ultraviolet region.

Beta structures of alternating polypeptides and their possible prebiotic significance

A survey of the commonest amino acids formed in prebiotic conditions suggests that the earliest form of genetic coding may have specified polypeptides with a strong tendency to form stable Beta-sheet structure. Poly(Val-Lys), like other polypeptides in which hydrophobic and hydrophilic residues alternate, tends to form Beta structures. We show that bilayers with a hydrophobic interior and a hydrophilic exterior may be present in aqueous solution.

The secondary structure of human Hageman factor (factor XII) and its alteration by activating agents

Hageman factor (factor XII) is activated by exposure to surfaces such as glass or by solutions of certain compounds, notably ellagic acid. Changes in the structure of Hageman factor accompanying activation have been examined in this study by circular dichroism spectroscopy. The spectrum of unactivated Hageman factor in aqueous solutions suggests that its conformation is mainly aperiodic. Various perturbants altered the conformation of Hageman factor in differing ways, demonstrating the sensitivity of Hageman factor to its environment. After activation of Hageman factor with solutions of ellagic acid, a negative trough appeared in the region of the circular dichroism spectrum commonly assigned to tyrosine residues, along with other minor changes in the peptide spectral region. Some of these changes are similar to changes that occurred upon partial neutralization of the basic residues at alkali pH. Activation of Hageman factor by adsorption to quartz surfaces (in an aqueous environment) also produced changes similar to those in the ellagic acid-activated Hageman factor, including the negative ellipticity in the tyrosine region. These observations suggest that the activation process may be related to a change in status of some of the basic amino acid residues, coupled with a specific change in the environment of some tyrosine residues. The importance of these changes during the activation process remains to be determined. The sensitivity of Hageman factor to its environment is consistent with the view that the initiation of clotting by exposure of plasma to appropriate agents is brought about by alterations in the conformation of Hageman factor that occur in the apparent absence of Fletcher factor or other recognized clotting factors.

Physical and chemical properties of Trichoplusia ni granulosis virus granulin

The protein solubilized from the proteinic crystalline structure surrounding the granulosis virus of Trichoplusia ni by use of a carbonate buffer (pH 10.7) gives a major component, as analyzed by ultracentrifugation, with a molecular weight of 180,000. This protein has heterogeneous subunit structure as demonstrated by estimates of molecular weights by use of gel electrophoresis, amino-, and carboxy-terminal analyses, and peptide mapping of enzyme digests of the protein. The amino acid composition shows that the protein is acidic with a high percentage of amino acids with hydrophobic side groups. Optical rotatory dispersion studies reveal the presence of beta-structure in the protein complex. The conversion of the beta-structure to alpha-helix with sodium lauryl sulfate and to a random coil state with strong alkaline treatment are observed.