Beta-Keratin

FIGURE 10.5 Beta-keratin sheet for the copolymer derived from glycine and alanine. 

The beta-keratin structure is also foimd in the feathers and scales of birds and reptiles. 

Alpha keratin is found in sheep wool. The springy nature of wool is based on its composition of alpha helices that are coiled around and cross-linked to each other through cystine residues. Chemical reduction of the cystine in keratin to form cysteines breaks the cross-links. Subsequent oxidation of the cysteines allows new cross-links to form. This simple chemical reaction sequence is used in beauty shops and home permanent products to restructure the curl of human hair—the reducing agent accounts for the characteristic odor of these products. Beta keratin is found in bird feathers and human fingernails. The more brittle, flat structure of these body parts is determined by beta keratin being composed of beta sheets almost exclusively. 

The apparent identity distance in the fiber X-ray diagrams of silk is somewhat smaller than corresponds to a completely extended polypeptide chain. We accordingly concluded that the polypeptide chains have a zigzag conformation in silk and the beta-keratin structure. We reported in detail three proposed sheet structures. The first one, which we... 

Fraser and MacRae have reported important absorption bands for natural product proteins and nylon polyamide at 4870 cm- (2188 nm) resulting from a combination of the peptide absorptions at 3305 cm- (3026 nm) and 1540 cm- (6494 nm). For feather shafts, the authors report absorption bands at 4970 cm- (2012 nm) and 5040 cm (1984 nm). A shoulder at 5040 cm (1984 nm) was reported as resulting from side chain amide groups. For beta-keratin, the paper reports important absorption bands at 4600 cm- (2174 nm), 4850 cm- (2062 tun), and 4970 cm (2012 nm). The authors were interested in studying the amorphous components of naturally occurring protein structures. 

In wet hair the hydrogen bonding of alpha keratin can break to transform to beta keratin and then hair can be shaped by brushing or with hair-curlers. After drying, hydrogen bindings return to their initial position and the form of the hair is temporary maintained. (Diaz-Sales, 1998). 

T. Kai, T. Isami, Y. Kurosaki, T. Nakayama, and T. Kimura. Keratinized epithelial transport of beta-blocking agents. II. Evaluation of barrier property of stratum corneum by using model lipid systems. Biol. Pharm. Bull. 16 284—287 (1993). 

Both the 5.1-A spacing characteristic of the spacing of helix turns and the 9.8 A of the lateral helical spacing decrease in intensity at 210° and 230 °C (49). The fiber also loses its birefringence in this temperature range. These reflections are not present in fibers which have been super-contracted or otherwise rendered amorphous. Similarly, the x-ray diffraction pattern of wool heated to 130 °C while immersed in water shows the total disappearance of the alpha-keratin reflections and the appearance of a disordered beta pattern (66). When either the dry or the wet fiber is heated above these melting temperatures, the transformation is irreversible. 

X-ray diffraction and IR dichroism studies suggest that the long-range elasticity of wool is related to a reversible molecular transformation of the alpha-keratin to an extended beta form (66). No convincing evidence supports this mechanism in stratum corneum viscoelasticity. In fact, the available evidence suggests that the elastic behavior of corneum is primarily entropic in origin. At low deformations, the mechanical properties of hydrated stratum corneum is best described as the behavior of a lightly-crosslinked rubber. 

When wool is stretched, a-keratin is converted into /9-keratin, with a change in the x-ray diffraction pattern. It is believed that the helixes are uncoiled and the chains stretched side by side to give a sheet structure of the beta type. The hydrogen... 

Great advances have been made in protein chemistry since the time of Astbury s pioneering work. Polyalanine, like keratin, exists in an alpha form which may be transformed into the beta conhguration by stretching. It is much easier to interpret X-ray diffraction diagrams of substances of known constitution, such as polyalanine, than those obtained from the infinitely more complex keratin molecule. 

As mentioned before, hydrolytic cleavage of peptide bonds in keratins, as well as formation of lanthionyl residues, can also occur in alkali. In addition to lanthionine, lysinoalanine [59] and beta-aminoalanine [60] residues can be formed in some keratins under alkaline conditions. 

Earland and Raven [65] have examined the reaction of A-(mercap-tomethyl) polyhexamethyleneadipamide disulfide (XV) with alkali. Under alkaline conditions that produce lanthionyl residues in wool, no thioether is formed from this polymeric disulfide however, cyanide readily produces thioether from either (XV) or wool fiber. Therefore, the mechanism for thioether formation must be different in these two reactions. Because this polymeric disulfide (XV) contains no beta-hydrogen atoms (beta to the disulfide group), a likely mechanism for formation of lanthionyl residues in keratins, under alkaline conditions, is the beta-elimination scheme [64] (the reaction depicted by Equation F). Other mechanisms that have been suggested for this reaction have been summarized by Danehy and Kreuz [66]. 

The formation of lanthionine in keratin libers is believed to involve two reaction sequences. The first sequence consists of beta-elimination to form dehydroalanine residues in hair ... 

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