180 KERATIN AND KERATINIZATION 



structural analysis. Chemically they are rather simple. Bombyx silk and 

 Tussah silk fibroin are alike in that glycine, alanine, serine and tyrosine 

 comprise over 90% of the amino acids. The classical studies of Meyer and 

 Mark and Astbury, which have been referred to above, made it clear that 

 in certain fibres, yielding the j3-type pattern with a fibre period of 7 A, 

 the polypeptide chains must be almost fully extended. The completely 

 extended chain is calculated to have a repeat spacing of 7*27 A which is 

 significantly greater than that actually found. 



Marsh, Corey and Pauling (1955) have made a thorough comparison 

 between the diffraction data and the predictions based on the anti-parallel 

 sheet model and have established beyond question that this structure 

 exists in Bombyx silk. In their model the chains of fibroin consisting of (in 

 the main) alternate glycine and alanine residues, are arranged so that the 

 CH 3 groups of the alanines stick out entirely on one side of the sheets. 

 Pairs of sheets are thus separated alternately by distances of 3*5 A and 

 5*6-7 A, spacings which figure prominently in the X-ray patterns. There 

 are minor points still disputed. Peptides comprising the bulk of the pro- 

 tein and containing, apparently, long sequences of glycyl-alanine pairs 

 (Lucas et al., 1956 and 1958) and sequences in which serine substitutes for 

 glycine or alanine residue (Waldschmidt-Leitz and Zeiss, 1955) have been 

 separated from digest of Bombyx fibroin. These glycyl-alanine clusters 

 probably join together to form the crystallites. The solution of War- 

 wicker (1954) is essentially the same. 



There is no place in these models for the more bulky side chains, such 

 as that of tyrosine, which together constitute some 18% of the protein. It 

 is supposed therefore that they are inserted at intervals (regularly or at 

 random) in the chains, thus buckling the sheets and causing some of the 

 diffuse reflections always present. Since in the case of keratins the residues 

 with larger side chains form a far greater part of the protein, distortions 

 arising from such causes must be far more common and perhaps account 

 for the larger amount of diffuse scattering from these materials. 



Tussah silk yields a /^-pattern distinct from Bombyx silk and is isomor- 

 phous with polyalanine (Bamford et al., 1954). It contains more alanine 

 than Bombyx and thus, assuming a similar antiparallel-sheet arrangement, 

 both faces must contain methyl groups and all pairs of sheets must be 

 similarly spaced at 5*3 A. Kay et al. (1956) have shown the existence of 

 " cores " of peptides containing long sequences of alanine. 



No attempt has yet been made to discuss rigorously the diffraction data 

 of the ^3-keratins of stretched hair and wool in terms of the pleated-sheet 

 models and it would seem, in view of the small amount of information to 

 be obtained from the actual patterns themselves, that the attempt would 

 not be immediately fruitful. The general similarity of all these patterns 

 leaves little doubt, however, that in the crystalline regions of the materials 



