a PROTEINS 345 



and Zahn, 1942). The side chains, they say, stand perpendicular to 

 the planes of the frame and it is therefore of no consequence if they 

 differ in length. It is assumed that the grids form a laminar structure 

 parallel to the surface of the hair or nail. 



Other arguments against the salt link theory are advanced by 

 LiNDLEY (1950). The basic amino acid residues arginine, lysine and 

 histidine have a constant ratio 12:4:1, whereas the other members 

 of the keratin polypeptide (cystine, tyrosine, glutamic and aspartic 

 acid) show considerable fluctuations depending on the wool sample 

 chosen (Block, 1939). Fractional hydrolysis yields peptides of low 

 molecular weight with numerous acid residues clustered together, 

 whilst the basic amino acids are regularly distributed along the poly- 

 peptide chains. 



MiDDLEBROOK (1951) thinks that cystine which amounts to about 

 1/8-1/4 of the total number of amino acid residues is concentrated in 

 definite regions along the polypeptide chains, and that these regions, 

 w^ith a periodicity of about 200 A, cannot assume the a-folds because 

 of steric hindrance. Therefore, a-keratin would always contain peri- 

 odical short segments with straight /^-constellation. 



If a hair, which has been stretched 100% and temporarily set, is 

 placed, free, in a steam bath for a short time, it will contract, not 

 only to its original length, but considerably further; a super con- 

 traction takes place. This fact implies that the polypeptide chains in 

 the a-keratin are not entirely free and independent of each other; 

 rather, it would seem that they too are mutually stabilized by certain 

 bridges. Apparently, however, the treatment breaks up these linkages 

 and enables the polypeptide chains to fold far more than before. 

 Restrictive lateral bridges of this kind are also supposed to be re- 

 sponsible for the fact that a hair is only 50-70% extensible in cold 

 water; they weaken in hot water and the polypeptide chains can then 

 be fully stretched (about 100%). 



If keratin is exposed for a short time to the action of hot water or 

 vapour, connecting bridges between protein chains are evidently 

 broken down. Since dilute caustic soda similarly loosens the chains, 

 this might be a case of hydrolytic decomposition of acid amide 

 bridges. Yet the self-same treatment, if more prolonged, will facilitate 

 the formation of new, stronger bonds. In view of the theory regarding 

 the structure of cytoplasm developed in this book, this behaviour is 



