234 KERATIN AND KERATINIZATION 



presumably form the first stage of synthesis. And further, it is meaningful to 

 ask whether or not there exists a hierarchy of definite molecular association 

 of increasing molecular weight whose formation precedes the appearance 

 of the finest filaments which can be seen microscopically (diameter 

 ~ 60 A). 



There are two ways in which these problems have been attacked: (a) 

 extracts have been made of the growing tissues with the object of dissolving 

 the proteins before they become keratinized, and (b) attempts have been 

 made, starting with the hardened tissues, to reverse the keratinization 

 process and to obtain soluble macromolecular products from the hardened 

 tissue. This degradation could obviously be continued until small pep- 

 tides and amino acids were obtained. The full analysis of the complete 

 mixtures resulting from such a partial hydrolysis would clearly do much to 

 elucidate the amino acid sequence of the original polypeptides and it is 

 regrettable that, apart from the pioneering work of Consdon, Gordon, 

 Martin and Synge (Martin, 1946), little has been attempted. For the 

 present we are more concerned with the possible existence of high mole- 

 cular weight, intermediate polypeptides of a definite character. 



From a consideration of the composition and reactions of proteins in 

 general, and of the keratins in particular, the following kinds of bonds 

 might be supposed to participate in the consolidation of an insoluble 

 protein : 



(a) Hydrogen bonds, i.e. associations between neighbouring CO and NH 

 groups mediated by the hydrogen atom (see Chapter V). 



(b) Salt bridges, i.e. salt-like linkages formed between acid groups 

 ( — COOH) and amino groups (— NH 2 ). Speakman (1934) has 

 amassed evidence to show these are effective in hair and wool. (For 

 a contrary opinion, see Jacobsen and Linderstram-Lang (1949).) 



(c) Weaker and less well-defined forces referred to as Van der Waal's 

 forces. 



(d) Disulphide bridges (— S— S— ). 



(e) Other bonds have been proposed, e.g. between phenolic OH groups 

 and acid groups (Alexander and Hudson, 1954) but are not known to 

 exist for certain. 



All these bonds and perhaps others not yet discovered, may play a role in 

 stabilizing insoluble proteins; accordingly, the solvents used to effect a 

 solution or make extracts of keratinized tissues have been chosen because 

 of their specific effect on one or more of these bonds. In keratin the co- 

 valent disulphide bonds appear ultimately to prohibit solution and next in 

 importance on account of their number are the hydrogen bonds. These 

 appear particularly to influence the dry hardness and extensibility. The 

 histochemical experiments described above (Figs. 91 and 92) seem to show 

 that the H-bond sustains the structure in the early stages of keratinization 



