252 KERATIN AND KERATINIZATION 



Speakman and his collaborators. The dependence of the 30% index on 

 pH is shown in Fig. 106 (Speakman and Hirst, 1933; Speakman, 1934) 

 reveals a weakening of the fibre towards the extremes of pH suggestive of 

 "hydrolysis" of salt-linkages, that is a weakening of electrical attraction 

 following the discharge of ionized groups by combination with H + or 

 OH - . Speakman and Hirst (1933), showed that the reduction in work in 

 acid solutions was proportional to the amount of acid combined with the 

 fibre, i.e. proportional to the number of salt-links put out of action. The 

 amount of acid or base bound as a function of pH is shown in Fig. 105 and 

 it may be compared to the work-reduction in Fig. 106. 



There is room for some difference in opinion over the theoretical 

 interpretation of these findings or even whether the term " salt-link " is 

 really appropriate. Nevertheless, from the point of view of the effects of 

 keratinization, it is significant that the reduction in cohesion of the fibre 

 and its swelling (and its combination with acids and bases which is inter- 

 related with these) are minimal in a broad range of pH values including 

 neutrality. The material is in a sense " buffered " against environmental 

 changes in the range of variables which, biologically speaking, might well 

 be encountered by the integument. 



Disulphide Bonds 



In the literature dealing with the interpretation of the elastic properties 

 of wool the relative importance of disulphide bonds and hydrogen bonds 

 has been much debated. Speakman in particular insisted on the pre- 

 dominant role of the sulphur bridge; Alexander (1951), Elod and Zahn 

 (1944 and 1949) principally did much to direct attention to the importance 

 of hydrogen bonding. Speakman and collaborators (Speakman, 1934, 1936 

 and 1947) have demonstrated both the weakening of fibres when the 

 disulphide bond is broken and their recovery when it is reformed. 



The existence of disulphide bonds and their effect on fibre properties is 

 revealed most clearly by the series of elegant experiments of Harris and his 

 collaborators (Patterson et al., 1941; Harris and Brown, 1946) who used 

 thioglycollic acid to reduce cystine bridges: 



P— S— S— P + 2 HS.CHo.COOH -> 2 P.SH + CH.,COOH 



I 

 S 2 .CH 2 .COOH 



and investigated not only the properties of the reduced fibres, but also those 

 of the fibres in which the cross-linkages had been rebuilt (a) or blocked (b) 

 by reacting the reduced thiols with alkylhalides : 



2 P.SH + Br.CH 2 CH 2 Br -> P.S.CH 2 .CH 2 .S.P + 2 HBr (a) 



P.SH + Br.CH 2 .CH 3 -> P.S.CH 2 CH 3 + HBr (b) 



