254 



KERATIN AND KERATINIZATION 



The effect of the reduction of the disulphide bonds on the work of ex- 

 tension is shown in Fig. 107. Reduced wool or " blocked wool " (reaction 

 (b)) was found to be more easily stretched, more readily dissolved and less 

 resistant to enzymatic digestion than normal (Geiger et ai, 1941 and 1942). 

 Resistance was restored when new cross-linkages were introduced by 

 alkylation (reaction (a)). 



% Cystine bonds broken 



Fig. 107. Relationship between wet strength of wool fibres and the 

 number of disulphide bonds (Harris and Brown, 1946). 



These reactions form the basis of current hair-waving treatments in 

 which the hair, softened by thioglycollate reduction, is deformed into the 

 desired shape and held there until cross-linkages have reformed by 

 oxidation (McDonough, 1952). 



Hydrogen Bonds 



Even when the greater part of the disulphide bonds are broken, the 

 strength of dry fibres is not greatly reduced and the fibre form may be 

 retained when the fibre is placed in solutions of pH< 9 at normal temper- 

 ature. Above this pH much of the keratin may enter solution (see p. 240). 

 The stabilization persisting after reduction is attributed to hydrogen 

 bonding, the presence of which is directly revealed by infra-red absorption 

 spectra (Chapter V, p. 196). When steps are taken to break the hydrogen 

 bonding in a previously-reduced fibre, a characteristic contraction in 

 length occurs. Following Astbury and Woods, this is usually called 



