258 KERATIN AND KERATINIZATION 



feather structure was based on more or less fully-extended chains. Some- 

 what surprisingly the well-oriented /^-configuration of feather does not 

 supercontract in solvents which both reduce disulphide bonds and break 

 H-bonds although it swells and partly dissolves. When regenerated in 

 fibre form, the amorphous threads in water are rubber-like (Ward et al., 

 1946). 



Meyer et al. (1949, 1952) have criticized the broad picture developed 

 by Astbury and Woods and asserted that the long range elasticity is really 

 rubber-like. This criticism seems to be based on a misconception. Meyer's 

 experiments were carried out on hairs which were strongly swollen and 

 reduced, i.e. on hairs in which special steps had been taken to reduce the 

 internal energy factors, and under these conditions the entropy term may 

 be expected to predominate. The theories of Astbury and Woods were, in 

 fact, developed to give an account of the elastic behaviour of fibres in which 

 the internal constraints were effective. In their theory the swollen fibre 

 with its reduced interchain bonding and a relative absence of covalent 

 cross-linking should be essentially rubber-like. In fact this condition has 

 been demonstrated experimentally by Meyer and Haselbach (1949). 



Molecular Configuration in the Supercontracted State 



The molecular mechanism involved in supercontraction is complex. 

 In terms of the earlier interpretation of Astbury (1933) the shortening in 

 length (about 30%) is due to a further folding of the polypeptide chains 

 into a shorter configuration and this view still prevails although no precise 

 picture of how an a-helix shortens has been proposed. 



Strongly supercontracted fibres usually yield an unoriented £-type 

 X-ray pattern which is not very helpful. In some instances an oriented 

 a-pattern may persist in fibres shortened by 20%, showing that the 

 contraction is occurring in the non-crystalline phase ; in others all signs of 

 pattern may vanish. These findings suggest that the mechanism of 

 contraction may not always be the same. The numerous chemical treat- 

 ments which produce some degree of supercontraction have in common 

 only the property of reducing interchain bonding. 



A classification of types of supercontraction is, however, possible on 

 structural grounds. The simplest situation is perhaps the most drastic, 

 e.g. boiling in solutions of bisulphite, which leads to a disoriented /3-type 

 of configuration (Astbury and Woods, 1933; Whewell and Woods, 1946). 

 The explanation here is that as a result of a chemical weakening of inter- 

 chain interaction, the main chains, acting as mobile individuals under the 

 influence of thermal agitation, are able to assume a shorter and more 

 random configuration. On drying these randomized chains may in part 

 recrystallize in the ^-configuration. 



The second type of supercontraction is that which results from hydrogen 



