THE KERATINIZATION PROCESS 259 



bond rupture and the dispersion of the crystallites without rupture of 

 disulphide bonds. In this form the X-ray a-pattern fades and the con- 

 tracted fibre shows no wide-angle pattern. Characteristically the con- 

 traction may be reversed by washing out the reagent; the crystallites 

 reform, the fibre resumes its original length and the a-pattern returns. 

 Examples: supercontraction in cuprammonium solution (Whewell and 

 Woods, 1946), lithium bromide (Alexander, 1951), phenols (Zahn, 1947) 

 and formamide (Elod and Zahn, 1944). A third type of contraction occurs 

 when the disulphide bond system is destroyed and when the conditions of 

 contraction are not too violent (<20% contraction). In this case the 

 a-pattern may still be elicited from the contracted fibre. Examples: 

 contractions in dilute caustic soda (Whewell and Woods, 1946) or after 

 peracetic acid oxidation (Alexander, 1951). When conditions are more 

 drastic (higher temperature or longer treatments) the crystallites may be 

 affected and the more strongly-contracted fibre gives a jS-pattern. 



Astbury and Woods, Whewell and Woods and Alexander all recognized 

 that these results show that, in a sense, the keratin may be divided into two 

 parts which may be induced to contract more or less independently. This 

 separation can now be understood in terms of the filament-plus-matrix 

 model. Disulphide-bond destruction converts the matrix into a viscous 

 material which facilitates chain movement both in itself and in the filaments. 

 If a limited contraction of the non-crystalline chain segments occurs, the 

 fibre shortens and the a-crystallites persist. In the other case, when the 

 disulphide cross-linked matrix is intact while contraction is induced by 

 freeing the chains in the crystallites, a contraction results with crystallite 

 destruction which is reversible, because the over-ruling macromolecular 

 organization is preserved by the cross-linked matrix which envelops the 

 filaments. 



Another form of supercontraction is that which leads to the formation of 

 a cross-jS configuration in the contracted fibre. This may be the case when 

 contraction is produced under mild conditions in reagents which both 

 loosen the matrix by reducing disulphide bonds and also disperse the 

 crystallites by rupturing hydrogen bonds, e.g. strong solutions of urea 

 containing bisulphite (Mercer, 1949a). See Fig. 84 and Plate 2B. The 

 cross-/3 pattern has already been discussed on p. 200 et seq. 



The Setting of Hairs 



The study of the important phenomenon of set, by which is meant the 

 more-or-less permanent retention of a deformed state, has shown that the 

 same factors which stabilize a fibre in its natural state are also those which 

 operate to maintain a stretched state or set. A hair which is stretched in 

 water, relaxed and dried will partly retain the stretched length. It quickly 

 returns to its original length in water (Fig. 108) and more rapidly in 



