THE KERATINIZATION PROCESS 257 



The temperature at which contraction commences is a useful measure 

 of the degree of stabilization of a fibre. Most hard keratins require a 

 temperature above 100°C in water. It is lowered by treatments which 

 reduce the degree of cross-linking (H bonds, covalent bonds or salt- 

 linkages) and the temperature difference is a measure of the reduction in 

 internal stabilization. For comparison of different keratins it is more 

 convenient experimentally to use solutions of substances (phenol, forma- 

 mide, etc.) which weaken the internal cohesion due to hydrogen bonding 

 and bring the contraction-temperature below 100°C. Elod and Zahn (1946 

 and 1949) and Stoves (1947) examined various hairs in this way and found 

 that the temperature required to initiate contraction increased with the 

 cystine content and the fibre diameter. 



The careful analysis of the elastic behaviour of keratin fibres by Bull 

 (Bull and Gutman, 1944; Bull, 1945) and Woods (1946) has thrown 

 much light on the relation between the internal energy factors tending to 

 stabilize the oriented structure and the randomizing entropy factors. It is 

 shown in thermodynamics that, if the tension on a specimen is P and its 

 length L, then for reversible changes : 



p= \di)~ T \bi)r UZ/r + \Wl 



Where U is the internal energy, S the entropy and T the temperature. 



In these equations the tension P is expressed as the sum of the two 

 terms: one Pu depending on the internal energy changes and the other 

 Ps depending on the change in entropy when L is increased. Since the 

 latter can be measured by observing the temperature coefficient of the 

 tension at constant length (dPjdT) L , the two can be determined separately. 

 The dominant factor controlling the length was shown by Woods and 

 by Bull to be the internal energy term (Pu) ; even in swollen and relaxed 

 fibres the entropy term remains small but becomes more important in 

 supercontracted fibres in which the crystallites are dispersed and the 

 molecular arrangement more randomized. 



The long-range reversible elasticity shown by hairs should not be 

 thought of as a characteristic of keratin per se. It is rather a characteristic 

 of the a-type proteins and is related to the a-type molecular structure. The 

 definitive feature of a keratin is the stabilization based on cystine cross- 

 linking and this chemical device may be used to stabilize proteins of a 

 different structure and quite different elastic potentialities (see p. 24). 

 Feather cells, for example, are similar in fine structure to the cortical cells 

 of hair and the fibrillar system is also stabilized by cystine cross-linking ; 

 the keratin is, however, of jS-type which permits of extensions of only a few 

 per cent. This lack of long range extensibility was one of the direct 

 indications which led Astbury and Marwick (1932) to the view that the 



