256 



KERATIN AND KERATINIZATION 



The destruction of the crystallites in such a system, produced either by a 

 solvent able to penetrate the crystallites or by a rise in temperature suffi- 

 cient to " melt " them, releases the molecular chains which may then 

 assume a more probable, less oriented, and shorter, configuration. This 

 change may be visible as a change in shape of an oriented specimen, and 

 is accompanied by the loss of other signs of orientation, such as bi- 

 refringence and the fibre-type X-ray pattern (Fig. 109). The contracted 



(a) (b) (c) 



Fig. 109. To illustrate the relation between the X-ray pattern given by a 

 crystallite in an unswollen fibre (a); a fibre (b) in which swelling mole- 

 cules have penetrated only the amorphous phase and the X-ray pattern is 

 unaffected and (c) a fibre in which molecules have penetrated the crystal- 

 lite disorienting the molecular arrangement to give a non-oriented X-ray 

 pattern and causing a further shortening (supercontraction) of the fibre 

 length. 



material may also acquire rubber-like properties. Perhaps the most 

 striking example of this is provided by collagen fibres which have less 

 interchain bonding than keratin and may shorten to as much as one-third 

 of their original length when heated above a well-defined temperature in 

 water. This "contraction temperature" is the point at which the crystallites 

 disperse or " melt"; it is not as well defined in the keratins. On cooling, 

 contracted collagen spontaneously resumes its original length and the crys- 

 tallites reform. Clearly the normal triple helical structure is adequately 

 stabilized by internal energy considerations at normal temperatures. 



