PROTEINS 



341 



1933c; Mark and Philipp, 1957) that such sulphur bonds hold the 

 polypeptide chains together in keratin, for sulphur plays a similar 

 part in vulcanized rubber. It connects the free polyprene chains 

 of the raw rubber laterally, in this way producing a molecular frame, 

 and thus enhances the elastic properties of the raw rubber, while 



:i 







o) 



NH 



NH 



CH' 



I 



CO- 



I 



NH 



RCH 



ho 



I 



NH 



^: 



CO 



I 



CH, CO 



I 

 (^0 



NH 



CHy CH2 I 



^CHl'^.N'H'^f^^CtC '^"^ 



Glutamic I 

 acid 



-^0 

 Arginine j^^ 



I 



CM. 



CH. 



S S' 



Cystine 



ho 



NH 



-ho 



CHR 



ho 

 f 



RCH 



Fig. 172. a) Keratin frame as lattice grid; b) side chains of keratin. 



its plasticity deteriorates. If too many sulphur bridges are intro- 

 duced, however, the material will lose its elasticity, being "vulcanized 

 to death", and hard rubber or ebonite results. Now there is some 

 analogy between raw rubber and vulcanized rubber, on the one hand, 

 and actomyosin (free from sulphur, p. 3 5 2) and keratin (containing sul- 

 phur), on the other. By way of comparison, therefore, the tonofibrillae 

 have been termed "vulcanized' muscle fibres, which would explain 

 the loss of contractility and their great strength. 



Despite illuminating comparisons such as these, which are very 

 helpful to a qualitative interpretation, there remain serious quanti- 

 tative obstacles to a complete understanding of the submicroscopic 

 structure of keratin. Above all, the length of the cystine molecule does 

 not agree with the X-ray evidence as to the length of the keratin side 

 chains. As is apparent from Fig. 172 b, the sulphur bridge is by no 

 means long enough to span the distance of 9.8 A from primary chain 

 to primary chain. Hence the molecular frame cannot be as simple and 



