346 



FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES 



III 



N 



/ 



CH 



COH- 



I 

 N 



II 

 -COH 



N 



I 



HC 



I 



■CH-. 



\ 



'\ 



/ 



CH, 



CH. 



/ 



CH, 



co- 



COH- 



CHR 

 \ 



/ 



CH 



\ 



Profile 



COH- 



HOCH 

 \ 



CO 



I 



CH^ 



N 



II 

 - COH 



I 



CHR 



I 



N 



II 

 COH 



/"^^Ih 



Proline 



Glycine 



■ Amino acid R 



presence in the collagen of about 24% of proline and oxyproline in 

 addition to 34% of glycine. The many five-membered rings cannot, 

 of course, all act as chain end groups (see Fig. 88, p. 133); they 

 must surely be built into the primary chains (Fig. 173), causing con- 

 siderable primary folding (Astbury, 1940). Collagen, like the other 



frame proteins, appears after all to 

 be built up according to the diagram 

 of polypeptide chains of indeter- 

 minate length. 



Tautomeric rearrangements help 

 to explain the shortening of the 

 members, for if within the stretched 

 chain the hydrogen of every second 

 NH group is transferred to the 

 neighbouring CO group, double 

 bonds -N = C(OH)- are formed 

 which entail the stereoisomeric poss- 

 ibilities of the cis and trans configur- 

 ation. If the cis position is assumed, 

 the members of the chain are short- 

 ened to the value of 2.86 A ascert- 

 ained experimentally (cf. Halle, 

 1937; Ch ampetier and Faure - Fre- 

 MiET, 1938). The enolic peptenol 

 form [^ C (OH)] of the polypeptide 

 chain (Fig. 173) has been shown by UV absorption, since ^ C = C<^ 

 bonds yield a characteristic UV band between 2400 and 2600 A. Since 

 the aromatic amino acids tyrosine and tryptophane, which have over- 

 lapping absorption bands, are absent, the peptenol group can be studied 

 in collagen by this method better than in any other protein 

 (ScHAUENSTEiN and Stanke, 195 i). 



There is a similar small fibre period of 2.9 A in elastoidin (Cham- 

 petier and Faure-Fremiet, 1937), which is the frame substance of 

 the fin rays oiSelachii (Faure-Fremiet, 1936). Its thermal and sweUing 

 properties are comparable to those of collagen, from which elastoidin 

 is distinguished by slight chemical differences in resistivity to trypsin 

 and by sulphur content. 



Optical and swelling behaviour of tendons. Optically, tendons and 



CH:; 



I 



,N 



I 



Oxyproline 



Front view 



Fig. 173. Diagram of a gelatin chain. 



