356 



FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES 



III 



In profile the crystallized chains present the familiar picture of the 

 zig-zag line with consecutive CO, NH and CH groups (Fig. 170a). 

 AsTBURY (1935a) calls the distance between neighbouring chains the 

 "backbone spacing" and it measures 4.5 A, It may therefore be said 

 that the depth which a chain requires in the plane of the zig-zag line, 



\ 

 / 



RCH 



NH 



/ 



NH 



\ 



CO 



/ 



CO 



\ 



'CHR 



NH 



RCH 

 \ 



/ 



NH 

 \ 



CO 



Profile 



CO 



RCH 



/ 



NH 



\o- 



/ 



RCH 



\ 



NH 

 / 

 CO 



RCH 



/ 



NH 



\ 



CO- 

 / 



I 



-co- 

 I 



CH 

 I °^ 



NH ^ 



:;r 



CH, 



■CO- 



I 

 -CH 



I 

 NH 



C>5 



,„J 



CO 



CH-CH,—(~^0H 



I 

 NH 



I 



—-co 



I 



to 





> P 



Front 



a) b) 



Fig. 170. Molecular structure of silk fibroin. 



or the "backbone thickness", 

 amounts to 4.5 A. The side 

 chains of the CH groups are 

 not seen in the profile view of 

 Fig. 170 a, as they stand off, like 

 ribs, perpendicular to the back- 

 bone plane, suggestive of a 

 vertebrate skeleton. The thread 

 molecule has, therefore, to be 

 seen from the front to get the 

 side chains in their proper place 

 (Fig. 170b). Thus the kinked 

 chain appears as a straight 

 line with foreshortened valence 

 bonds, while the side chains lie 

 in the plane of the drawing. The glycine residue can scarcely be said to 

 have a side chain, which in this case is represented only by the insigni- 

 ficant H atom; but with the alanine residue it consists ofa methyl group. 

 The side chains are not fitted in pairs like actual ribs but point alter- 

 nately to left and right on consecutive CH groups, with the result that 

 every two neighbouring amino acid residues together form a morpho- 

 logical unit, which in the X-ray diagram becomes the fibre period. In 

 silk fibroinitamountsto6.95 A. From this it may be concluded that the 

 length of each backbone segment, or in other words the extension of 

 each individual amino acid member, is 3.5 A. This length is quite 

 irrespective of the nature of the amino acids in the primary valence 

 chain. Thus all the entirely different components, glycine, alanine and 

 even tyrosine, represent, as members of the chain, sections of the 

 thread molecule of exactly the same length (Astbury, 1933b). They 

 can therefore be interchanged without thereby causing any alteration 

 in the fibre period or the backbone thickness. 



The distance apart of the main chains depends on the length of the 

 side chains which, as may be seen in Fig. 170 b where tyrosine is 



