258 ORIGIN OF STRUCTURES AND FUNCTIONS 



proteins there is a structure of the type of the a-helix of 

 L. PauHng, R. B. Corey and H. R. Branson,"^ a diagram 

 of which is here reproduced (Fig. 20). 



A helix of this sort is obtained when the chain is twisted 



in such a way that each 

 group is united with the 

 third group away from it 

 by means of a hydrogen 

 bond. A complete turn 

 of the helix contains 3-7 

 amino acid residues. The 

 helix advances 5-44 A 

 for every turn ; each 

 amino acid residue there- 

 fore occupies 1-47 A 

 measured parallel to the 

 axis of the helix. This 

 helix is far more stable in 

 LINKAGESJ jj-g energy relations than 

 other suggested configura- 



26 A 



5^6 H BONDS 

 (18 PEPTIDE 



Fig. 20. 



System of hydrogen bonds in 

 the helical configuration of 

 the polypeptide chain having 

 3-7 residues per turn (after 

 Pauling, Corey and Branson). 



tions of peptide chains and corresponds most nearly to all 

 the theoretical and experimental data. 

 Nevertheless, B. W. Low"^ writes: 



The problem of protein structure is not simply, however, the 

 problem of polypeptide chain configurations. In all the native 

 protein structures examined in detail there appear to be several 

 chains or lengths of chain arranged in parallel close packed 

 array. The stability of the molecule as a whole must depend, 

 therefore, upon the nature of the interchain bonding. In the 

 helical structures the side-chain groups are thrown outwards 



