182 KERATIN AND KERATINIZATION 



The a-helix is based in the first place on geometrical requirements 

 deduced from the known structures of small peptides (see above) (Fig. 73). 

 When a model of a single polypeptide chain, built according to these 

 requirements, is formed into a spiral, it is found that one very compact 

 formation results, in which there are 3*6 amino acid residues per turn of 

 5*4 A so that each residue occupies 1*5 A of the length of the helix (Figs. 

 75 and 76). The stability of the helix is assured by the formation of 

 multiple internal H-bonds, and Donohue (1953) has shown that of the 

 several possible helices the a is the most stable. 



DIMENSIONS OF AN or HELIX 



Rise per 

 Residue 





26° ^&r r 



JL 3 



J„ 



5TH 

 TURN 



4TH 

 TURN 



3 RD 



TURN 



2 ND 

 TURN 



18 Residues 

 27 A 



5.4 A Pitch 1 1 ST 

 3.6 Residues [TURN 



Fig. 76. Diagram of an a-helix indicating the 1 *5 A rise per residue which 



gives rise to the characteristic axial X-ray spacing and the relation between 



the pitch 5-4 A (36 residues) and the 54 A spacing. (After Corey and 



Pauling.) 



The a-helix was put forward in the first place as a proposal. As a direct 

 experimental test for the presence of such a helix, Perutz (1951) pointed out 

 that such a structure should give a strong meridional reflection of 1*5 A 

 corresponding to the segment of the helix occupied by one residue (Fig. 

 76). He found that, in fact, this reflection was present in the patterns 

 given by hair, quill, muscle, various other proteins and polypeptides 

 (Perutz and Huxley, 1951). This reflection had also been recorded earlier 

 by Macarthur from porcupine quill (1943). Since no other of the chain 

 configurations proposed for the a-proteins can give a 1*5 A reflection, the 



