184 



KERATIN AND KERATINIZATION 



helix can only fit when it is tilted ^ 20° to the first. Crick suggests a 

 three-strand rope to give the 198 A longitudinal period but this does not 

 account for the strong 27 A reflection on the equator. The high density 

 (1*3 g/cm 3 ) remains a difficulty. 



3 



Pitch of 



large 

 helix 



d 



•^'.Pftch of 

 V .small helix 



A 



tJ 



la) 



(b) 



Fig. 77. Illustrating the curving of the a-helix into a super helix (a) and 

 (b) and two possible combinations of super helical structures to give com- 

 pound helices (coiled coils), (c) the seven-strand cable and (d) a three- 

 strand coiled coil (Pauling and Corey, 1956. Reproduced with permission). 



Pauling and Corey reached a similar conclusion on the grounds that an 

 a-helix composed of a repetition of different amino acids in a regular 

 pattern would not have a straight axis but one distorted into a large helix. 

 They suggest that a radius of 10 A for the a-helix would permit six 



