MOLECULAR AND MACROMOLECULAR STRUCTURE 



181 



yielding them, structures based on pleated sheets are present. There 

 are other silks with somewhat bulkier side-chain composition and hence 

 larger side spacings which approach more closely to the actual conditions 

 prevailing in the keratins. A knowledge of the particular peptide sequences 

 in keratins which participate in crystallite formation is still wanting. 



Helical Configurations — The <x-helix 



Helical structures were first proposed by Huggins (1943) and Taylor 

 (1941); those described by Pauling, Corey and Branson are, however, 

 more precisely specified and are thus capable of quantitative evaluation. 



Fig. 75. Diagram of the oc-helix of Corey and Pauling (reproduced by 

 permission). 



They discovered two helical (a and y) arrangements, and since that time 

 Low and Baybutt (1952) have described another. Only the a-helix has been 

 extensively studied since it does seem to provide the required structure. 



