MOLECULAR AND MACROMOLECULAR STRUCTURE 



209 



evidence is available is not incompatible with this, the frequency of 

 proline by analysis being 1 in 10, but direct evidence on the form of 

 isolated peptides is still wanting. It would not seem too difficult to observe 

 electron-microscopically the hexagonal packing of the compound cylinders 

 (diameter 67 A) and filaments of a diameter less than 100 A have in fact 

 been observed by the author (Mercer, 1956). One interesting result of this 

 work is the introduction of the idea of 8-helices. 



Fig. 89. Illustrating the proposal of Krimm and Schor for a feather 

 keratin structure. The drawing shows a projection on an 8 A cylinder of 

 one turn of their cylindrical unit X = propyl residue, O = non-propyl. 

 The master repeat period is 189 A and the 94-5 A is regarded as a 

 pseudo repeat unit (taken from Schor's thesis (1958). Reproduced by 

 permission. 



In a recent comment on feather keratin Fraser and MacRae (1959) reject 

 the proposals of Krimm and Schor as being inadequate to account for the 

 intensity distribution of the diffraction pattern and re-assert that some 

 variant of the older ^S-type structure will prove the correct solution. Since 

 none of these discussions attempts a strictly crystallographic treatment it is 

 difficult to assess their merits, but it is evident that a final solution of the 

 structure of feather keratin has yet to be proposed. 



