MOLECULAR AND MACROMOLECULAR STRUCTURE 



175 



further assumption that all parts of the fibre (and their component mole- 

 cules) were equally extensible, they were thus able to conclude that the 

 fully extended molecular chain (/3-form) was twice as long as the folded 

 chain (a-form). This was equivalent to assuming that the molecular 

 configurations in the non-crystalline fractions (phases I and III, Fig. 70) 

 were essentially similar to those prevailing in the crystalline phase II, but 

 for various steric reasons were unable to assume the perfection necessary 

 for crystallization. 



Fig. 71. Creep curves for Cotsvvold wool under constant load in steam 



(continuous line) and in a 1 % aqueous solution of caustic soda (broken 



line). The timescale has been contacted by X2 for curve (a) and expanded 



by X10 for curve (b) (Astbury and Woods, 1933). 



On the basis of models of polypeptide chains and the assumption that 

 the a-form is half the length of the j8-form, Astbury and his associates 

 advanced successively two definite proposals for an a-keratin structure. 

 The first of these (Astbury and Woods, 1933) was shown by Neurath (1940) 

 to be too tightly folded to accommodate the side groups. The second 

 (Fig. 72) (Astbury and Bell, 1941) gives more ample opportunities of 

 intrachain H-bond formation, appeared to satisfy most of the require- 

 ments. Even without a completely detailed description of atomic 

 positions this broad concept of a folded polypeptide chain linked by a 

 variety of side chains to other main chains proved, in the hands of the 



