MOLECULAR AND MACROMOLECULAR STRUCTURE 



189 



stretched into a straight-chain configuration, an extensibility of the right 

 order ( ~ 120%) to satisfy the requirements of the a-j8 transformation as 

 envisaged by Astbury (p. 174). No detailed proposal of the nature of 

 this transformation in terms of the helix has been given and there are 

 certainly formidable unsolved problems relating to side-chain movement 

 and of inter- and intra-chain cross-links to be considered. These become 

 increasingly difficult when multi-strand cables are introduced. 



An inspection of models shows that when two helices of the same sense 

 are joined at more than one point uncoiling is physically impossible without 



00 -helix?) 

 stretched 



a -helix 

 (ijnstretched) 



Fig. 82. Difficulties encountered in extending a bundle (a-filament) of 

 a-helices to yield a j8-fibril. The a-helices must be supposed to uncoil in 

 some manner and to reform to yield a jS-type structure in the stretched 

 filament. If rotation is restricted in any way this would seem geomet- 

 rically incompatible with the filament retaining its identity. 



rupture. If segments in which the sense changes from right to left-hand are 

 permitted, as Lindley (1955) has proposed for insulin, taking advantage of 

 the bends introduced by proline residues, straightening may be possible. 

 In multi-stranded filaments, it would seem that the strand structure must 

 persist during extension and again permanent cross-links must be severely 

 restricted or the individual helices will lack the mobility to open up. 

 Those who are convinced that coiled helical structures exist in keratin may 

 find in this a further argument for excluding sulphur cross-linkages from 

 the helical (fibrous) phase. 



Figure 82 has been constructed to emphasize these difficulties assuming 

 that the electron-microscopically visible filaments are bundles of a-helices. 



