MOLECULAR AND MACROMOLECULAR STRUCTURE 193 



derived from proline and hydroxyproline, cystine and glycine — have, 

 however, special consequences. Proline actually constrains the shape of 

 the polypeptide chain by introducing a " bend." Thus a chain containing 

 a proline residue cannot be straight and an a-helix cannot be maintained 

 at segments where a proline or hydroxyproline residue is inserted; 

 although Lindley (1955) has pointed out that, in conjunction with appro- 

 priate neighbours, it can change the sense of the helix from r.h. to l.h., a 

 feature which could be of importance. Glycine is unique in having no side 

 chain. It is a " space saver " and may permit a more compact packing of 

 adjacent chains. It is common in collagen (a frequency of 1 in 3) and its 

 smallness plays a special part in the construction of the triple helix assumed 

 by that protein. Cystine, by virtue of its power to form intra- or inter-chain 

 cross-links, would reduce drastically the mobility of any polypeptide 

 system. 



It is significant that most chemical reactions with wool and hair have 

 little effect on the X-ray diffraction pattern unless they are of the type that 

 leads to a dissolution of the H-bonds. Reactions not markedly affecting 

 the pattern include : absorption of water, neutralization of acid or basic 

 side chains, nitration, iodination, reduction and substitution of the cystine 

 bridges. It would seem then that many residues are either excluded from 

 or are to be found only on the surfaces of the crystallites. 



The picture originally (1933) presented by Astbury and Woods of the 

 crystalline and amorphous regions, stretching partly in series and partly 

 in parallel when the fibre is stretched, still commands general acceptance 

 in spite of a considerable evolution of views concerning the actual <x- 

 structure itself. There have been other proposals based mainly on 

 different interpretations of the course of events in stretching (see p. 176). 

 For example, Peters and Speakman (1949) and Burte and Halsey (1947) 

 have envisaged the possibility, even at zero extension, of an equilibrium 

 between portions of the molecular chains in the a- and ^-configurations in 

 the non-crystalline region and have developed this concept to give a 

 quantitative description of limited aspects of the elastic behaviour of 

 swollen wool. The existence of any considerable fraction of the molecules 

 in an extended form at zero extension would invalidate the basic assump- 

 tion of Astbury and Woods that ultimate extensibility of the whole fibre 

 (100%) is also that of the component molecular chains. According to 

 Elliott (p. 199) there is evidence from infra-red absorption spectra of 

 /3-configurations in unstretched hairs. 



In terms of the " matrix-plus-filament " model developed in Chapter 6 

 a certain amount of the diffuse scattering of X-rays must be produced also 

 by that fraction of the material described as y-keratin, which is considered 

 to exist outside of, and to envelop the fibrillar component proper. The 

 polypeptides in this region may be pictured as being so heavily cross-linked 



