194 KERATIN AND KERATINIZATION 



by the frequently-occurring disulphide bridges (a frequency of from 1 in 2 

 to 1 in 3) that crystallization is impossible and that, as in liquids, a 

 diffraction pattern of diffuse haloes is produced simply because certain 

 interatomic spacings occur with a higher frequency. This " liquid-like 

 order " of the non-crystalline fraction is to be distinguished from an 

 unoriented ^-arrangement. The /3-form is a well-defined structure 

 maintained by H-bonds of a perfectly definite length. The y-structure is 

 pictured as being structurally amorphous since the packing of the chains 

 in either a- or /?-forms is hindered by either cross-links or awkwardly- 

 shaped side chains. A certain amount of the non-crystalline material 

 within filaments themselves may also be pictured either in this form or as 

 a disordered a-structure, since all a-type protein patterns are characterized 

 by the same broad halo of average spacing of 4*5 A irrespective of their 

 fine histological structure. This is the case for natural fibres, keratinized or 

 not, and also for regenerated protein fibres devoid of histological or fine 

 structure. When no crystalline material is present, as in some regenerated 

 fibres, only the broad haloes appear. It seems probable that, to a degree 

 limited by the steric hindrance of side chains and cross-links, short 

 segments in the a-fraction approximate more-or-less closely to the a-helix 

 favoured on energetic grounds. 



The radial distribution patterns to be expected from the unoriented 

 a-helix have been calculated by Donohue (1954) and some comparison with 

 the experimental scattering curves made by Arndt and Riley (1955), but 

 according to Kendrew and Perutz (1957) the radical distribution function 

 is not a sensitive test of configuration. 



There appears to be some connexion between crystallinity and density. 

 The densities of horn, wool and porcupine quill (1-28, 1-302 and 1*32) 

 are in order of the degree of crystallinity; but human hair, far less 

 crystalline than quill, has much the same density (1*317). This discrepancy 

 may rise from the rather higher cystine content of the hair (Fraser and 

 MacRae, 1957). 



Other methods of determining chain configuration 



Optical Rotation and Rotary Dispersion 



During the last few years there has been a renewed interest in the 

 measurement of optical rotation [a] A and its dependence on wavelength 

 (rotary dispersion) as an added tool for the evaluation of configurational 

 changes and for estimating the " helical content " of protein preparations. 

 While little work has been attempted on keratin preparations, a good deal 

 of information has been gathered concerning proteins in general and of 

 various well-characterized a-type fibrous proteins which is relevant to the 

 keratin problem. 



