186 KERATIN AND KERATINIZATION 



When models of structures of this size are attempted either by forming 

 concentric shells of a-helices or by arranging these on helical sheets, 

 difficulties are met in effectively filling the centre of the filament without 

 some disorder (Fig. 79). Perhaps an indication that the nature of the 



helix =1 



10 20 JO A 



Fig. 79. Relation between the a-helix and the a-filament. The a-helix 

 (diameter 10 A) is the structural unit deduced from model building and 

 from the data afforded by actual X-ray patterns. The a-filament l.h.s. 

 (diameter <— ' 60 A) is the smallest structural unit observed electron- 

 microscopically. The filament could be a twisted yarn of a-filaments 

 with a prevailing angle relative to the axis of about 20° but the internal 

 arrangements are obscure. 



2000-l0,000A| 

 I0A 60A £| I 0-2-1/* 



(0) (b) 



Fig. 80. Relative dimensions in cross-section of the molecular, macro- 

 molecular and histological units of hair keratin, (a) the a-helix of 

 diameter 10 A {molecular level); (b) the a-filament diameter ~ 60 A 

 whose internal structure in terms of a-helices is not known {macro- 

 molecular level) ; (c) the fibril visible in the light microscope composed of 

 large numbers of filaments {histological level). Cross-sections of fibrils 

 are to be seen in Figs. 102 and Plates 15 and 16. 



packing alters (or that an element of disorder enters) is given by the fact 

 that the centres of the filaments seen electron-microscopically are some- 

 what more stained than their peripheries (Rogers, 1959). There seems no 

 structural principle that we can appeal to limit the number of helices in a 

 filament, yet the evidence is that the filaments are a uniform and definite 

 structural element. 



