188 KERATIN AND KERATINIZATION 



that compact arrangements can result either from flat sheets lying over each 

 other at a suitable angle (Fig. 87) as Rudall has proposed for the ootheca 

 protein (p. 205), or as in hair, where they lie on the circumference of 

 concentric circles (or helically-wound sheets) with an appropriate increase 

 in the angle of pitch between successive layers. 



The degree to which these initial aggregations would persist into the 

 finally-hardened state could easily depend on the speed with which the 

 final changes are effected. For, if we regard the cystine-rich matrix protein 

 (p. 248) as an interpolation between the filaments (or as an addition to 



0GX3GX5 



Fig. 81. Illustrating the lateral aggregation of rodlets (a-filaments) 



to give (a) a spiral compound or (b) a sheet. The rodlets are here shown 



in cross-section. 



their surfaces) this could, by rapidly cross-linking, maintain an earlier 

 condition; on the other hand, were hardening more slow, it would facilitate 

 a rearrangement of the filaments into the most compact condition, 

 hexagonal packing. This may be pictured more plausibly by thinking of 

 the filaments as being separated by a viscous but fluid medium which both 

 reduces the close interfilamentous contacts responsible for the earlier 

 packing and also permits the movements of readjustment. Fraser et al. 

 have appropriately likened the matrix in keratin to the interfilamentous 

 water of other fibrous systems (1959). In skin we may suppose these 

 changes occur too rapidly to permit of much readjustment. Rogers (1959b) 

 has shown that hexagonal packing prevails in the more stable fraction of 

 wool (paracortex). 



The a-/? transformation in terms of the a-helix 



One of the commendable features of the a-helix is that, while it was 

 developed primarily to satisfy structural principles derived from a study 

 of simple peptides, etc., by crystallographic means, it has also, on being 



