206 KERATIN AND KERATINIZATION 



that essentially the same basic features are exhibited by patterns from 

 many hard parts of birds and reptiles (1949). The feather pattern is 

 perhaps more promising for detailed analysis than any other given by a 

 keratin, but a complete solution of the structure has not yet been given. 



According to Astbury and Marwick (1932) the wide-angle pattern is 

 typically jS with the definitive axial repeat of 6*2 (2 X 3*1) A and complex 

 side spacings centred around 10 and 4*5 A (Plate 2A). Feathers may be 

 stretched by about 7% of their length before breaking. This does not alter 

 the nature of the pattern, the various longitudinal spacings increasing by 

 the same order; this behaviour contrasts with that of the a-keratins. The 

 axial long spacings have been regarded as orders of a main spacing of 

 95 A, cf. in a-keratin 198 A. The existence of definite and strong lateral 

 spacings at low angles distinguished the feather pattern sharply in type 

 from the pattern of collagen. Collagen fibrils give no lateral reflections 

 indicative of structures of greater thickness than the basic monomeric 

 filament. It was the lateral spacings of the feather pattern which early led 

 to the idea that in this material we were concerned with what were 

 essentially long crystals of precisely-constructed protein molecules not 

 essentially different in the detailed nature of their internal structure from 

 molecules of soluble proteins (Astbury and Marwick, 1932). This idea 

 persists in the more recent attempts to elucidate the structure made by 

 Bear and Rugo (1951). 



In these attempts, Bear and Rugo (1951), while not proposing a solution 

 of the small-scale structure, have drawn attention to the implication at the 

 macromolecular level of the characteristic manner in which the pattern 

 degenerates when feather is subjected to the disintegrative action of water 

 and heat (" heat-moisture treatment "). After a prolonged treatment many 

 of the details of the pattern become blurred and fade and are replaced by a 

 much simpler pattern (" net-pattern ") which can be derived from a net of 

 the type shown in Fig. 88. What seems to have happened is that the heat- 

 moisture treatment has disturbed the fine-scale structural order to the 

 point that it is no longer capable of coherent reflections leaving only the 

 large particles (now effectively internally amorphous) centred about the 

 nodal points of the net to dominate the scattering. Two arrangements of 

 these large particles (macromolecules), are envisaged by Bear and Rugo, 

 and it will be seen that the major axial-repeating distance of 95 A may be 

 referred to the length of two or four of the participating particles. 



These proposals illustrate very clearly the tendency shown in connexion 

 with the structure of protein fibrils, and already referred to above (p. 165), 

 to separate the X-ray diffraction pattern into two parts : (a) that given by 

 the small-scale molecular spacings (i.e. smaller than 20 A), in this case the 

 /3-pattern; and (b) that to be referred to the ordered packing of macro- 

 molecular units. 



