KERATIN AND MOLECULAR BIOLOGY 



15 



(ii) two very strong diffuse extensive areas of reflections symmetrically 

 disposed on the equator and centred about 10 A referred to as 

 " side spacings "; 

 (iii) a sharp meridional reflection at 1-5 A not usually recorded on flat 



photographic plates with an arrangement such as that of Fig. 3 ; 

 (iv) a strong diffuse halo centred about 4-2 A forms a background to 

 the sharper fibre pattern. 

 A definite feature of a material giving an a-pattern is that by stretching, 

 pressing, or heating in water, it can be transformed into an isomer giving 

 a jS-type X-ray pattern (see below). 



The number of proteins capable of giving an a-pattern is large and 

 includes, in addition to all types of mammalian keratins, actomyosin, the 



Fig. 7. Diagnostic reflections of the ft pattern. 



contractile muscle protein, the blood proteins, fibrinogen and its insoluble 

 form fibrin, bacterial flagella, etc. There is reason to suppose that most 

 intracellular fibrous proteins are of the a-type. Astbury refers to the group 

 as the kmf proteins (keratin-myosin-fibrinogen). 



(b) The fi-pattern (Fig. 7 and Plate IB). Type materials, silk fibroin, 

 stretched hair. Characteristic features : 

 (i) axial repeat spacing about 3-5 A; 



(ii) strong side-chain reflections (in the keratin-type) centred around 

 10 A resembling those of the a-form. In the simpler silks the 

 spacing may be much less (3 — 5 A) ; 

 (iii) strong broad symmetrically disposed spots on the equator at 



4-5 A called the " back-bone spacing ". 

 (iv) a diffuse halo about 4-2 A apparently identical with that of a- 



patterns, but less well developed in the well-crystalline silks. 

 The numerous silks are usually, but not always, of the /3-type. 



