16 KERATIN AND KERATINIZATION 



(c) Avian fi-keratin pattern (Plate 2A). This pattern is related to the 

 normal /S-pattern in having most of its characteristics. It is distinct in 

 having a shorter (apparent) axial repeat spacing of 3-1 A and in being 

 generally far more elaborately developed. Type material, bird feather 

 calamus or rachis. 



(d) The collagen pattern (Fig. 8). Type material, rat tail tendon. 

 Characteristic features : 



(i) axial repeat ^ 2-8 A; 



(ii) diffuse side-chain spacing at ~ 12 A. Unlike the corresponding 

 spacing at 10 A in the a-patterns, the side spacing of the collagens 

 is sensitive to hydration and may increase to 15 A and more in 

 swollen materials. 



#••# 



Fig. 8. Principle features of the collagen diagram. 



On stretching, collagen fibrils usually break at about 10 per cent extension 

 without any change in the type of X-ray pattern yielded. The reflections 

 are considerably sharpened (Randall, 1953). Heating, swelling and other 

 destructive influences likewise do not change the nature of the pattern 

 although reducing its intensity. Collagen fibres are common in vertebrate 

 connective tissue and have a wide distribution elsewhere (see later). 



Notes. Numerous other reflections are visible in all patterns and are 

 shown for a- and ^-patterns in Figs. 9 and 10; they have been variously 

 described and are indexed in Astbury and Bell (1939). They are usually 

 less strongly developed and may not be detectable. When the specimen 

 is poorly oriented the various reflections are drawn out into arcs and are 

 less readily recognized. The completely disoriented condition is not 

 uncommonly met with, particularly in artificial preparations and the 

 corresponding patterns may be recognized as follows : 



(e) Disoriented oc-pattern. Type example, regenerated precipitates of 



