14 KERATIN AND KERATINIZATION 



other senses. Thus when the fibre is mounted at right-angles to the 

 beam (Fig. 3) every possible orientation will be present and reflections 

 from all sets of planes will be possible without rotating the fibre. Such 

 patterns are called " fibre-type patterns " (Astbury, 1933) and from them 

 the most important characteristic of a fibre, the distance along the axis at 

 which the molecular pattern repeats, can be immediately calculated. The 

 diffracted rays emerge from a fibre (or a rotated crystal) on a series of 

 cones about the fibre axis, and since these intersect a flat photographic 

 plate, mounted as in Fig. 3, in a series of hyperbolae, we find the spots 

 lying on these hyperbolae (Fig. 5) which are referred to as layer lines 

 (Bunn, 1946). 



Fig. 6. Diagnostic reflections of the oc pattern. 



The patterns given by biological fibres are not usually very sharp and as 

 detailed as those of well-formed crystals. This is in part due to the small- 

 ness of the crystallites and in part to their imperfect orientation which 

 has the effect of drawing each spot into an arc. These defects render a 

 strict crystallographic determination of structure well nigh impossible. 

 but in no way hinder the use of the patterns for recognition purposes. 



The a, j8 and Collagen Patterns 



The principal features of the main, fibre-type, wide-angle X-ray patterns, 

 which constantly recur in discussions on biomolecular structure, are 

 summarized below. The classification and characterization are largely due 

 to Astbury (Astbury and Bell, 1939). 



(a) The oc-pattern (Plate 1A, Fig. 6). Type material, mammalian hair. 

 Characteristic features : 



(i) the axial repeat spacing appears as a strong sharp meridional arc 

 corresponding to 5-1 A (5-05 — 5-15 AV 



