KERATIN AND MOLECULAR BIOLOGY 



11 



X-Ray Methods 



With the growing hope of actually determining the molecular structure 

 of proteins by means of X-ray diffraction, or at least of obtaining certain 

 experimental criteria of structure even when this may not be determinable 

 in detail, the possibility arises of devising a rational system of classification 

 based on molecular structure. The simple procedures based on X-ray 



n 



Photographic plate 



Fig. 3. The principal features of an X-ray diffraction apparatus. 

 T, X-ray generator consisting of a copper target on which impinge 

 high-speed electrons emitted in vacuo from the heated cathode E; W 

 is the window from which the rays emerge and are collimated into a 

 narrow beam at C to fall on the specimen mounted at S. The diffracted 

 X-rays are recorded on a flat photographic plate placed at right angles 

 to the beam at P. Other forms of plate may be used but the flat plate 

 is commonly employed in fibre studies. The diffracted rays within a 

 few degrees of the plate centre are referred to as the low-angle pattern ; 

 the more widely-scattered reflections constitute the wide-angle pattern 

 used to characterize the fibre-type. 



diffraction have been of value for the rapid survey of material as is required 

 in comparative biochemistry replacing the laborious chemical methods. 

 Another advantage is that the material is not destroyed and need not 

 necessarily be isolated pure. Its biological applications were pioneered 

 by Astbury and developed in particular by Rudall. It has played a great 

 part in the study of keratins. 



The first X-ray patterns of biological materials were in fact obtained 

 from natural fibres, among them hairs. The commonly-occurring fibrous 

 materials have each been shown to give characteristic X-ray patterns of 



