12 KERATIN AND KERATINIZATION 



the fibre-type when examined by monochromatic X-rays (Astbury, 1933). 

 It is significant and, at first sight, surprising in view of the astronomical 

 numbers of stereoisomers possible for a polypeptide chain, that, judging 

 from X-ray patterns, very few fundamental arrangements of chains are 

 actually found in nature. 



The material in a suitable fibrous form is mounted at right-angles to a 

 narrowly-collimated beam of X-rays (usually CuK a radiation) and the 

 diffraction pattern is recorded on a photographic plate as shown in Fig. 3. 



Fig. 4. Illustrating Bragg's law for the reflection of an X-ray beam, 

 i is the incident beam and r the reflected beam. The crystal consists 

 of many parallel planes containing atoms which can scatter the incident 

 radiation. The set shown consists of planes a distance d apart. When 

 the crystal planes are so oriented with respect to the incident beam that 

 the angle 6 satisfies the Bragg equation : nA = 2d sin d, reinforcement of 

 the scattered radiation occurs giving a definite reflected beam. In any 

 natural fibre a great many crystallites having the appropriate direction 

 will occur. 



The diffraction pattern is immediate evidence of a characteristic 

 arrangement of the atoms in the specimen, but we need not pause at this 

 point to discuss its interpretation (see Chapter 5). It is sufficient for our 

 present purposes to accept each pattern as a sort of "finger print" revealing 

 the presence of the fibre-type in question. The method is simple and, 

 when a positive result is obtained, reliable. It has the drawback that it 

 tends to overemphasize the crystalline fibrous components, which alone 

 give recognizable patterns, and to overlook non-crystalline components 

 which may contribute importantly to the properties of the material. 

 Patterns are recognized partly on sight, using standard examples such as 

 are reproduced in Plates 1 , 2 and 3 as guides. The positions of a limited 

 number of characteristic reflections are also measured and a corresponding 

 lattice spacing calculated from the Bragg equation. Crystals consist of a 

 large number of parallel planes containing atoms each of which may 

 scatter X-rays from an incident beam. It was shown by Bragg, that only 

 when certain geometrical conditions are satisfied, is a definite reflected 

 beam of X-rays obtained. This condition may be understood from Fig. 4. 



