200 



KERATIN AND KERATINIZATION 



configuration assumed by the polypeptide chains is of considerable interest 

 from the viewpoint of the biosynthesis (Chapter III) of the various poly- 

 peptide configurations. 



Deuterium Exchange 



A further method of estimating helical content (or at least the content of 

 tightly packed chain segments) is that of deuterium exchange which is based 

 on the finding that, when a protein is dissolved in deuterium oxide (D 2 0), 

 certain of the hydrogen atoms participating in hydrogen bonding are 

 exchanged with deuterium atoms and are given off again at varying rates 

 when the protein is returned to water. Thus one can distinguish instan- 

 taneous, rapidly and slowly exchangeable hydrogens. The slowly 

 exchangeable hydrogens are interpreted by Linderstrom-Lang (1955) to 

 be the hydrogen atoms in the tight helically-folded segments and thus the 

 helical content can be estimated by the proportion of these to the total 

 H-bond content. The method is being actively applied. Fraser and 

 MacRae (1958) have shown that in fibrous keratin the irreplaceable H- 

 bonds (inaccessible) were in the crystalline phase, i.e. presumably the 

 a-helices. 



Fig. 84. Characteristic reflections of the parallel-/? (||) and the cross-/? 



(X/?) patterns. The " backbone spacing " 4-65 A in the ||/? pattern occurs 



on the equator; in the cross-/? it is found on the meridian. 



The cross-/? pattern 



In X-ray photographs of denatured proteins, such as the muscle 

 proteins and egg white (Astbury et ai, 1935) and some contracted keratins 

 (epidermin (Rudall, 1946), wool (Mercer, 1949a; Peacock, 1959)), 

 there may be found a /?-type pattern in which the (4-65 A) reflection usually 

 associated with interchain CO . . . NH linkages is not in its normal place 

 on the equator (see Plate 2B) but is on the meridian (Fig. 84). To 

 distinguish this pattern from the usual ^-pattern, or parallel-/? (|| /?), it is 

 referred to, by Rudall and his associates, as the cross-/? pattern (X/?). 



The stretching of boiled egg white at first produces an oriented pattern 



