192 KERATIN AND KERATINIZATION 



1949c) (p. 10) show that the original value is of the order of N u — N x = 

 0*01, and that it increases in the range of stretching from to 40% sug- 

 gesting an increased ordering of the structure. It reaches its highest value 

 in fibres stretched to 40-50% extension and held under tension, and falls 

 when these are steamed to relax the tension and to produce the /2-forms 

 causing " set." From this it may be deduced that set involves a decreased 

 average molecular alignment parallel to the axis of main chains plus side 

 chains and that the /?-form itself is not necessarily more birefringent 

 than the a-form. 



All methods combine to suggest that a very considerable fraction 

 of the material is in a somewhat disordered form. Such non-crystalline 

 regions could be pictured in various ways. Astbury and Woods (1933) 

 originally suggested that the non-crystalline regions approximate to the 

 crystalline in structure and thus that conclusions drawn from the dif- 

 fraction pattern of the crystallites are applicable broadly to the whole 

 keratin system. This assumption necessarily underlies their interpretation 

 of fibre elasticity (p. 174) since all " phases " must be capable of the same 

 ultimate extensibility of 100% if one is to infer molecular extensibility 

 from whole fibre extensibility. The sequence of events during extension 

 is still under investigation by X-rays. (Bendit, 1957 ; Skertchly and Woods, 

 1960). 



Whatever the solution may be, it seems not unlikely that, as in the 

 fibroins, the poorly-organized regions will be associated with side chains 

 which are difficult to pack. Fibroin crystallites contain predominately the 

 shorter side chains which pack readily ; the acids with longer chains seem 

 to be excluded from these crystalline clusters and their interpolation 

 elsewhere impairs the chain alignment and thus introduces a non- 

 crystalline region. There are many more amino acids with bulky side 

 chains in keratin, a fact which might be associated with the lower crystalline/ 

 amorphous ratio, and the crystalline regions might well be those where 

 clusters of the smaller acid residues (glycine, alanine, leucine, serine) 

 occur. The isolation from a natural keratin of peptides containing such 

 sequences, as has been analogously effected with fibroin, would be a 

 valuable indication. Large side chains do not in themselves preclude 

 crystallization. Among the synthetic polypeptides containing a-helices, 

 excellent ordering is possible even with large side chains since the homo- 

 genity favours packing even when the side chains are long. The helices are 

 spaced further apart in this event (Bamford et al., 1956). Certain natural 

 silks contain crystalline regions with long side chains (Warwicker, 1959). 



The presence of certain residues in the crystalline region may be doubted 

 on other grounds than size. Model building shows that most residues have 

 no special importance in the sense that their side chains do not influence 

 the possible configurations of the main chain. Four residues — those 



