190 KERATIN AND KERATINIZATION 



Individual helices must have freedom to uncoil and the j9-form remains a 

 multi-stranded structure in which the jS-chains may be conceived to form 

 stacks of pleated sheets which are slightly twisted to form concentric, 

 closed cylindrical pleated sheets (|8-helices) such as has been recently 

 proposed for feather keratin (p. 209). 



It would seem that the structure at the macromolecular level of the forms 

 of a-proteins found in hair is far from settled. The purely theoretical 

 discussion given by Lindley (1955) on model making and the packing of 

 helices shows that the possibilities are far from being exhausted. Lindley's 

 models are based strictly on the dimensional criteria of Corey and Pauling 

 but, by skillfully exploiting the possibilities of helices of left-hand and 

 right-hand sign and the discontinuities introduced by the presence of such 

 residues as proline and cystine, he showed that a number of unexpected 

 packings could be achieved. 



The non-crystalline fraction 



The structures we have been discussing up to this point are those of the 

 crystalline material occurring in a keratinized tissue, i.e. that part of the 

 tissue in which the molecules are sufficiently well arranged to yield a 

 definite X-ray diffraction pattern in the form of discrete spots. There is no 

 question but that a large part of these tissues is not in such a well-organized 

 form and that, moreover, many of the important properties of the materials, 

 such as their elastic and chemical behaviour towards mild reagents, is 

 influenced by this fraction. It is not very useful to regard the keratins 

 as perverse molecules which may some day be persuaded to assume a 

 perfectly crystalline form and give the crystallographers their chance. 

 Ordered crystalline regions certainly exist in the large masses of hardened 

 protein, and for these precise structures may be described; but equally 

 certainly disordered non-crystalline regions exist too, and these imperfect 

 regions must be regarded as an essential part of the whole formation since 

 they confer on it certain properties required for a performance of its 

 biological role. For these reasons it is necessary to consider as a separate 

 problem the type of structure which prevails in these regions and to 

 estimate what fraction it forms of the entire tissue. For this purpose there 

 are available in addition to X-ray diffraction techniques, various other 

 methods of a physicochemical nature. It should be emphasized that we 

 are not considering here the non-keratinous constituents (p. 270) but 

 rather that portion of the keratin itself which, since it does not contribute 

 to the fibre-type X-ray pattern, may be referred to as " non-crystalline", 

 a term more exact than " amorphous". 



The amount of this material, usually expressed as a crystalline/amor- 

 phous ratio, may well vary from cell to cell and from tissue to tissue. The 

 estimates of its value as found in the literature are mostly for wool or hair 



