172 KERATIN AND KERATINIZATION 



The first and most essential experimental step was the discovery by 

 Astbury and Street (1931), that stretched hair gave a different pattern 

 from unstretched hair, a pattern of the j8-type similar in fundamentals to 

 that yielded by silk. Meyer (Meyer and Mark, 1930) had given a broadly 

 satisfactory interpretation of the structure of silk in terms of a model 

 structure composed of a bundle of parallel straight polypeptide chains 

 oriented parallel to the fibre axis. Astbury recognized that, allowing for 

 the differences in amino acid composition between the rather simple 

 fibroin and the more complex keratin containing large bulky amino acid 

 groups, the same type of model would account for the major features of 

 the /3-type keratin pattern. This close similarity between silk fibroin and 

 /?-keratin still forms the basis of present-day proposals for the structure 

 of the latter. The structure of fibroin is now known with great precision, 

 (Marsh, Pauling and Corey, 1955) and accordingly the proposed structures 

 for /?-keratin are formulated with equal precision and are thus more 

 capable of being experimentally checked. These will be described later. 



Astbury's second step was the far-reaching proposal that the a-structure 

 developed from the shortening by folding of the straight polypeptide chains 

 of /3-keratin. This idea was suggested by a comparison with rubber, the 

 elasticity of which was currently being explained in terms of the coiling up 

 of the polyisoprene chains and by the proposal of Meyer and Mark (1930) 

 that other forms of " protoplasmic " elasticity, such as muscle contraction 

 might rest on chain coiling. In Astbury's hands this concept of polypep- 

 tide chain folding was extensively exploited and today forms the basis of 

 most ideas about protein structure and of polypeptide behaviour. 



The elastic properties and the structure of hair 



If a-keratin is a shortened form of /3-keratin, then it would seem possible 

 that the degree of folding of the polypeptide chains in a-keratin could be 

 inferred from the increase in length necessary to produce the a-/? trans- 

 formation. This possibility led Astbury and Woods (1933) to an extensive 

 analysis of the elastic properties of wool and hair. 



A description of some aspects of the complex rheological behaviour of 

 hair and wool will be given later (p. 249). Here we are concerned with one 

 particular problem: the relation between molecular extensibility and the 

 change in length of the whole fibre, and the possibility of inferring the 

 extensibility of the molecular chains from the extension of the whole 

 fibre. For this purpose it is necessary to analyse in greater detail the 

 sequence of events which occur during the stretching of a hair. Fig. 70 is 

 an idealized version of a stress-strain curve for a wool fibre or hair as 

 described by Speakman (1928) and by Astbury and Woods (1933). It will 

 be seen that it appears to divide into several distinct steps : 



OA (0-2%): In this section the curve is closely linear and is for this 



