MOLECULAR AND MACROMO LECU L AH STRUCTURE 199 



well-oriented X-ray pattern, a low crystalline amorphous ratio is indicated. 

 The spectra in this case may provide data concerning chain configuration 

 in the important amorphous phase which is not accessible to X-ray 

 methods. In particular the a- and ^-configurations may be recognized by 

 their distinct dichroic effects as demonstrated in several polypeptides. 

 Ambrose and Elliott (1951) examined sections of elephant tail hair (a- 

 keratin) and obtained spectra such as shown in Fig. 83. The NH and CO 

 directions seem to be more parallel to the fibre axes (parallel dichroism), 

 although the dichroic ratio is very low. Robinson and Ambrose (1952) 

 claimed that this evidence excludes the Astbury-Bell model. 



A structure based on the a-helix would have a high parallel dichroism, 

 and the low figure found is therefore only possible if we assume a large 

 admixture of amorphous material. Parker (1955) modified the amorphous 

 regions by replacing the H atoms by D atoms and thus displaced the 

 (OH and NH) bands to much lower wave-numbers enabling the dichroism 

 of the crystallite to be observed alone. The parallel dichroism rose from 

 1*5 to 4*5 showing more certainly that the low value usually obtained is 

 due to the large fraction of amorphous material and lending support to 

 structures of the a-helix type. 



Elliott (1952) has also concluded that there are significant amounts of 

 |8-keratin in the amorphous phase of unstretched hair and of the a-form 

 in hair stretched 100%. This deduction is now uncertain. It was held that 

 these observations show that care must be exercised when attempting to 

 correlate molecular and whole-fibre extensions. The chief value of these 

 infra-red observations remains now that they allow some investigation of 

 the often large amounts of non-crystalline constituents not revealed by 

 X-ray diffraction and indicate whether or not this is in the /3-form with 

 some measure of its orientation. 



The important fact that reversible a-/? transformations can be obtained 

 from purely synthetic polypeptides was established by the Courtauld 

 group using indications provided by infra-red absorption spectra (Bam- 

 ford et ai, 1956). For examples, fibres of a-poly-L-alanine stretched in 

 steam give a highly-oriented /3-pattern and somewhat similar results are 

 obtained with poly-a-amino-w-butyric acid. Reversible conversions from 

 one form to another are also obtained by immersing specimens in various 

 swelling solvents. Concentrated formic acid often produces a /3-form and 

 chloroform or w-cressol may reconvert to the a-form. 



formic acid 



m-cresol 



The conversions are reversible and there is no loss or degradation of 

 material. This demonstration of the influence of the solvent on the 



