MOLECULAR AND MACROMOLECULAR STRUCTURE 



197 



parallel to the fibre and then at right angles, is called the dichroic ratio. 

 Infra-red spectra have proved fruitful in the study of the synthetic poly- 

 peptides particularly in the hands of the Courtauld group and reference 

 may be made to their book (Bamford et at. 1956). 



The frequencies associated with the CO and NH groups may show small 

 variations depending on whether they are H-bonded in an inter- or 

 intra-molecular configuration. The wave-number associated with a group 



i-5r 



10 



5 



1500 1600 1700 



Wav e number (cm7 7 ) 



Fig. 83. Infra-red spectra of a-keratin (porcupine quill), upper curves, 

 jS-keratin (swan quill), lower curves. Full line, light polarized perpen- 

 dicular to fibre axis; broken curve, light polarized parallel to fibre axis. 

 (Ambrose and Elliott, 1952. Reproduced by permission.) 



bonded in a j8- configuration is somewhat lower than in the other forms. 

 These relationships have been particularly studied by Ambrose and 

 Elliott (1951 and 1952) who deduced a correlation between structure and 

 absorption from a study of certain synthetic polypeptides known on X-ray 

 grounds to exist in more than one form. In Table 12 some key frequencies 

 as determined by them are given. For example, poly-1-glutamicbenzyl 

 ester shows amide absorption as a single band at 1658 cm -1 and the X-ray 

 diffraction photograph shows it to be in the a-form. When the methyl 



