196 KERATIN AND KERATINIZATION 



Woods (1959) found that the " low sulphur extracts " of wool resemble 

 the synthetic polypeptides in their optical behaviour. In 8 M urea the 

 molecule is in a completely random form with [a] D = — 105°; in some 

 organic solvents [a] D is approximately zero, i.e. the molecule is largely 

 helical. In aqueous solution [a]^ is of the order of —60° which, according 

 to some forms of calculation, would mean a helical content of 30-40%. 

 Undoubtedly the helical content of a keratin will be restricted by its 

 proline and cystine content. When there is more than 8% proline dis- 

 tributed along a polypeptide chain (wool keratin 9*5% and feather keratin 

 10%) it is possible that there will be no segments long enough to be stable 

 in the helical configuration. Very high proline contents may favour 

 another structure such as that proposed for feather by Krimm and Schor 

 (p. 208). 



Infra-red Spectra and Structure 



Infra-red spectra arise from changes in the vibrational energy ot mole- 

 cules produced by the absorption of infra-red radiation. Their value in 

 structural studies is due to the fact that the absorption effects the move- 

 ment of nuclei in the field of the interatomic binding forces, and the 

 examination of large numbers of substances of known structure has shown 

 that certain frequencies are associated with particular valency bonds 

 whose presence in other compounds of unknown structure may thus be 

 deduced from their spectra. Fortunately for protein studies, bonds of 

 hydrogen atoms with other atoms give characteristic absorptions and their 

 study provides a method of investigating associations between groups 

 which are mediated by H-bonds. 



When the molecules are oriented as in crystals or in fibres it is also 

 possible to obtain information concerning the direction of some valency 

 bonds by using polarized infra-red radiation (Ambrose and Elliott, 1951 

 and 1952). The absorption coefficient is proportional to the square of the 

 cosine of the angle between the E- vector of the radiation and the direction 

 of the rate of change of the dipole moment of a normal mode of vibration 

 of the molecule. If the bond associated with this mode of vibration is 

 already known, it may be possible to infer its direction in the fibre (see 

 Fig. 83). 



The bands in infra-red spectra are usually given as wave-numbers, 

 rather than frequencies, where the wave-number is the reciprocal of the 

 wavelength in centimetres (cm -1 ); and the spectra are presented graphi- 

 cally by plotting wave-number against optical density: 



intensity of incident radiation 

 s 10 intensity of transmitted radiation 

 The ratio of the optical density, measured first with the electric vector 



