178 KERATIN AND KERATINIZATION 



(2) The following bond distances and angles found in small peptides, 

 etc., will occur in polypeptides: 



aC— C = 1-53 A 



C— O = 1-24 A 



C— N = 1-32 A 



N— aC = 1-47 A (all ± 0-01 A) 



These dimensions are illustrated in Fig. 73 taken from Pauling and 

 Corey. The planarity of the amide group and the shortening of the C — N 

 distance to 1*32 A is attributed to resonance. 



(3) Hydrogen bonds NH — O will form where possible with an N — O 

 distance of 1*79 ± 0*1 A and the O lies close to the NH axis. 



(4) The /raws-configuration of the amide group is significantly more 

 stable than the m-configuration. 



Fig. 73. The planar amide group according to Pauling and Corey (1955). 



This structural information was systematically employed to construct 

 polypeptide models and two types of structure were derived depending on 

 whether the attempt was made to form the H-bonds by intrachain or, 

 on the other hand, by interchain association. The first led to helical, 

 folded chains, the second to sheets of associated polypeptides. 



Not all crystallographers accept the Pauling-Corey conditions. Huggins 

 (1958) in particular, points out that a departure of 30° from planarity of 

 the amide group does not involve a marked instability and might well 



