MOLECULAR AND MACROMOLECULAR STRUCTURE 179 



exist if other structural or energetic requirements were better satisfied. 

 There would seem to be reason to expect that structural irregularities 

 arising from this and other causes will be found in the polypeptide chains 

 in the amorphous regions. 



Pleated- Sheet Configurations — Silk fibroin 



Figure 74 (a) is a view parallel to the planes of the planar amide groups in 

 a polypeptide in which all of the side chains (j8-carbons) project away from 

 the amide groups. The planes of the amides slant alternatively to right and 

 left and thus may be fitted on to a " pleated sheet " with the pleats running 

 out from the paper. Successive chains may be placed on the sheet in either 



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-OQ~ ~ ^ -Oo-OQ- ~ Aj} 



^«£^ 



(a) 



lb) 



Fig. 74. The structure of silk fibroin: (a) view parallel to the planes of 

 the planar amide groups ; (b) the pleated sheet. 



of two ways depending on whether adjacent chains are parallel or anti- 

 parallel in direction. By making small adjustments of the parameters of 

 the atoms in order to achieve near straight H-bonds, it was found that two 

 configurations had significantly different identity distances along the 

 polypeptide chains : 7*0 A for the antiparallel sheet and 6'5 A for the 

 parallel pleated sheet. Thus the two sheets might well serve as models for 

 silk fibroin (axial repeat distance 7 A) and /8-keratin ( ~6'5 A), respectively. 

 Among the fibrous proteins the silk fibroins are the most promising for 



