PROTEIN STRUCTURE 



to the a-carbon atom (78). In the fully extended planar poly- 

 peptide chain configuration (Figure 3) the predicted repeat 

 distance is 7.23 A. When two such chains are packed side by 

 side to give interchain hydrogen bonds there is steric hindrance 

 between opposing side chains unless these are (7) both glycyl 

 residues ( — H) or (2) one glycyl and the other alanyl ( — CH3) or 

 seryl ( — CHoOH). Both the other two structures form lateral 

 interchain hydrogen bonds, even if the side chains are bulky. 

 In one arrangement, the parallel pleated sheet structure, all the 

 chains have the same direction. In the antiparallel pleated 

 sheet structure, alternate chains are opposed in direction. The 

 predicted repeat distances along the chain direction in both 

 structures are somewhat shorter than the 7.23 A of the fully 

 extended structure. The general pattern of the pleated sheet 

 structure is susceptible of some modifications in details of 

 arrangement. In their earlier calculations (78) Pauling and 

 Corey assumed that certain preferred orientations around single 

 bonds in the chains would give the most stable configurations. 

 Later (80) they concluded that this factor was of secondary 

 importance and that configurations would be favored which 

 gave approximately linear hydrogen bonds — that is, bonds in 

 which the N — H. . . .O angle is 180° or nearly so. The models 

 constructed on this assumption gave a fiber axis period of 7.00 

 A for the antiparallel, and 6.50 A for the parallel, chain pleated 

 sheet. The structure of silk fibroin has been studied in great 

 detail by Marsh, Corey, and Pauling (72); the x-ray patterns 

 obtained were in good agreement with expectation for an anti- 

 parallel chain pleated sheet structure, in which the predominant 

 chain sequence is Gly.X.Gly .X.Gly, where X is alanyl or 

 seryl. The structure may be modified so as to include the larger 

 side chains, such as tyrosyl, in the crystalline component of 

 structure, instead of placing them in amorphous regions as did 

 most earlier investigators who considered this problem. 



In the /3-keratin structure the parallel pleated sheet arrange- 

 ment appears the more reasonable model. The hydrogen- 

 bonded sheets in both silk fibroin and /?-keratin must be held 



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