MOLECULAR AND M ACROMOLECULAR STRUCTURE 177 



Pauling, Corey and Branson (1951). Up to this time it had seemed 

 difficult to propose structures which would be capable of quantitative test. 

 However, stimulated by the precise structures proposed by Pauling and 

 Corey, a new attack opened, with the result that at the present time 

 probable structures for most of the fibrous proteins are known. 



These impressive developments stemmed in the first place from methods 

 which were in principle the same as those originally applied by Astbury 

 to the keratin problem. That is, from the study of a number of small and 

 simple compounds, such as amino acids and peptides, which permit of a 

 full crystallographic analysis, accurate information about the bond lengths 

 and angles found in unit structural elements occurring in proteins is 

 obtained. From these data models of the structural units are constructed, 

 and from these the probable conformations of peptide chains are inferred. 

 In this way Pauling and Corey, using the very accurate data relating to 

 bond lengths and angles which had accumulated in their hands by 1953, 

 predicted the existence of two helical structures, which could serve as 

 models of folded chains in fibrous proteins, and of other structures built 

 from extended chains which could form the basis of models for £-type 

 structures. 



A later important development, increasing the purely crystallographic 

 element in this work, was the publication by Cochran, Crick and Vand 

 (1952) of a theory of X-ray diffraction from helical structures which greatly 

 facilitates the interpretation of the patterns. The most fruitful result of 

 this theory was the elucidation of the structure of DNA (p. 109). It has, 

 however, also allowed a detailed comparison to be made between the 

 observed X-ray pattern of several synthetic polypeptides, prepared as 

 simpler models of proteins, and that predicted on the basis of helices, with 

 the result that the structures of these polypeptides are no longer in doubt. 



The basis of any model of the polypeptide chain is the amide unit and 

 originally Pauling, Corey and Branson (1951) formed their conclusions 

 concerning its structure on these three compounds: A^-acetyl glycine, 

 /3-glyclyglycine and diketo-piperazine. Since that time several more 

 glycylpeptides have been solved and also the tripeptide, glutathione. 

 These analyses combine to give as the probable value for the C — N 

 distance 1-32 A and show that the amide group is planar with the CO and 

 NH groups in the ^raws-configuration. These features now figure in all 

 models of polypeptides and proteins (Fig. 73). 



The structural principles formulated by Pauling, Corey and their 

 associates (Pauling, Corey and Branson, 1951; Pauling and Corey, 1951a 

 and b; Pauling and Corey, 1953; Pauling, 1958) as necessary conditions 

 for the precise prediction of stable polypeptide configurations are as 

 follows : 



(1) The amide group ( — NH— CO— ) (Fig. 73) is planar. 





V 



