BARBARA W. LOW AND JOHN T. EDSALL 



The Q!-helix has been shown to be the dominant feature of 

 the configuration of long chain synthetic polypeptides with 

 identical nonpolar side chains. Similarly, once the coiled coil 

 modification is introduced, the a-helix structure satisfactorily 

 describes the major features of the ordered regions in the a 

 fibrous proteins. In the globular proteins, however, the a-helix 

 model needs more extensive revision and modification. In this 

 group of proteins a folded or coiled peptide chain is further folded 

 back on itself making many complete (180°) turns. A single 

 molecule appears either to have considerable regions of dis- 

 ordered chain or to be made up of rather short and poorly 

 aligned lengths of coiled chain. 



Whatever the mechanism which controls protein synthesis, 

 the stability of a single molecule must depend upon the overall 

 intramolecular three-dimensional packing. Apart from the 

 intrachain amide group interactions (hydrogen bonds) of a 

 coiled chain, only three other types of intramolecular interactions 

 are possible: (7) Interchain amide hydrogen bonds. Ordered 

 regions with regular interchain hydrogen bonds are regions of 

 j8 configuration. There is no evidence for this type of bonding 

 in globular proteins. (2) Side-chain-side-chain interactions. 

 Both inter- and intrachain covalent (e.g., cystine — S — S — ) 

 linkages are known to exist. Electrostatic interaction will 

 depend upon the distribution of polar groups. Interaction 

 between adjacent nonpolar side chains may also be expected. 

 These interactions must be responsible for the associations 

 between peptide chains which hold the molecule in its unique 

 compact configuration. (J) Side-chain-main-chain interactions 

 are theoretically possible; for example hydrogen bonds may be 

 formed between polar and hydroxyl-containing side-chain and 

 peptide-chain amide GO.NH groups. 



In brief, the kinds of intramolecular bonds possible will be 

 dependent on the precise sequences of amino acid residues along 

 the chains and the details of the interchain arrangement. This 

 puts the cart before the horse; rather we should say that the 

 specific molecular configuration will be determined by the amino 



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