BARBARA W. LOW AND JOHN T. EDSALL 



These arguments may obviously be applied to other helical 

 peptide chain structures. In so far as these are initially less 

 stable than the a-helix the values for the critical length will be 

 different from the a-helix value. Donohue (33) has studied the 

 relative stabilities of a group of several helices which includes 

 the first four members of the a series 



H O 



I II 



N(CHCO.NH)„C 



where n = 1, 2, 3, and 4. 



The a-helix structure shows excellent van der Waals packing 

 across the interior of the coil. None of the other three helices 

 packs so well. Donohue has not included the contributions of 

 van der Waals forces in his estimates of the order of relative 

 stability within the group. He finds that the two most reasonable 

 alternative structures, the 7r-helix (n = 4) and a ribbon structure 

 (49) with n = \ are less stable than the a-helix by an amount 

 which is approximately equal to 500 cal. per residue per mole. 

 This finding may be of significance in considering the absence 

 of experimental evidence for any helical configuration except 

 the a-helix. On the other hand, it should be remembered that 

 most globular proteins are labile and denature easily. The most 

 stable molecular configuration may not be that of the native 

 protein molecule. 



The experimental evidence on the configuration of long- 

 chain polypeptides in water solution is still meager. Bamford, 

 Elliott, Trotter, and their associates — see for instance (10,11,41) 

 — have made extensive studies of synthetic polypeptide films or 

 fibers cast from various solvents. In general it is true that the 

 long-chain peptides are more likely to be found in the compact 

 a configuration, as judged by x-ray and infrared studies; 

 unfolded chains commonly aggregate to form fibers aligned in 

 the j3 configuration. The tendency to assume the a or /? form — 

 or sometimes a mixture of the two — is naturally much influenced, 

 not only by the chain length, but also by the solvent from which 

 the fibers are cast. Recently Doty, Holtzer, Bradbury, and Blout 



418 



