The molecular association of polypeptides in many solvents has heretofore prevented the molecular characterization of these substances. It is shown here that there are two classes of solvents in which poly-7-benzyl-L-glutamates are not associated. In one group (dimethylformamide, cresol, chloroform-fonnamide) the intrinsic viscosities were found to be the same for a given sample and to increase very strongly with molecular weight. In the other group (dichloroacetic acid) the dependence of the intrinsic viscosity on molecular weight was like that typical of randomly coiled polymers. Light scattering studies in chloroform-formamide of very high molecular weight samples showed that their lengths were proportional to their molecular weights and that the length per residue was 1.5 Á. These features demonstrate that the configuration is that of the -helix of Pauling and Corey. Further support for this conclusion was obtained by showing that the intrinsic viscosity-molecular weight relation could be fitted to Simha's equation for ellipsoids by assuming only that the minor axis was 18.2 A. This corresponds to 14.9 A. for the diameter of the solvated helix. The helical configuration appears to be rigid and rod-like up to 300,000 molecular weight but a few observations extending to 800,000 indicate the onset of a very small amount of flexibility. The helix is no longer stable at sufficiently low molecular weights: it is likely that this lower limit depends on the solvent and temperature. The randomly coiled configuration in dichloroacetic acid is due to the unusually strong secondary bonding that this solvent can form with the polypeptide chain. Molecular association occurs in solvents with very low or zero hydrogen bonding potential (purified chloroform, dioxane, benzene). In these cases the viscosity is greatly increased as a result of the association and it is shown that the association is probably of the end-to-end type arising from the formation of intermolecular hydrogen prevent association by solvating the helix-ends.

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The intrinsic mixed acidity constant (pK,) of glutamic acid polymers has been determined from potentiometric titrations of poly-DL-glutamic acid in aqueous solutions of various NaCl concentrations and at two temperatures (25 and 48 "), thus making the difficult extrapolations of poly-L-glutamic acid titration curves more certain. These experiments indicate that p& is nearly independent of temperature but varies from 4.58 in 0.01 M NaCl to 4.32 in 0.40 M NaC1. Similar studies of the titration of poly-L-glutamic acid as a function of temperature and NaCl concentration give the standard Gibbs free-energy change per amino acid residue for the transition from un-ionized CY helix to un-ionized random coil (AGOIN) for each of the various conditions; AGOjN depends on temperature, but only slightly, if at all, on salt concentration. From the temperature dependence of the free energy, we find that AH"/N is 975 & 50 cal/(residue mole), and that AS"/N is 2.67 =t 0.1 cal/(residue mole deg). We find that there is no measurable effect of polymer concentration on these thermodynamic parameters and conclude that nonequilibrium aggregation is not present. It is demonstrated that, for poly-L-glutamic acid, three independent measures of helix content agree, namely ultraviolet absorption, titration, and optical rotatory dispersion. The helix content of un-ionized poly-DL-glutamic acid is estimated, from its extinction coefficient at 200 mp, to be 62%. The implications of these experiments for molecular theories of the conformation of polyamino acids in aqueous solutions are examined. It is found that assignment of the contribution of individual molecular forces (e.g., hydrogen bonding, hydrophobic bonding) to the over-all free energy of transition, whether that assignment is semiemprical or a priori, will have to be made with considerably more precision than has thus far been possible, if such theories are to be meaningful. n view of the pervasiveness of the a-helical conforma-

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