Experimental theromodynamics of the helix-random coil transition. I. Influence of polymer concentration and solvent composition in the PBG-DCA-EDC system.

The course of the reversible helix formation of poly(7-benzyl L-glutamate) (PBG) dissolved in a mixture of dichloroacetic acid (DCA) and 1,2-dichloroethane (EDC) was followed by measuring the heat capacity and the optical rotation of the system through the transition region. The resulix of these measurements indicate that the transition enthalpy AH, the transition temperature T,, and the Zimm-Bragg parameter u depend considerably on the PBG concentration as well as on the composition of the solvent. For the standard state of infinite dilution, however, a linear extrapolation of the measured AH values results in a standard value AHo = 950 cal./mole, independent of the solvent composition. The results of the calorimetric measurements are discussed in relationship to changes in optical rotation. Some peculiarities in the measured thermodynamic and optical properties in solutions with relatively high content of dichloroacetic arid are reported.