Predictions of Conformational Enthalpy and Heat Capacity from the Zimm−Bragg Theory

Abstract

Statistical mechanical calculations are presented of the conformational enthalpy and heat capacity of a solution of poly-γ-benzyl-l-glutamate using the partition function of Zimm and Bragg for helix−coil transitions. We determine the temperature dependence of the statistical parameter s from optical data and treat the parameter σ as a fitting parameter. We note, however, that consistency with the Zimm−Bragg theory requires that σ not be less than the square of the reciprocal of the number of residues per molecule. We utilize that data given by Ackermann and Ruterjans [Ackermann, Th.; Ruterjans, H. Ber. Bunsen-Ges. Phys. Chem. 1964, 68, 850] and find the same value for σ as they, σ = 2.5 × 10-5 for their solution of 0.257 mol of residue per liter in a solvent of dichloracetic acid and 1,2-dichlorethane. We consider our calculation to be in satisfactory agreement with that of the experiment but point out some problems associated with the determination of the temperature dependence of s.