Application of the nearest-neighbor ising model to the helix-coil transition of polypeptides in mixed organic solvents

Using the formalism of nearest‐neighbor Ising model and assuming that the allowed states for a monomeric unity of a polypeptide chain in solutions containing strong acids are E (helix), C (coil), and CS (solvent‐bonded coil), the partition function of the system was deduced analytically. Equations were obtained which permitted the prediction of the characteristic thermodynamic behavior of the helix–coil transition under these conditions. These equations were used to examine critically the possible correlations between experimental data obtained using different techniques. Particular attention was devoted to quantities called “transition enthalpies,” obtained from the slope of the transition curves at the point where the helix fraction is one‐half (ΔH optexp), or for measurements of the heat of solution of the polymer over the total range of solvent composition (ΔH solexp), or from heat capacity measurements taken at various temperatures (ΔH calexp). Literature data of ΔH jexp(j = opt, sol, cal) for the system poly‐γ‐benzyl‐L‐glutamate in mixtures of dichloroacetic acid and 1,2‐dichloroethane were carefully analyzed.