Solution properties of synthetic polypeptides. XII. Enthalpy changes accompanying helix‐coil transition of polypeptide

For helix‐coil transitions of polypeptide in binary mixtures consisting of helix‐forming solvent and coil solvent, the transition enthalpy Δ H ( T , x ) has been found to depend significantly on temperature ( T ) and solvent composition ( x ). For such systems, calorimetric measurements may yield some averages of Δ H ( T , x ) which are no longer amenable to direct comparison with Δ H itself. Theoretical equations relating calorimetric data to Δ H ( T , x ) are derived and tested favorably with experimental data. It is demonstrated that the transition enthaply from heat capacity measurements is approximately equal to Δ H cfm , while those from heat of dilution and heat of solution measurements are equal to Δ H c . Here Δ H c denotes the value of Δ H at the transition point and f m represents the maximum helical content attained in a thermally induced transition. The discrepancies among calorimetric data are also discussed.