The observed change in heat capacity accompanying the thermal unfolding of proteins depends on the composition of the solution and on the method employed to change the temperature of unfolding.

Abstract

The apparent change in heat capacity, delta C(p), accompanying the thermally induced unfolding of lysozyme and of ribonuclease A was determined by means of differential scanning calorimetry in dilute aqueous buffer containing one of the following added solutes: 0.5 M or 1.0 M sucrose, 1.0 M glycine, 0.5 M, 1.0 M, or 2.0 M guanidinium chloride, 10% glycerol, or 0.5 M NaCl over a pH range. In each system the temperature of half-completion, t1/2, of the unfolding transition was varied by varying the pH. The resulting enthalpies of denaturation were linearly dependent on t1/2 for each solvent system. The resulting values of delta C(p) for each protein showed variation of almost 2-fold. Such large variations in the sensitivity of the proteins to temperature changes are not readily interpreted.