Thermodynamics of the thermal unfolding of eglin c in the presence and absence of guanidinium chloride.

The thermal unfolding of eglin c, a small proteinase inhibitor of molecular weight 8.1 kDa, is studied by means of high sensitivity scanning calorimetry over a wide pH range in dilute buffer solutions, and in the presence of varying concentrations of guanidinium chloride at pH 7.00 and 10.55. The temperature of half-completion of the unfolding transition, t1/2, in dilute buffer varies from 41 degrees C at pH 1.1 to 86 degrees C at pH 7.0 to 10.55, with corresponding enthalpy changes of approximately 40 kcal mol-1 and 71 kcal mol-1. This latter enthalpy change, amounting to 8.7 cal g-1, is unusually large for a protein, especially for one of unusually small molecular weight. Addition of 3.3 M guanidinium chloride at pH 10.55 lowered t1/2 from 86 degrees C to 40 degrees C and decreased the enthalpy change from approximately 71 kcal mol-1 to 25 kcal mol-1.