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 mole cular weight 8.1 kDa, is studied by means of high sensitivity scanning calorimetry over a wide pH range in dilute buffer solutions, and in t he presence of varying concentrations of guanidinium chloride at pH 7. 00 and 10.55. The temperature of half-completion of the unfolding tran sition, t(1/2), 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 enth alpy change, amounting to 8.7 cal g(-1), is unusually large for a prot ein, especially for one of unusually small molecular weight. Addition of 3.3 M guanidinium chloride at pH 10.55 lowered t(1/2) from 86 degre es C to 40 degrees C and decreased the enthalpy change from approximat ely 71 kcal mol(-1) to 25 kcal mol(-1).