New thermodynamic studies on ribonuclease A at low pH.

The Planck-Benzinger thermal work function, delta W zero(T), represents the heat-flux term which is responsible for breaking or forming the noncovalent bonds in macro-molecular interactions, while the temperature-invariant chemical bond energy, delta H zero(T zero), gives molecules the cohesiveness to form macromolecular structures. A method is described for evaluating delta H zero(T zero) at different temperatures for ribonuclease A at low pH in the standard state, in order to determine the effect of pH on the thermodynamic stability of this protein. Ribonuclease A at pH 1.13 has a delta H zero(T zero) value of 58 kcal mol-1; at pH 2.50, delta H zero(T zero) is 58, at pH 2.77, 59, and at pH 3.15, 60 kcal mol-1. In the conformational thermal transition of ribonuclease A at pH 2.8, the compensatory temperature ranges from 50 to 320 K, and delta H zero(T zero) is approximately 5-6 kcal mol-1. This widening temperature range is typical of the unfolding process. Such differences in the magnitude of the temperature-invariant chemical bond energy can be attributed to specific changes in the solvent ordering in the immediate domain of ribonuclease A.