Thermal unfolding of staphylococcal nuclease and several mutant forms thereof studied by differential scanning calorimetry

The effects of eight mutations on the thermodynamics of the reversible thermal unfolding of staphylococcal nuclease have been determined over a range of pH and protein concentration by means of differential scanning calorimetry. Variation of the protein concentration was included in our study because we found a significant dependence of the thermodynamics of protein unfolding on concentration. Values for the change in the standard free energy of unfolding, ΔΔG_d⁰, produced by the mutations in the pH range 5.0–7.0 varied from 1.9 kcal mol⁻¹ (apparent stabilization) for H124L to ‐2.8 kcal mol⁻¹ (apparent destabilization) for L25A. As has been observed in numerous other cases, there is no correlation in magnitude or sign between ΔΔG_d⁰ and the corresponding values for ΔΔH_d and TΔΔS_d⁰, the latter quantities being in most cases much larger in magnitude than ΔΔG_d⁰. This fact emphasizes the difficulty in attempting to correlate the thermodynamic changes with structural changes observed by X‐ray crystallography.