An analysis of the hydration thermodynamics of the CONH group

The hydration thermodynamics of the CONH group play a fundamental role for the stability of the native conformation of globular proteins, but cannot b e measured in a direct manner. The values of the thermodynamic functions ha ve to be extracted from experimental measurements on model compounds using group additivity approaches. The estimates determined by Makhatadze and Pri valov in the temperature range 5-100 degreesC are used in the present study in view of their qualitative reliability. They are analyzed by means of a suitable approach that couples scaled particle theory calculations with the application of the modified Muller's model. It results that the negative e ntropy change is caused by the excluded volume effect for cavity creation, exaggerated in liquid water by the small size of water molecules themselves ; the negative enthalpy change is determined by the H-bond energetics, form ation of CONH-water H-bonds, and reorganization of water-water H-bonds. The negative heat capacity change, a striking feature of CONH hydration thermo dynamics, is because the H-bonds in the hydration shell of the CONH group a re less broken than those in bulk water in the temperature range examined.