Solvent isotope effect and protein stability.

Here we present a comparative study of the stability of several proteins in H2O and D2O as a function of pH/pH*. We show that the substitution of D2O for H2O leads to an increase in the transition temperature and a decrease in the enthalpy of unfolding. The stability of the proteins, however, appears to be largely unchanged as a result of entropic compensation for the decrease in enthalpy. This enthalpy-entropy compensation is attributed to changes in hydration of proteins in D2O compared to H2O. Analysis of thermodynamic data for the transfer of model compounds from H2O to D2O shows that almost all the changes in the enthalpy of unfolding and in the protein-ligand interactions due to water isotopic substitution can be rationalized by changes in hydration of the buried non-polar groups.