Enthalpic stabilization of an SH3 domain by D O.

The stability of a protein is vital for its biological function, and proper folding is partially driven by intermolecular interactions between protein and water. In many studies, H O is replaced by D O because H O interferes with the protein signal. Even this small perturbation, however, affects protein stability. Studies in isotopic waters also might provide insight into the role of solvation and hydrogen bonding in protein folding. Here, we report a complete thermodynamic analysis of the reversible, two-state, thermal unfolding of the metastable, 7-kDa N-terminal src-homology 3 domain of the Drosophila signal transduction protein drk in H O and D O using one-dimensional F NMR spectroscopy. The stabilizing effect of D O compared with H O is enthalpic and has a small to insignificant effect on the temperature of maximum stability, the entropy, and the heat capacity of unfolding. We also provide a concise summary of the literature about the effects of D O on protein stability and integrate our results into this body of data.