Water Destabilizes Protein-Hydrophobic Ligand Complexes

The first quantitative comparison of dissociation rates of solvated and nonsolvated complexes of proteins with hydrophobic ligands gives surprising results. Water was believed to stabilize such complexes. But John S. Klassen at the University of Alberta and coworkers there and at Amgen report that water molecules destabilize the complexes instead, allowing them to dissociate more easily (J. Am. Chem. Soc., DOI: 10.1021/ja106731e). The findings have implications for better understanding solvent effects on chemical reactions. The researchers used surface plaon resonance spectroscopy and time-resolved blackbody infrared radiative dissociation mass spectrometry, respectively, to measure dissociation rate constants in aqueous solution and in the gas phase of fatty acid complexes of bovine β-lactoglobulin. Although earlier work suggested that desolvation reduced the stability of such complexes, in this study it made them more stable and caused them to dissociate less readily. Klassen and coworkers propose that this occurs because reactants and dissociative transition states are hydrated to different extents. But follow-up studies “are needed to establish the generality of the current findings and to further elucidate the origin of the enhanced stability of the gaseous complexes,” they write.