Hydration of gas phase proteins: Folded +5 and unfolded +7 charge states of cytochrome c

Free energy changes for the first steps in the hydration of the +5 and +7 charge states of gas-phase cytochrome c have been determined from equilibrium constant measurements at 271 K. Previous ion mobility studies (Shelimov, K. B.; Jarrold, M. F. J. Am. Chem. Sec. 1996, 118, 10313) indicate that the +5 charge state is tightly folded in the gas phase, while the +7 charge state is partially unfolded under the conditions employed. Free energy changes have been determined for the adsorption of the first nine water molecules on the folded +5 charge state and the first five water molecules on the unfolded +7 charge state. The free energy changes for transfer from bulk water to the hydration shell of the protein are remarkably small, between -2 and -7 kJ mol(-1), indicating that the charge is very effectively shielded in both the unfolded and folded conformations. The free energy changes for initial hydration of the folded +5 charge state are significantly more negative than for the unfolded +7 state. This may indicate that the charge is more effectively shielded in the unfolded +7 charge state, or the incorporation of some of the adsorbed water molecules as structural water molecules in the folded +5 charge state.