Hydration of Gas Phase Proteins: Folded +5 and Unfolded +7 Charge States of Cytochrome c

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Abstract

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. Soc. 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.

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