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

• Published in: The journal of physical chemistry. B 1997-02, Vol.101 (6), p.847-851
• Main Authors: Woenckhaus, Jürgen, Mao, Yi, Jarrold, Martin F
• Format: Article
• Language: English
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp963389e

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.