Hydration of gas-phase gramicidin S (M+2H)(2+) ions formed by electrospray: The transition from solution to gas-phase structure

The hydration of doubly protonated gas-phase ions of gramicidin S formed by electrospray ionization was investigated. Under ''gentle'' electrospray conditions, a near Gaussian distribution of (M + 2H + nH(2)O)(2+) ions with n up to 50 can be readily formed. These extensively hydrated gas-phase ions should have structures similar to those in solution. For intermediate extents of hydration, the ''naked'' or unsolvated ion is present in unusually high abundance. This is attributed to a competition between solvation of the charges by water vs intramolecular self-solvation via hydrogen bonding. In addition, ''magic'' numbers of attached water molecules are observed for n = 8, 11, and 14. These magic numbers are attributed to favorable arrangements of water molecules surrounding the charge and surface of the peptide in the gas phase. These results are indicative of a gentle stepwise transformation from the solution-phase structure of the ion to the preferred gas-phase structure as solvent evaporates from the hydrated ions. (C) 1997 American Society for Mass Spectrometry.