HYDRATION OF GAS-PHASE GRAMICIDIN-S (M- THE TRANSITION FROM SOLUTION TO GAS-PHASE STRUCTURE(2H)(2+) IONS FORMED BY ELECTROSPRAY )

The hydration of doubly protonated gas-phase ions of gramicidin S form ed by electrospray ionization was investigated. Under ''gentle'' elect rospray conditions, a near Gaussian distribution of (M + 2H + nH(2)O)( 2+) ions with n up to 50 can be readily formed. These extensively hydr ated gas-phase ions should have structures similar to those in solutio n. For intermediate extents of hydration, the ''naked'' or unsolvated ion is present in unusually high abundance. This is attributed to a co mpetition 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 ma gic numbers are attributed to favorable arrangements of water molecule s 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 str ucture as solvent evaporates from the hydrated ions. (C) 1997 American Society for Mass Spectrometry.