Gas phase hydration of electrospray ions from small peptides

Electrospray ions of several small peptides were dispersed at atmospheric pressure into nitrogen containing precisely controlled concentrations of water vapor. The resulting gas-ion mixture was passed through a supersonic free jet expansion into a vacuum chamber containing a quadruple mass analyzer. Mass spectra of the product ions revealed sequences of peaks due to ions of the parent peptide with varying numbers of adduct water molecules. The extent of hydration seemed to relate to the "hydrophobicity" of the parent peptide. In their hydration behavior the ions of the complex comprising a couplet of cyclosporin A (CSA) and gramicidin S (GRS) resembled ions of the former more than those of the latter. The simplest explanation for this similarity is that in bonding with the GRS molecule the CSA molecule blocks the subsequent hydration of hydrophilic sites on that GRS molecule. Thus, the overall hydrophilicity is less for the complex than for the sum of its parts. This result suggests that the solvation behavior of ions of non-covalent complexes may provide clues to their conformations. (C) 2002 Elsevier Science B.V. All rights reserved.