Measurement of the enthalpy change for the successive additions of water molecules to a hydrated proton cluster have been studied using an adaptation of an existing thermospray ionization source. Results to date show that clusters with as many as 7 water molecules can be observed, and that the relative intensities of the clusters observed are consistent with ion source pressures determined by direct measurement. At present, we observe rather substantial changes in Keq for sequential additions as a function of pressure. Because the ion intensities of the clusters observed are reasonably consistent with direct measurement of source pressures, we believe that the pressure dependent changes in Keq are a consequence of perturbations external to the ion source. In particular, we feel this is a consequence of ion optic electric fields inducing collisional dissociation of clusters. 2. We have taken the Cu(II) glycine and glycyl-glycine interactions as a model of metal/peptide interaction. Complexes of [Cu(II)<-gly]+ and [Cu(II)<-gly-gly]+ and [gly- >Cu(II)<-gly-gly]+, having the correct 63Cu/65Cu isotope ratio, are observed in the electrospray spectrum of solutions of these species. These singly charged ions can be explained on the basis of the presence of gly and gly-gly as anions in solution, rather than as complexes of neutral gly or gly-gly formed in the gas phase. The solution ions are postulated to be 5-membered ring structures resulting from complexes of the Cu(II) ions and non-bonding electrons of the amino acid residues. If such ions were formed in a gas phase reaction after electrospraying, one would expect to see either doubly charged ions corresponding to a Cu(II)- gly complex, i.e., no anion, and/or singly charged ions having an additional proton, corresponding to a Cu(II)-gly. Therefore we have both a model of solution processes and proof of the origin of at least some ions of electrospray mass spectrometry
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