Structures and fragmentations of zinc(II) complexes of amino acids in the gas phase. I. Electrosprayed ions which are structurally different from their liquid phase precursors

Zinc complexes of deprotonated amino acids (AA), denoted [AA - H + Zn]+, are readily formed in the gas phase by electrospray. Their fragmentations can be studied by low energy collisional activation, yielding structural information about the gaseous ions. In this article we show that such ions may not have a structure similar to that of their liquid phase precursors. The simple case of the deprotonated methanol complex [ZnOCH3](+) is first studied in detail. The precursor of this ion in the desolvation process is [(CH3OH)ZnOCH3](+). Accurate ab initio calculations show that the direct desolvation of this ion via methanol evaporation is a costly reaction, while rearrangement via beta - H transfer to [(CH3OH)ZnH(OCH2)](+) is much more favorable. Competitive evaporation of either methanol or methanal from the rearranged ion is also mon favorable. Thus the [Zn, O, C, H-3](+) ion observed corresponds to a two-ligand, hydride complex [ZnH(OCH2)](+). These computational results are fully consistent with experiments, either direct source or collision induced dissociation (CID) spectra. Analogous observations hold for the zinc complex of deprotonated glycine [Gly - H + Zn](+). Detailed computations of the various possible structures of [Gly-H + Zn](+) and its precursor are consistent with the formation of rearranged structures, which are more stable than those of the initially formed species in solution. These structures are also adequate for explaining the low energy CID spectrum of [(Gly - H)Zn](+). It is concluded that for electrosprayed ions in general, whenever the last desolvation steps are energetically costly, rearrangements may occur before evaporation of the last solvent molecule. The common wisdom that electrospray ionization is a gentle process which produces faithful images of solution phase structures is shown not to apply to certain categories of metal ions. (C) 2000 Elsevier Science B.V.