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]+, ar e readily formed in the gas phase by electrospray. Their fragmentations can be studied by low energy collisional activation, yielding structural infor mation about the gaseous ions. In this article we show that such ions may n ot have a structure similar to that of their liquid phase precursors. The s imple case of the deprotonated methanol complex [ZnOCH3](+) is first studie d in detail. The precursor of this ion in the desolvation process is [(CH3O H)ZnOCH3](+). Accurate ab initio calculations show that the direct desolvat ion of this ion via methanol evaporation is a costly reaction, while rearra ngement via beta - H transfer to [(CH3OH)ZnH(OCH2)](+) is much more favorab le. Competitive evaporation of either methanol or methanal from the rearran ged ion is also mon favorable. Thus the [Zn, O, C, H-3](+) ion observed cor responds to a two-ligand, hydride complex [ZnH(OCH2)](+). These computation al results are fully consistent with experiments, either direct source or c ollision induced dissociation (CID) spectra. Analogous observations hold fo r the zinc complex of deprotonated glycine [Gly - H + Zn](+). Detailed comp utations of the various possible structures of [Gly-H + Zn](+) and its prec ursor 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, rearrangement s may occur before evaporation of the last solvent molecule. The common wis dom that electrospray ionization is a gentle process which produces faithfu l images of solution phase structures is shown not to apply to certain cate gories of metal ions. (C) 2000 Elsevier Science B.V.