Structures and fragmentations of zinc(II) complexes of amino acids in the gas phase. II. Decompositions of glycine–Zn(II) complexes

Abstract

The zinc complex of glycinate [Gly–H + Zn] + has been formed by electrospray of a glycine/ZnCl 2 mixture in a 50:50 vol. water/methanol solution. In this article, the precursors and the fragments of [Gly–H + Zn] + ions are studied by means of collisional induced decomposition (CID) experiments including H/D exchanges and accurate ab initio calculations. Two precursors were identified: [Gly + CH 3OH–H + Zn] + ( A) and [Gly + Gly–H + Zn] + ( B), A being much more abundant than B. The three main fragmentations of [Gly–H + Zn] + are loss of carbon dioxyde, loss of carbon monoxyde, and successive losses of water and carbon monoxyde. To interpret these fragmentations four structures were chosen to describe [Gly–H + Zn] +. These structures are complexes between Zn(II) and glycine deprotonated either on the carboxylic group [NH 2CH 2COOZn] + ( 1) or on the amine function [ZnNHCH 2COOH] + ( 2) or isomeric forms involving ZnH + i.e. either [NH 2CHCOOZnH] + ( 3) or [HZn ⋯ NHCHCOOH] + ( 4) respectively. None of the fragmentations is interpretable directly from structures 1 and 2. Loss of carbon dioxyde occurs from 3, loss of carbon monoxyde from a complex CX where HOZn + interacts with CO and NHCH 2, a rearranged form of 2. Successive losses of water and carbon monoxyde can take place from 4. The non occurrence of structures 1 and 2 during the fragmentation of [Gly–H + Zn] + ions is interpreted by isomerizations within A before evaporation of the last molecule of solvent. These isomerizations are energetically easier than the last step of desolvation.