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

The zinc complex of glycinate [Gly-H + Zn]⁺ has been formed by electrospray of a glycine/ZnCl₂ 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 + CH₃OH-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 [NH₂CH₂COOZn]⁺ (1) or on the amine function [ZnNHCH₂COOH]⁺ (2) or isomeric forms involving ZnH⁺ i.e. either [NH₂CHCOOZnH]⁺ (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=CH₂, 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.