Characterization of [M-H] cations, radicals and anions of glycine in the gas phase: a combined experimental and ab initio study

The gas phase structures of the [M-H] cations and anions of glycine have been studied by using a combination of ab initio calculations (at the MP2(FC)/6-31+G* level of theory) and tandem mass spectrometry (MS/MS). It was found that the ab initio stability order for the anions is [H2NCH2CO2](-) > [H2NCHCO2H](-) > [HNCH2O2H](-). In contrast, the cations exhibit different behaviour, whereas [H2NCHCO2H](+) is predicted to be a stable structure, [H2NCH2CO2](+) spontaneously fragments to the ion-molecule complex [H2NCH2+ ... (OCO)] and the singlet [HNCH2CO2H](+) isomer is predicted to undergo a skeletal rearrangement to form [CH2NHCO2H](+). MS/MS spectra of [M-H](+) cations of various glycine isotopomers were obtained via: (i) collisional activation of electron impact generated cations and (ii) charge reversal of anions formed via HO- negative ion chemical ionization. The resulting spectra were significantly different, suggesting different structures were involved. Neutralization-reionization experiments were performed on [M-H]- anions in order to gain insights into the structures of the intermediate radicals. (C) 1999 Elsevier Science B.V.