C-C AND C-H BOND ACTIVATION IN THE FRAGMENTATION OF THE [M-ACIDS(NI](+) ADDUCTS OF ALIPHATIC AMINO)

The major metal-containing species formed upon fast atom bombardment o f amino acid/Ni2+ mixtures is the [M + Ni](+) adduct, involving reduct ion of the Ni+2 to the +1 oxidation state. By contrast, electrospray i onization of amino acid/Ni+2 mixtures produces predominantly [Ni(M - H )M](+); this species, on collisional activation, produces predominantl y [M + Ni](+) by elimination of [M - H], presumably a carboxylate radi cal. The unimolecular fragmentation reactions occurring on the metasta ble ion time scale for the [M + Ni](+) adducts of a variety of alpha-a mino acids have been recorded. The adducts with phenylalanine, alpha-a minoisobutyric acid and alpha-aminobutyric acid fragment by eliminatio n of H2O, H2O + CO and, to a minor extent, by elimination of CO2. Thes e reactions are similar to those observed for the [M + Cu](+) adducts of alpha-amino acids. A reaction distinctive for the [M + Ni](+) adduc ts involves formation of the immonium ion RCH=NH2+. By contrast, the [ M + Ni](+) adducts with leucine, isoleucine, and norleucine show exten sive metastable ion fragmentation by elimination of H-2, CH4, C2H4, C3 H6, and C4H8, with the relative importance of the different fragmentat ion channels depending on the configuration of the C4H9 side chain. Th ese results are interpreted in terms of C-C and C-H bond activation of the C4H9 side chain by the Ni+. The adducts with valine and norvaline fragment in a fashion similar to the adduct with phenylalanine, excep t that minor elimination of C3H6 is observed. (C) 1997 American Societ y for Mass Spectrometry.