C-C and C-H bond activation in the fragmentation of the [M + Ni] + adducts of aliphatic amino acids

Abstract

The major metal-containing species formed upon fast atom bombardment of amino acid/Ni+2 mixtures is the [M + Ni]+ adduct, involving reduction of the Ni+2 to the +1 oxidation state. By contrast, electrospray ionization of amino acid/Ni+2 mixtures produces predominantly [Ni(M − H)M]+; this species, on collisional activation, produces predominantly [M + Ni]+ by elimination of [M - H], presumably a carboxylate radical. The unimolecular fragmentation reactions occurring on the metastable ion time scale for the [M + Ni]+ adducts of a variety of α-amino acids have been recorded. The adducts with phenylalanine, α-aminoisobutyric acid and α-aminobutyric acid fragment by elimination of H2O, H2O + CO and, to a minor extent, by elimination of CO2. These reactions are similar to those observed for the [M + Cu]+ adducts of α-amino acids. A reaction distinctive for the [M + Ni]+ adducts involves formation of the immonium ion RCH=NH 2 + . By contrast, the [M + Ni]+ adducts with leucine, isoleucine, and norleucine show extensive metastable ion fragmentation by elimination of H2, CH4, C2H4, C3H6, and C4H8, with the relative importance of the different fragmentation channels depending on the configuration of the C4H9 side chain. These 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, except that minor elimination of C3H6 is observed.