GAS-PHASE COMPLEXES OF AMINO-ACIDS WITH CU(II) AND DIIMINE LIGANDS .2. AMINO-ACIDS WITH O, N AND S FUNCTIONAL-GROUPS IN THE SIDE-CHAIN

Ternary complexes of amino acids, Cu(II) and diimine ligand 2,2'-bipyridyl (bpy) are obtained in the gas phase by electrospray ionization (ESI). The amino acids are found to bind to Cu(II) by their deprotonated carboxylate groups in the gas-phase complexes. Collisionally activated dissociations at low ion kinetic energies of the [Cu(II)(AA - H)(bpy)](+.) complexes show decarboxylation followed by cleavages of the amino acid side-chains that reveal interactions with the Cu atom of the side-chain functional groups. Hydroxy, sulfide and disulfide groups are transferred on to the Cu atom following collisional activation, but do not displace the bpy ligand. The side-chain functional groups and the CbetaH2=CalphaHNH2 fragment are found to compete for coordination to Cu(bpy)(+) in the products. The epsilon-amino group in lysine and the guanidyl group in arginine displace the bpy ligand following decarboxylation. The fragmentations of the gas-phase [Cu(II(AA - H)(bpy)](+.) complexes are highly specific for the amino acid residues. Arginine and lysine also form doubly charged ions by ESI that distinguish lysine from the isobaric glutamine.