Gas‐phase complexes of amino acids with Cu(II) and diimine ligands. Part I. Aliphatic and aromatic amino acids

Gas‐phase complexes of Cu(II) with aliphatic and aromatic α‐amino acids (AA) and diimine ligands, 2,2′‐bipyridyl (bpy) and phenanthroline (phen) are obtained by electrospray ionization of their aqueous‐methanol solutions. The amino acids are found to bind to Cu(II) by their COO − and amino groups in the gas‐phase complexes. The [Cu(II)(AA – H)(bpy)] +· complexes (M +· ) show distinct dissociations upon collisional activation. Decarboxylation produces stable (M – CO 2 ) +· ions containing α‐amino alkyl residues coordinated to copper. Further dissociations of radicals derived from aliphatic amino acids involve fission of the C β C γ bond in the alkyl chain that reveals its branching. Leucine and isoleucine are readily distinguished by their different dissociation products. Complexes of aromatic amino acids undergo facile amine group transfer on to copper. Electron distributions in several copper complexes were obtained by ab initio calculations and used to discuss the dissociation products formed from gas‐phase [Cu(II)(AA – H)(bpy)] +· ions.