Structure, energetics and reactivity of ternary complexes of amino acids with Cu(II) and 2,2'-bipyridine by density functional theory. A combination of radical-induced and spin-remote fragmentations.

The structures, electron distributions and dissociation energies of gas-phase ternary complexes of Cu(II) with 2,2'-bipyridine and leucine, isoleucine and lysine were addressed by density functional theory using the hybrid B3LYP functional, effective core potentials and the 6-31 + G(d) and LANL2DZ basis sets. The calculations confirm the previously suggested structures, in which amino acid carboxylates coordinate to the Cu atom by the carboxylate and alpha-amino groups in square-planar complexes. The dissociation energies for consecutive eliminations of CO(2) and alkyl radicals from isomeric singly charged complexes of leucine and isoleucine correlate with the ion relative abundances observed in collisionally activated dissociation mass spectra. Doubly charged lysine complexes show extremely low dissociation energies that are consistent with the <1 eV center-of-mass collision energies that were used in previous CAD studies. The calculated charge and spin densities point to radical-induced dissociations of singly charged complexes with an open-electron shell. In contrast, the unpaired electron is virtually inert in doubly charged, open-shell complexes that undergo charge-induced, spin-remote dissociations in the amino acid residues.