Metal-mediated formation of gas-phase amino acid radical cations

The results from an investigation of the collision-induced dissociation (CID) of the ternary complexes [Cu ^II(terpy)(AA)] ²⁺ are presented (terpy = 2,2′:6′,2″-terpyridine; AA = one of the twenty common amino acids). These complexes show a rich gas-phase chemistry, which depends on the identity of the amino acid. For the histidine-, lysine- and tryptophan-containing complexes, oxidative dissociation of the amino acid is observed, yielding the amino acid radical cation. The results of further mass selection and CID of these amino acid radical cations are presented. The CID of the series [Fe ^III(salen)(AA)] ⁺ (where salen = N,N′-ethylenebis(salicylideneaminato)) is also examined. These complexes undergo loss of the neutral amino acid in all cases, although the radical cation of arginine is also produced and its subsequent fragmentation examined. B3-LYP/6-31_G(d) computations were carried out to test aspects of the proposed fragmentation mechanism of the histidine and arginine radical cations.