Elucidation of Fragmentation Pathways for the Collision-Induced Dissociation of the Binary Ag(I) Complex with Phenylalanine

During our ongoing investigation of the formation and reactivity of gas-phase complex ions composed of Ag(I) and various α-amino acids, we discovered that the mass-to-charge ratio for the major collision-induced dissociation (CID) product generated from a binary Ag + complex with phenylalanine was consistent with the formation of an Ag + complex with an aldehyde. In this study we investigated and compared the fragmentation pathways for complexes of Ag + with phenylalanine, phenylalanine with exchangeable protium replaced with deuterium, phenylalanine with the carboxylic acid group labeled with 13 C, and phenylalanine with the benzylic group labeled with deuterium. The reaction pathways were determined using multidimensional dissociation steps in an ion-trap mass spectrometer. The dissociation experiments provide clear evidence for the formation of several novel product species, including the Ag + complex with phenylacetaldehyde, as well as the formation of an Ag + complex with either a benzyl carbene or styrene. These dissociation products are markedly different from those observed following the fragmentation of other transition and alkali metal adducts of phenylalanine. On the basis of the dissociation of the various isotope-exchanged and -labeled versions of phenylalanine, we propose several reaction pathways that implicate the formation of an Ag + complex with an aziridinone (α-lactam), for which a peak at the correct mass-to-charge ratio was observed in the MS/MS spectrum of the (M + Ag) + ion. A comparison of the apparent reactivity toward water and methanol in the ion-trap mass spectrometer of the Ag + -containing product ions to Ag + complexes with various low-mass organic molecules provided further evidence to support the proposed formation of the aldehyde and styrene complexes with Ag + ions. For instance, the apparent reactivity of the Ag + /aldehyde product ion generated from the CID of the (M + Ag) + ion is identical to that observed for a complex produced by the electrospray ionization of a solution containing Ag + ions and neat phenylacetaldehyde. Similar results were obtained for a dissociation product ion assumed to be a complex composed of Ag + ions and styrene.