Gas-Phase Fragmentation Reactions of Protonated Aromatic Amino Acids:  Concomitant and Consecutive Neutral Eliminations and Radical Cation Formations

Abstract

Gas-phase dissociation reactions of protonated amino acidsphenylalanine, tyrosine, tryptophan, and histidineare rich and diverse. Considerable similarities exist among the four amino acids, but there are also significant differences. Facile reactions include the elimination of NH3, common to all aromatic amino acids except histidine, and the concomitant elimination of H2O and CO. Labeling experiments with deuteriums show considerable H/D scrambling prior to dissociation involving N−H, O−H, and C−H (both aliphatic and aromatic hydrogens). Mechanisms of this scrambling are proposed. At higher collision energies, eliminations of H2O, CO, CO2, and CH2CO occur after that of NH3. Similarly, eliminations of HCN, HCNH2, and NH3 occur after that of H2O and CO. The elimination of CH2CO is preceded by migration of the hydroxyl ion from the carboxylic group to the exocyclic carbon on the side chain. Aromatic amino acids, with the exception of tyrosine, were observed to yield cationic radical fragments by eliminating small radicals, including H•, CH3 •, and NHCH•.

UV photoinduced dynamics in protonated aromatic amino acid

“…The fragmentation channels will thus be similar to the ones observed in CID 3,19,22 . However, one might expect that the branching ratio between the different fragmentation channels changes drastically.…”

“…3 The absence of pump/probe signal on this channel clearly indicates that this is not the case and that the kinetics of fragmentation is faster than the proton exchange and total energy redistribution. It should be stressed that the coincidence experiment gives an upper limit for the kinetics of dissociation at less than 10 ns.…”

“…This internal energy is large enough to induce fragmentations. For instance, less than 2 eV only are required for an ammonia loss 3 . This excess energy is randomized and consequently leads to a statistical-type dissociation of the ion on the ground state potential energy surface, as also observed in collision-induced dissociation (CID) experiments.…”

Dissociations of Complexes Between Monovalent Metal Ions and Aromatic Amino Acid or Histidine

“…+ upon IR photodissociation [35]. In an earlier publication, we reported detailed fragmentation reactions of protonated aromatic AAs [43]. H/D exchange and precursor ion scan experiments show that some of the ions formed from protonated and metalated aromatic AAs are identical.…”

New evidence for H/D scrambling of tryptophan and its analogues in the gas phase

“…There were a large amount of reports on the studies of the tryptophan radical which can take part in electron and radical transfers in proteins in the gas phase [2][3][4][5][6]. For the protonated tryptophan, eliminations of some small molecules such as H 2 O, CO, CO 2 or C 2 H 2 O can be observed after losing NH 3 under higher collision energy [7][8][9][10].…”