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

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 NH 3, common to all aromatic amino acids except histidine, and the concomitant elimination of H 2 O 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 H 2 O, CO, CO 2, and CH 2 CO occur after that of NH 3 . Similarly, eliminations of HCN, HCNH 2, and NH 3 occur after that of H 2 O and CO. The elimination of CH 2 CO 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 •, CH 3 •, and NH CH • .