Fragmentation Reactions of Protonated α-Amino Acids

The metastable ion fragmentation reactions of protonated α-amino acids were recorded. In addition, the low-energy collision-induced dissociation (CID) reactions were studied as a function of collision energy and breakdown graphs, expressing the energy dependence of the fragmentation reactions, were established for a variety of protonated amino acids. The fragmentation reactions observed depend strongly on the identity of the R group in H2NCH(R)COOH. Protonated amino acids containing only alkyl groups in the side-chain fragment primarily by elimination of (H2O+CO) in both metastable and CID reactions. Hydroxylic and acidic amino acids show loss of H2O and loss of (H2O+CO) from MH+ with the H2O loss occurring from the side-chain and (H2O+CO) loss occurring from the α-carbohydroxy group. Amidic amino acids show NH3 loss from the side-chain and (H2O+CO) loss from the carbohydroxy group. Aromatic and sulfur-containing amino acids show loss of NH3 from MH+, as does lysine. Protonated arginine shows a variety of fragmentation pathways, including elimination of NH3, elimination of neutral guanidine and formation of protonated guanidine. The energy-dependent breakdown graphs elucidate a variety of secondary fragmentation reactions of the primary fragment ions.