Dissociation Chemistry of Protonated α-, β- and Other Amino Acids Using Multiple Stage Tandem Mass Spectrometry

To discriminate amino acid isomers by multiple stage tandem mass spectrometry (MSn), the fragmentation of protonated amino acids were investigated by MSn with collision-induced dissociation (CID) and density functional theory calculations. The CID of protonated alpha-amino acids results in a loss of 46 Da, corresponding to H2O and CO, and iminium ions appear as resultant fragments. The CID of protonated beta-amino acids also produces iminium ions, but the corresponding loss is 60 Da instead of 46 Da. H2O loss initiates the fragmentation of protonated beta-amino acids, producing protonated beta-lactams as an intermediate. Subsequently, protonated beta-lactams are easily converted to iminium ions and CH2CO. By contrast, H2O loss from the protonated forms of gamma- and epsilon-amino acids provides protonated lactams with 5- and 7-membered rings, respectively. Protonated lactams with more than 5-membered rings provide stable fragments and do not undergo further degradation during CID. In addition, protonated forms of gamma- and epsilon-amino acids undergo NH3 loss as a competitive fragmentation pathway of H2O loss, producing protonated lactones. Because the fragmentation of protonated amino acid by CID depends on the position of amino and carboxyl groups, the tandem mass spectrometry with CID can discriminate alpha-, beta-, and other amino acids.