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 α-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 β-amino acids also produces iminium ions, but the corresponding loss is 60 Da instead of 46 Da. H₂O loss initiates the fragmentation of protonated β-amino acids, producing protonated β-lactams as an intermediate. Subsequently, protonated β-lactams are easily converted to iminium ions and CH₂CO. By contrast, H₂O loss from the protonated forms of γ- and ε-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 γ- and ε-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 α-, β-, and other amino acids.