Hydride rearrangements lead to different decomposition pathways for leucine and isoleucine†
Abstract
The mechanism of gas-phase fragmentation of the isomeric amino acids leucine (Leu) and isoleucine (Ile) remains an area of active investigation. In this study, we carried out collision-induced dissociation (CID) on the protonated amino acids and observed that the characteristic ion at m/z 69 exhibited different abundances in their CID tandem mass spectra. These variations can be attributed to competitive reactions involving hydride rearrangement and β-elimination. For Ile, the 1,2-hydride shift is favored over β-elimination, resulting in the ion peak at m/z 69 being the most intense peak. In contrast, for Leu, the 1,3-hydride rearrangement is less favorable than the corresponding β-elimination, leading to a near-complete disappearance of the ion at m/z 69 in the mass spectra (MS). The energies associated with these fragmentation processes were calculated using density functional theory (DFT). The proposed fragmentation pathways were further validated through MS3 experiments, isotope labeling, and calculations of fragment ion yields produced by electrospray ionization (ESI) in-source CID. These findings provide a rational basis for the differences observed in the CID tandem mass spectra of the isomeric amino acids Leu and Ile, thereby facilitating peptide sequencing.