Fragmentation of Protonated Histamine and Histidine by Electrospray Ionization In-Source Collision-Induced Dissociation

Electrospray ionization (ESI) generally produces intact gas-phase ions without extensive fragmentation; however, for histamine and histidine, ESI provides fragment ions through in-source collision-induced dissociation (CID). In this study, we investigated the fragmentation of these compounds both experimentally and using density functional theory calculations. We found that histamine undergoes protonation with subsequent NH3 loss by ESI in-source CID. The corresponding fragmentation mainly produces bicyclo and spiro compounds. In contrast, the ESI in-source CID of protonated histidine preferentially results in H2O loss rather than NH3 loss. However, the corresponding fragment ion is not observed in the ESI mass spectrum of histidine, because it undergoes further CO loss within 100 ps. Consequently, protonated histidine produces a fragment ion arising from a 46 Da loss, which corresponds to the masses of H2O and CO, by ESI in-source CID. The fragment ion yields of histamine and histidine produced by ESI in-source CID are then estimated from the dissociation rate constant and internal energy of the analyte ion, respectively. The dissociation rate constant and internal energy of the analyte ion were determined by double-hybrid density functional theory calculations and the survival yield method using benzylpyridinium thermometer ions, respectively. Because intense fragment ion signals are present in the ESI mass spectrum, the analysis of the fragment ions produced by ESI in-source CID facilitates the identification of metabolites originating from aromatic amino acids, such as histamine.

Automated summary

This study investigated the fragmentation patterns of histamine and histidine under electrospray ionization (ESI) using both experimental and theoretical approaches. The results showed that histamine undergoes NH3 loss, producing bicyclo and spiro compounds, while protonated histidine primarily loses H2O. The study also demonstrated that the analysis of these fragmentation ions can facilitate the identification of metabolites derived from aromatic amino acids.