High-energy collision-induced dissociation of histidine ions [His + H]+ and [His - H]- and histidine dimer [His2 + H

Abstract

Histidine (His) is an essential amino acid, whose side group consists of an aromatic imidazole moiety that can bind a proton or metal cation and act as a donor in intermolecular interactions in many biological processes. While the dissociation of His monomer ions is well known, information on the kinetic energy released in the dissociation is missing. Using a new home-built electrospray ionization (ESI) source adapted to a double-focusing mass spectrometer of BE geometry, we investigated the fragmentation reactions of protonated and deprotonated His, [His + H]+ and [His - H]- , and the protonated His dimer [His2 +H]+ , accelerated to 6keV in a high-energy collision with helium gas. We evaluated the kinetic energy release (KER) for the observed dissociation channels. ESI of His solution in positive mode led to the formation of His clusters [Hisn + H]+ , n = 1-6, with notably enhanced stability of the tetramer. [His + H]+ dissociates predominantly by loss of (H2 O + CO) with a KER of 278meV, while the dominant dissociation channel of [His - H]- involves loss of NH3 with a high KER of 769meV. Dissociation of [His2 + H]+ is dominated by loss of the monomer but smaller losses are also observed. The KER for HCOOH loss from both [His + H]+ and [His - H]- is similar at 278 and 249meV, respectively, which suggests that the collision-induced dissociation takes place via a similar mechanism. The loss of COOH and C2 H5 NO2 from the dimer suggests that the dimer of His binds through a shared proton between the imidazole moieties.