Infrared spectra of the protonated neurotransmitterhistamine: competition between imidazolium and ammonium isomers in the gas phase

The infrared (IR) spectrum of protonated histamine (histamineH+) was recorded in the 575–1900 cm−1 fingerprint range by means of IR multiple photon dissociation (IRMPD) spectroscopy. The IRMPD spectrum of mass-selected histamineH+ ions was obtained in a Fourier transform ion cyclotron resonance mass spectrometer coupled to an electrospray ionization source and an IR free electron laser. A variety of isomers were identified and characterized by quantum chemical calculations at the B3LYP and MP2 levels of theory using the cc-pVDZ basis set. The low-energy isomers are derived from various favourable protonation sites—all of which are N atoms—and different orientations of the ethylamine side chain with respect to the heterocyclic imidazole ring. The measured IRMPD spectrum was monitored in the NH3 loss channel and exhibits 14 bands in the investigated spectral range, which were assigned to vibrational transitions of the most stable isomer, denoted A. This imidazolium-type isomer A with protonation at the imidazole ring and gauche conformation of the ethylamine side chain is significantly stabilized by an intramolecular ionic Nπ–H+⋯Nα hydrogen bond to the ethylamino group. The slightly less stable ammonium-type isomer B with protonation at the ethylamino group is only a few kJ mol−1 higher in energy and may also provide a minor contribution to the observed IRMPD spectrum. Isomer B is derived from A by simple proton transfer from imidazole to the ethylamino group along the intramolecular Nπ–H+⋯Nα hydrogen bond via a low barrier, which is calculated to be of the order of 5–15 kJ mol−1. Significantly, the most stable structure of isolated histamineH+ differs from that in the condensed phase by both the protonation site and the conformation of the side chain, emphasizing the important effects of solvation on the structure and function of this neurotransmitter. The effects of protonation on the geometric and electronic structure of histamine are evaluated by comparing the calculated properties of isomer A with those of the most stable structure of neutral histamine A(n).