Infrared spectra of protonated neurotransmitters: dopamine

Abstract

The infrared (IR) spectrum of the isolated protonated neurotransmitter dopamine was recorded in the fingerprint range (570-1880 cm À1) by means of IR multiple photon dissociation (IRMPD) spectroscopy. The spectrum was obtained in a Fourier transform ion cyclotron resonance mass spectrometer equipped with an electrospray ionization source, which was coupled to a free electron laser (FEL). The spectroscopic studies are complemented by quantum chemical calculations at the B3LYP and MP2 levels of theory using the cc-pVDZ basis set. Several low-energy isomers with protonation occurring at the amino group are predicted in the energy range 0-50 kJ mol À1. Good agreement between the measured IRMPD spectrum and the calculated linear absorption spectra is observed for the two gauche conformers lowest in energy (DE) and free energy (DG) at both levels of theory, denoted gÀ1 and g+1. Minor contributions of higher lying gauche isomers cannot be ruled out spectroscopically but their calculated energies suggest only minor population in the sampled ion cloud. In all these gauche structures, one of the three protons of the ammonium group is pointing toward the catechol subunit, thereby maximizing the intramolecular NH-p interaction of the positive charge with the aromatic ring. In total, 16 distinct vibrational bands are observed in the IRMPD spectrum and assigned to individual normal modes of the energetically most stable gÀ1 conformer, with deviations of less than 24 cm À1 (average 11 cm À1) between measured and calculated frequencies. Comparison with neutral dopamine reveals the effects of protonation on the geometric and electronic structure.

Infrared multiple photon dissociation (IRMPD) spectroscopy of oxazine dyes

“…The frequency was calibrated using a grating spectrometer and the dissociation yield was normalized for laser power fluctuations, assuming linear power dependence, as described previously. 22 …”

Weak hydrogen bonding motifs of ethylamino neurotransmitter radical cations in a hydrophobic environment: infrared spectra of tryptamine+–(N2)n clusters (n ≤ 6)

“…Comparison of the open-shell TRA + radical cation with previous studies on protonated closed-shell aromatic ethylamino neurotransmitters (e.g., serotonin, dopamine, histamine) [47][48][49] will reveal the effects of the charge distribution on the orientation of the side chain and the intramolecular NH-p interaction. The present study provides a detailed analysis of the IRPD spectra of TRA + -(N 2 ) n (n = 1-6) with quantum chemical calculations including a natural bond orbital (NBO) analysis to obtain insights into the intermolecular interactions acting on this prototypical aromatic neurotransmitter cation in a nonpolar environment.…”

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

“…The first spectroscopic data for this ion establish that the preferred protonation site of this fundamental neurotransmitter molecule in the gas phase is at the imidazole ring and not at the ethylamine side chain. In this respect, protonated histamine differs from all related neurotransmitterH + ions studied so far, 12,13,[54][55][56] which exclusively prefer protonation at the alkylamino side chain. …”

“…The IRMPD spectrum of mass-selected histamineH + ions was obtained in the 575-1900 cm À1 fingerprint range using the same strategy employed previously for related protonated neurotransmitters 12,13 and polycyclic aromatic hydrocarbon molecules. [68][69][70] Briefly, the IRMPD spectrum of histamineH + was recorded in a FT-ICR mass spectrometer coupled to the IR beamline of FELIX.…”

“…83 Reported relative (free) energies DE and DG (298 K) include harmonic vibrational zero-point energy corrections scaled with a factor of 0.98 (B3LYP) and 0.97 (MP2). 12,13 If not stated otherwise, all coordinates were relaxed to locate stationary points on the potential energy surface. For all stationary points, frequency analysis ensured their nature as a minimum or transition state.…”