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⁻¹) 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⁻¹. Good agreement between the measured IRMPD spectrum and the calculated linear absorption spectra is observed for the two gauche conformers lowest in energy (ΔE) and free energy (ΔG) at both levels of theory, denoted g−1 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–π 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 g−1 conformer, with deviations of less than 24 cm⁻¹ (average 11 cm⁻¹) between measured and calculated frequencies. Comparison with neutral dopamine reveals the effects of protonation on the geometric and electronic structure.