Intramolecular n-type proton/hydrogen network in basic structures of vitamin B6 investigated by IRMPD spectroscopy

A combined IRMPD and DFT study of the isomeric forms of protonated pyridoxine, pyridoxamine and pyridoxal has allowed to establish the spectroscopic and some peculiar structural features of these basic components of the vitamin B6 group in the gas phase, with the aim to offer a standpoint free from the environmental effects, which could help in understanding their behavior in the condensed phase typical of biological systems. The imine-enamine and the neutral-zwitterion tautomerism have been considered. Analyzing the experimental and theoretical data a detailed characterization of the protonation sites and intramolecular proton/hydrogen bonding interactions emerged. The comparison between experimental and theoretical signals of free N-H and O-H bonds and when they are involved as proton/hydrogen bond donors towards n-acceptors suggests a correspondence between their anharmonicity and the intensity of the IRMPD signals, up to the so called IRMPD transparency. (C) 2019 Elsevier B.V. All rights reserved.