Quantum-chemical modelling of the spectral-luminescent properties of pyridoxine in water

Abstract

This paper presents a theoretical interpretation of the electronic spectra of pyridoxine in water using quantum-mechanical calculations of vertical optical transitions within the framework of density functional theory and an analysis of electronic state changes. The absorption and fluorescence spectra of pyridoxine were calculated for three structures (isolated pyridoxine, in a continuous aqueous solution, and a complex with three water molecules). It was found that the excited state of isolated pyridoxine is formed mainly by a change in electron density near the heterocyclic nitrogen atom. The nitrogen and oxygen atoms of the hydroxyl group are involved in the formation of the excited state of the pyridoxine molecule surrounded by water in the continuous model and the complex with three water molecules in an equal amount. It is shown that the water environment of the pyridoxine molecule does not describe its spectral and luminescent properties even with explicit consideration of nearby water molecules, where no noticeable difference is observed in the absorption spectra and electron density redistribution of the lower state from the isolated structure. It has been established that the experimental absorption spectrum of pyridoxine in water is a superposition of the excitation of both the neutral and negatively charged forms of the molecule, where the longest wavelength transition is formed by the anionic form.