Conformational analysis of riboflavin in aqueous solution: The influence of hydrogen bonding in riboflavin and FAD(2-) molecules on fluorescence parameters

The influence of the hydrogen bonding between the isoalloxazine ring system and the ribityl chain on the fluorescence decay times is shown. A conformational analysis of the riboflavin molecule in an aqueous solution was carried out. Ab initio calculations were carried out using density functional theory, where functional were extended with Grimme’s dispersion correction. Conformers of riboflavin in the neutral form with the lowest energy values in water were determined. These conformers have the hydrogen bonding between the ribityl chain and the nitrogen atom in the isoalloxazine ring system. It was shown that for riboflavin and FAD(2-) molecules there are similar hydrogen bonding between the isoalloxazine ring system and the ribityl chain. The process of the isoalloxazine ring protonation in riboflavin and FAD(2-) conformers with the hydrogen bonding upon interaction with hydronium can be carried out through the proton transfer process from the oxygen atom at the ribityl chain.