A theoretical study on the quenching mechanisms of triplet state riboflavin by tryptophan and tyrosine

The reactions of tryptophan (Trp) and tyrosine (Tyr) with endogenous photosensitizer riboflavin (RF) have gained much interest for their crucial roles in various photobiological processes. In this paper, the quenching mechanisms of triplet state RF by Trp and Tyr have been explored employing density functional theory calculations. It is revealed that the H-atom transfer reaction from Trp and Tyr to triplet state RF is more favorable on thermodynamic grounds compared with direct energy transfer or direct electron transfer pathways. During the photosensitization, RF can photogenerate various reactive oxygen species (ROS) as intermediates, while the present study provides some deeper insights into the photosensitizing behaviors of triplet state RF by reacting directly with Trp and Tyr.