Singlet oxygen quenching by riboflavin

Derivatives of riboflavin, specifically flavin mononucleotide, have recently been in the spotlight as the active component in protein-encased optogenetic sensitizers to produce singlet oxygen, O-2(a(1)Delta g). The extent to which riboflavin deactivates singlet oxygen is a key aspect of these studies. Surprisingly, there is a dearth of information on the rate constant for riboflavin-mediated singlet oxygen removal. We now report that, when riboflavin is dissolved in both aqueous and methanol solutions, the rate constant for riboflavin-mediated removal of singlet oxygen cannot be greater than similar to 2 x 10(5) s- 1 M-1. This number is appreciably smaller than what has been reported in the literature and, as such, our results can be significant for mechanistic interpretations of photoprocesses that involve flavins in oxygenated systems.