Theoretical study of the efficient fluorescence quenching process of the firefly luciferin

•Firefly luciferin presents an efficient fluorescence quenching.•This is caused by excited state protonation of the benzothiazole nitrogen heteroatom.•This protonation leads to intersystem crossing with triplet states.•These triplet states are dark or nearly dark excited states.

The firefly oxyluciferin family of fluorophores has been attracting attention from the research community, due to their role on firefly bioluminescence. Moreover, this family exhibits a very efficient fluorescence quenching process. The elucidation of this process is very important, as it may occur inside of the luciferase active site, thereby decreasing the life time of bioluminescence. To this end we have used a computational approach to study the fluorescence quenching of firefly luciferin, the most studied member of the oxyluciferin family of fluorophores. We have found that the fluorescence quenching is due to the excited state protonation of the nitrogen heteroatom of the benzothiazole moiety. This protonation leads to a singlet to triplet intersystem crossing to triplet excited states, which explains the quenching of fluorescence.

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