Interaction of singlet molecular oxygen with double fluorescent and spin sensors

Double fluorescent and spin sensors were recently used to detect transient oxidants via simultaneous fluorescence change and production of the nitroxide radical detected by electron paramagnetic resonance. One such oxidant, singlet molecular oxygen (1O2), was detected in thylakoid membrane using these probes. In the present study, we investigated the total (physical and chemical) quenching of 1O2 phosphorescence by sensors composed of the 2,5-dihydro-2,2,5,5-tetramethyl-1H-pyrrole moiety attached to xanthene or dansyl fluorophores. We found that the quenching rate constants were in the range (2–7) × 107 M−1s−1 in acetonitrile and D2O. Quenching of 1O2 is usually an additive process in which different functional groups may contribute. We estimated that the 1O2 quenching by the amine fragments was ca. one to two orders of magnitude lower than that for the complete molecules. Our data suggest that the incorporation of a fluorescent chromophore results in additional strong quenching of 1O2, which may in turn decrease the nitroxide yield via the 1O2 chemical path, possibly having an effect on quantitative interpretations. We have also found that probes with the dansyl fluorophore photosensitized 1O2 upon UV excitation with the quantum yield of 0.087 in acetonitrile at 366 nm. This result shows that care must be taken when the dansyl-based sensors are used in experiments requiring UV irradiation. We hope that our results will contribute to a better characterization and wider use of these novel double sensors.