Time-correlated single photon counting was used to observe dynamic quenching of the hypericin and stentorin excited singlet states. The fluorescence quenching data for hypericin and stentorin were interpreted in terms of electron transfer. The observed correlation between free energy change of electron transfer and quenching rate constant suggests that quenching proceeds via electron transfer from hypericin and stentorin to the quenchers. EPR spectra for hypericin, stentorin, and stentorin chromoprotein demonstrated that free radical formation was initiated or enhanced by visible light and that similar radical species were produced in each sample. Furthermore, the EPR signal for stentorin was significantly enhanced by 1,4-benzoquinone, but the overall shape and g-value was unchanged. We suggest that electron transfer in the excited state of these chromophores results in the formation of a cation radical. This electron transfer is a rapid and efficient pathway for deactivation of hypericin and stentorin excited singlet states and should be considered when discussing the photoreactivity of hypericin as a photodynamic agent and of stentorin as the Stentor coeruleus photoreceptor.
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