ELECTRON-TRANSFER QUENCHING AND PHOTOINDUCED EPR OF HYPERICIN AND THECILIATE PHOTORECEPTOR STENTORIN

Time-correlated single photon counting was used to observe dynamic que nching of the hypericin and stentorin excited singlet states. The fluo rescence quenching data for hypericin and stentorin were interpreted i n terms of electron transfer. The observed correlation between free en ergy change of electron transfer and quenching rate constant suggests that quenching proceeds via electron transfer from hypericin and stent orin to the quenchers. EPR spectra for hypericin, stentorin, and stent orin chromoprotein demonstrated that free radical formation was initia ted or enhanced by visible light and that similar radical species were produced in each sample. Furthermore, the EPR signal for stentorin wa s significantly enhanced by 1,4-benzoquinone, but the overall shape an d g-value was unchanged. We suggest that electron transfer in the exci ted state of these chromophores results in the formation of a cation r adical. This electron transfer is a rapid and efficient pathway for de activation of hypericin and stentorin excited singlet states and shoul d be considered when discussing the photoreactivity of hypericin as a photodynamic agent and of stentorin as the Stentor coeruleus photorece ptor.