Electron Transfer Quenching and Photoinduced EPR of Hypericin and the Ciliate Photoreceptor Stentorin

Abstract

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.

Concerning the Deprotonation of the Photooxidized 3-Hypericinate Ion

“…Figure 2 compares the calculated values of this set with the data obtained above. Clearly, (3,6) 1 . À (À963.6 kJ Á mol À1 ) and (3,4) 1 .…”

“…is most acidic at positions 4 and 6 yielding the most stable radical anions (3,4) 1 . and (3,6) 1 . .…”

“…It has been recently hypothesized [6] that in close analogy to the case of the tyrosin radical cation [7] this intermolecular proton transfer could be due to the enhanced acidity displayed by the radical cation formed by electron transfer from an excited state. However, there is obviously an inconsistency in this reasoning.…”

Quenching of Excited Triplet State Hypericin with Energy Acceptors and Donors and Acceptors of Electrons

“…[17][18][19] Superoxide and the radical anion (Hyp •-) and/or radical cation (Hyp •+ ) may be directly or indirectly responsible for the photobiological activity of hypericin, and electron-transfer quenching of the fluorescent singlet state has been observed. [20][21][22] The literature lacks data about the photophysical parameters of Hyp •-, and the rate constant of the reduction reaction 1 is unknown, though the reaction of anionic aggregates of hypericin with O 2 has been reported. 23 The quenching of hypericin fluorescence by electron acceptors such as quinones, nitro-and azobenzene in DMSO occurs via electron transfer, as established by EPR.…”

Concerning Regioselective Photochemical Intermolecular Proton Transfer from Hypericin

“…Indeed, intermolecular proton transfer from the bay-hypericinate ion has been deduced from¯uorescence studies of hypericinate embedded in lipid vesicles with an encapsulated pH indicator solution [3]. Based on quenching experiments and electron paramagnetic resonance studies, it has been argued that a cation radical formed by electron transfer from the hypericin triplet state could serve as the strongly acidic proton source [6,7].…”