Electron transfer fluorescence quenching of Blepharisma japonicum photoreceptor pigments

The hypericin analogs blepharismin (BP), oxyblepharismin (OxyBP) and stento rin (ST), the photosensing chromophores responsible for photomotile reactio ns in the ciliates Blepharisma japonicum (red and blue cells) and Stentor c oeruleus, represent a new class of photoreceptor pigments whose chemical st ructures have recently been determined. In the case of ST it has been shown that the first excited singlet state can be deactivated by donation of an electron to an appropriate acceptor molecule (e.g. a quinone molecule). Thi s charge transfer can be considered a possible mechanism for the primary ph otoprocess for the photomotile responses in S. coeruleus. To determine whet her an electron transfer process also occurs in the deactivation of excited blepharismin, we studied the fluorescence quenching of OxyBP in dimethyl-s ulfoxide (DMSO) and in ethanol using electron accepters with different redu ction potentials. Under our experimental conditions ground state and excite d state complexes (like fluorescent exciplexes) are not formed between the fluorophore and the quenchers. In DMSO the bimolecular quenching constant v alues (k(q)) calculated on the basis of the best fitting procedures clearly show that the quenching efficiency decreases with the quencher negative re duction potential, EO. The k(q) (M-1 s(-1)) and E-0 (V) values are, respect ively, 7.8 x 10(9) and -0.134 for 1,4-benzoquinone, 8.9 x 10(9) and -0.309 for 1,4-naphthoquinone, 2.4 x 10(9) and -0.8 for nitrobenzene, 0.009 x 10(9 ) and -1,022 for azobenzene and 0 and -1.448 for benzophenone, These findin gs point to the conclusion that upon formation of the encounter complex bet ween OxyBP and the quencher, an electron is released from excited OxyBP to the quencher, similar to what happens in ST. It is suggested that in the pi gment granules such a light-induced charge transfer from excited blepharism in to a suitable electron acceptor triggers sensory transduction processes in B. japonicum.