Electron transfer fluorescence quenching of Blepharisma japonicum photoreceptor pigments

The hypericin analogs blepharismin (BP), oxyblepharismin (OxyBP) and stentorin (ST), the photosensing chromophores responsible for photomotile reactions in the ciliates Blepharisma japonicum (red and blue cells) and Stentor coeruleus, represent a new class of photoreceptor pigments whose chemical structures 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). This charge transfer can be considered a possible mechanism for the primary photoprocess for the photomotile responses in S. coeruleus. To determine whether an electron transfer process also occurs in the deactivation of excited blepharismin, we studied the fluorescence quenching of OxyBP in dimethyl-sulfoxide (DMSO) and in ethanol using electron accepters with different reduction potentials. Under our experimental conditions ground state and excited state complexes (like fluorescent exciplexes) are not formed between the fluorophore and the quenchers. In DMSO the bimolecular quenching constant values (k(q)) calculated on the basis of the best fitting procedures clearly show that the quenching efficiency decreases with the quencher negative reduction potential, EO. The k(q) (M-1 s(-1)) and E-0 (V) values are, respectively, 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 findings point to the conclusion that upon formation of the encounter complex between 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 pigment granules such a light-induced charge transfer from excited blepharismin to a suitable electron acceptor triggers sensory transduction processes in B. japonicum.