Low potential oxidation-reduction titration of absorbance changes induced by pulsed laser in chromatophores of photosynthesizing bacteria Ectothiorhodospira shaposhnikovii

The photoreactions activated by pulsed laser (694.3 nm, 30 ns, 1-3 mJ cm-2) were studied in isolated Ectothiorhodospira shaposhnikovii chromatophores poised at different redox potentials over a range of +200 mV to -160 mV. Within Esub(h) ranging from +60 to 0 mV the laser-induced oxidation reaction is turned over from high-potential cytochrome Csub(H) (Esub(m) = +290 mV) to low-potential cytochrome Csub(L) the midpoint potential of which (Esub(m)) was estimated to be within +10 - 0 mV at pH 7.0. From the decrease in the extent of the laser-induced oxidation of reaction centre bacteriochlorophyll Esub(m) = +390 mV) and cytochrome Csub(L), seen as absorbance changes on lowering Esub(h), it was estimated that the midpoint potential of the primary electron acceptor Esub(m) = -83 +- 9 mV at pH 7.0. At Esub(h) below -120 mV chromatophores demonstrated absorbance changes with a maximum at 425 nm and recovery half-time of 5 - 7 μs. The nature of the latter remains to be discussed. (author)