Picosecond spectroscopy of isolated membranes of the photosynthetic green sulfur bacterium Prosthecochloris aestuarii upon selective excitation of the primary electron donor

Abstract

Picosecond excitation at 850 nm of isolated membranes of the photosynthetic green sulfur bacterium Prosthecochloris aestuarii, when the primary electron donor P-840 was in the oxidized state, caused the formation of excited states of bacteriochlorophyll (BChl) a in the antenna, but not those of other pigment molecules, as had been observed upon excitation at shorter wavelengths. When, however, P-840 was initially in the reduced state, the flashes produced in addition absorption changes that could be attributed to reaction center components. These changes included bleachings of the absorption bands of P-840 and of BChl c. The latter bleaching, caused by BChl c reduction, decayed in 700 ± 70 ps, due to electron transfer to the next electron acceptor, whereas the bleaching of the bands of P-840 did not reverse in the time region studied (up to 3 ns). Analysis of the absorbance changes appeared to indicate a 1:1 stoichiometry for P-840 oxidation and BChl c reduction. The primary charge separation occurred in less than 10 ps.