Excited states and primary charge separation in the pigment system of the green photosynthetic bacterium Prosthecochloris aestuarii as studied by picosecond absorbance difference spectroscopy

Abstract

Picosecond absorbance difference spectra at a number of delay times after a 35 ps excitation flash and kinetics of absorbance changes were measured of the membrane vesicle preparation Complex I from the photosynthetic green sulfur bacterium Prosthecochloris aestuarii. After chemical oxidation of the primary donor the excitation pulse produced singlet and triplet excited states of carotenoid and bacteriochlorophyll a. With active reaction centers present also the flash-induced primary charge separation and subsequent electron transfer were observed. The singlet excited state of the carotenoid, formed by direct excitation at 532 nm, is characterized by an absorbance band peaking at 590 nm. Its average lifetime was calculated to be about 1 ps. Excited singlet states of bacteriochlorophyll a were characterized by a bleaching of their ground state Q y absorption bands. Singlet excited states, localized on the so-called core complex, were produced by energy transfer from excited carotenoid. Their lifetime was about 70 ps. A decay component of about 280 ps was ascribed to singlet excited bacteriochlorophyll a in the bacteriochlorophyll a protein. These singlet excitations were partly converted to the triplet state. With active reaction centers, oxidation of the primary donor, P-840, characterized by the bleaching of its Q y and Q x absorption bands, was observed. This oxidation was accompanied by a bleaching between 650 and 680 nm and an absorbance increase between 680 and 750 nm. These changes, presumably due to reduction of bacteriopheophytin c (Van Bochove, A.C., Swarthoff, T., Kingma, H., Hof, R.M., Van Grondelle, R., Duysens, L.N.M. and Amesz, J. (1984) Biochim. Biophys. Acta 764, 343–346), were attributed to the reduction of the primary electron acceptor. Electron transfer to a secondary acceptor occurred with a time-constant of 550 ± 50 ps. Since no absorbance changes due to reduction of this acceptor were observed in the red or infrared region, we tentatively assume that this acceptor is an iron-sulfur center.