A picosecond-absorption study on bacteriochlorophyll excitation, trapping and primary-charge separation in chromatophores of Rhodospirillum rubrum

Abstract

Absorbance-difference spectra and kinetics of absorbance changes were measured of chromatophores of Rhodospirillum rubrum by means of picosecond-absorption spectroscopy. A 35 ps excitation pulse at 532 nm produced absorbance changes due to the formation and decay of excited states of antenna pigments (Nuijs, A.M., Van Grondelle, R., Joppe, H.L.P., Van Bochove, A.C. and Duysens, L.N.M. (1985) Biochim. Biophys. Acta 810, 94–105), and, when open reaction centers were present, also those due to charge separation and primary electron transport. At low excitation energy density the lifetime of singlet-excited antenna bacteriochlorophyll was 80 ± 10 ps when the reaction centers were initially open and 200–400 ps when the primary electron donor was oxidized. Under the former conditions photooxidation of the primary donor occurred with a time constant of 70 ± 10 ps. Reduction of an electron-acceptor complex in the reaction center, probably involving both bacteriochlorophyll and bacteriopheophytin, was observed. Reoxidation of this acceptor occurred with a time constant of 200–300 ps. When the ubiquinone acceptor was reduced chemically, the primary radical pair decayed by recombination with a time constant of about 4 ns at high flash-energy densities, and of about 10 ns at lower energy densities. This dependence of the lifetime of the radical pair on the flash intensity was explained in terms of quenching processes by carotenoid triplet states in the antenna, and indicated a standard free-energy difference between the radical pair and the singlet-excited state of antenna bacteriochlorophyll of about 160 meV.