Combined fluorescence and photovoltage studies on chlorosome containing bacteria: Whole cells of Chloroflexus aurantiacus*

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Abstract

Energy transfer kinetics, primary charge separation, antenna size and excitonic connectivity of photosynthetic units (PSU) in whole cells of Chloroflexus aurantiacus were studied at room temperature by ps-fluorescence and ps-photovoltage as well as by stationary fluorescence-spectroscopy and fluorescence induction measurements. The fluorescence decay kinetics measured at different wavelengths are in accordance with the currently accepted sequential energy transfer from the chlorosomes via the baseplates to the B808–866 complexes and the final trapping in the RC with time constants of 19 ± 2 ps, 40 ± 4 ps and 90 ± 9 ps, respectively. However, the quantitative analysis of fluorescence spectra and the occurrence of slow phases in the fluorescence decays reveal that in whole cells a significant fraction of BChl c in the chlorosome and of BChl a in the baseplate is unconnected. The photovoltage kinetics consisted of two electrogenic phases with time constants of 118 ± 5 ps and 326 ± 35 ps and comparable electrogenicities. The first phase is ascribed to trapping from the B808-866 complexes by P+H_A- formation and the second one to charge stabilization on a quinone acceptor. Fluorescence induction curves displayed a pronounced sigmoidicity, indicating efficient lateral energy transfer between neighbored PSUs and a dense packing of ≈19 reaction centers (RC) beneath one chlorosome. A quantitative analysis of the fluorescence-induction curves at different excitation wavelengths allows the estimation of pigment stoichiometries (i.e. antenna sizes): BChl c/RC ≈ 794 and B808/RC ≈ 15.

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