Combined fluorescence and photovoltage studies on chlorosome containing bacteria - I. Whole cells of Chloroflexus aurantiacus

Energy transfer kinetics, primary charge separation, antenna size and excit onic connectivity of photosynthetic units (PSU) in whole cells of Chlorofle xus aurantiacus were studied at room temperature by ps-fluorescence and ps- photovoltage as well as by stationary fluorescence-spectroscopy and fluores cence induction measurements. The fluorescence decay kinetics measured at d ifferent wavelengths are in accordance with the currently accepted sequenti al 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 an alysis of fluorescence spectra and the occurrence of slow phases in the flu orescence 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 phot ovoltage kinetics consisted of two electrogenic phases with time constants of 118 +/- 5 ps and 326 +/- 35 ps and comparable electrogenicities. The fir st phase is ascribed to trapping from the B808-866 complexes by P+HA- forma tion and the second one to charge stabilization on a quinone acceptor. Fluo rescence induction curves displayed a pronounced sigmoidicity, indicating e fficient lateral energy transfer between neighbored PSUs and a dense packin g of approximate to 19 reaction centers (RC) beneath one chlorosome. A quan titative analysis of the fluorescence-induction curves at different excitat ion wavelengths allows the estimation of pigment stoichiometries (i.e. ante nna sizes): BChl c/RC approximate to 794 and B808/RC approximate to 15.