Combined fluorescence and photovoltage studies on chlorosome containing bacteria

Abstract

Energy transfer kinetics and primary charge separation were studied in whole cells and in chlorosome-depleted membranes of Chlorobium limicola by ps-fluorescence and ps-photovoltage as well as by stationary fluorescence spectroscopy. The fluorescence decay kinetics of whole cells indicate a sequential energy transfer from the chlorosomes via the baseplates and the Fenna–Matthews–Olson-protein (FMO) to the core-complexes with time constants of 35 ± 4 ps and 95 ± 10 ps, respectively. 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-FMO-protein is poorly connected to the core-complexes. The photovoltage kinetics of whole cells upon excitation in the chlorosome (λex = 532 nm) consisted of two electrogenic phases with time constants of 121 ± 10 ps and 575 ± 140 ps. The ≈ 120 ps phase is composed of energy transfer from the baseplate-FMO-protein to the core-complex and trapping from the core-complexes by P+A0- formation. The second phase (relative electrogenicity of 23%) is a charge stabilization step, probably from the first electron acceptor, A0, to the secondary electron acceptor FX. When the core-complexes were excited (λex = 840 nm) the photovoltage kinetics also consisted of two electrogenic phases, but the first phase showed a time constant of 23 ± 6 ps. This phase reflects exclusively trapping from the core-complexes by P+A0- formation. In dark-adapted whole cells the fluorescence yields of the peripheral antenna complexes increased strongly upon background illumination. This observation indicates the disappearance of endogenous quenchers, probably quinones.