PICOSECOND ENERGY-TRANSFER AND TRAPPING KINETICS IN LIVING CELLS OF THE GREEN BACTERIUM CHLOROFLEXUS-AURANTIACUS

The excitation energy transfer and trapping processes in intact cells of Chloroflexus aurantiacus were studied by picosecond time-resolved f luorescence spectroscopy. The fluorescence decay kinetics is investiga ted over the near infrared emission range between 730 nm and 920 nm us ing various excitation wavelengths and excitation intensities. The dat a were analyzed by global decay analysis and are presented as decay-as sociated spectra (DAS). The specific dependence of the decay kinetics on the excitation wavelength and on the photochemical redox state of t he reaction center (RC) allows the identification of the energy transf er and trapping components. The DAS provide evidence for two chlorosom al energy transfer processes. The first one occurs between the chloros omal bacteriochlorophyll (BChl)-c and the BChl-a792 complex (B792) in the chlorosomal baseplate with an equilibration time constant of 15-16 ps, while the second one occurs from the B792 pigments to the BChl-a8 06 pigments in the B806-866complex with a time constant of 35-40 ps. T he overall energy trapping process in whole cells is mainly determined by the kinetics of the primary charge separation process in the RCs. With open RCs (Q(A) oxidized) the trapping time constant is 70-90 ps, while the trapping process with closed RCs (Q(A) reduced) takes as lon g as 180-200 ps. The results on whole cells reported here are interpre ted in conjunction with those reported earlier for the various isolate d complexes, i.c., two different chlorosome preparations (Holzwarth, A .R., Muller, M.G. and Griebenow, K. (1990) J. Photochem. Photobiol. B 5, 457-465), the B806-866 complex (Griebenow, K., Muller, M.G. and Hol zwarth, A.R. (1991) Biochim. Biophys. Acta 1059, 226-232) and isolated reaction centers (Muller, M.G., Griebenow, K. and Holzwarth, A.R. (19 91) Biochim. Biophys. Acta 1098, 1-12). Based on these data, a unified and self-consistent scheme for the primary processes in the whole pho tosynthetic system of C aurantiacus is presented. The BChl antenna pig ment groups are arranged to form a linear energy transfer cascade with four energy transfer steps from shorter-wavelength- to longer-wavelen gth-absorbing antenna pools. The overall fluorescence decay kinetics o f the photosynthetic system of C aurantiacus turns out to be 'trap-lim ited' by the reaction center rather than 'diffusion-limited' by the en ergy transfer processes.