FEMTOSECOND SPECTROSCOPY OF CHLOROSOME ANTENNAS FROM THE GREEN PHOTOSYNTHETIC BACTERIUM CHLOROFLEXUS-AURANTIACUS

The antenna kinetics of bacteriochlorophyll (BChl) c- and a-containing chlorosomes from the thermophilic filamentous green photosynthetic ba cterium Chloroflexus aurantiacus were investigated using two independe nt pump-probe techniques with subpicosecond resolution. Isotropic one- and two-color absorption difference experiments using probe wavelengt hs between 710 and 770 nm reveal BChl c photobleaching (PB) and stimul ated emission (SE) decay kinetics with major lifetime components of 50 -100 fs, 1-2 ps, and 7-10 ps. Two-color PB/SE profiles pumped at 770 n m and probed at 800 nm (where BChl a pigments absorb) exhibit no detec table rise time. However, two-color experiments using 790 and 820 nm p ump and probe wavelengths, respectively, yield PB/SE rise components o f 100 fs, 2 ps, and 10 ps. Upon excitation at 720 nm, the BChl c PB/SE spectrum observed using a broad-band probe continuum displays surpris ingly little spectral evolution during the first 2 ps. Upon 760 nm exc itation, the BChl c PB/SE spectrum experiences a small blue shift (fro m similar to 750 to similar to 744 nm) during the first picosecond. Th e one-color anisotropies r(t) in the BChl c spectrum initialize very c lose to 0.4 and subsequently exhibit little decay (r(infinity) similar to 0.36). Single-exponential analyses of one- and two-color anisotrop ies probed at 800 nm yield a component with a lifetime similar to 10 p s; the final anisotropies are generally small, indicating that the ene rgy transfers in this region are accompanied by a large reorientation of Q, transition moments. The absorption difference profiles in the BC hl c (but not in the BCN a) region of the chlorosome spectrum contain oscillating components at early times, which are damped within similar to 1 ps. They likely arise from vibrational coherences in the BChl c aggregate. Our results do not appear to be consistent with subpicoseco nd spectral equilibration among inequivalent BCN c pigments within the main BChl c absorption band. The essentially prompt PB/SE absorbance changes at wavelengths up to 800 nm suggest that some pigments absorbi ng near the latter wavelength are kinetically strongly coupled to the main BChl c pigment aggregates. This is accompanied by apparent energy transfer and depolarization (with kinetics that exhibit a 10 ps compo nent) to longer-wavelength BChl a pigments that exhibit PB/SE in the 8 20 nm region. The latter BChl a pigments have orientations that differ considerably from those of the 800 nm pigments. Whether the 800 nm ab sorbing pigments are BChl a or long wavelength forms of BChl c is not yet clear.