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 bacterium Chloroflexus aurantiacus were investigated using two independent pump-probe techniques with subpicosecond resolution. Isotropic one- and two-color absorption difference experiments using probe wavelengths between 710 and 770 nm reveal BChl c photobleaching (PB) and stimulated 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 nm and probed at 800 nm (where BChl a pigments absorb) exhibit no detectable rise time. However, two-color experiments using 790 and 820 nm pump and probe wavelengths, respectively, yield PB/SE rise components of 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 surprisingly little spectral evolution during the first 2 ps. Upon 760 nm excitation, the BChl c PB/SE spectrum experiences a small blue shift (from ∼750 to ∼744 nm) during the first picosecond. The one-color anisotropies r(t) in the BChl c spectrum initialize very close to 0.4 and subsequently exhibit little decay (r(∞) ∼ 0.36). Single-exponential analyses of one- and two-color anisotropies probed at 800 nm yield a component with a lifetime ∼10 ps; the final anisotropies are generally small, indicating that the energy transfers in this region are accompanied by a large reorientation of Q_y transition moments. The absorption difference profiles in the BChl c (but not in the BChl a) region of the chlorosome spectrum contain oscillating components at early times, which are damped within ∼1 ps. They likely arise from vibrational coherences in the BChl c aggregate. Our results do not appear to be consistent with subpicosecond spectral equilibration among inequivalent BChl 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 absorbing 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 component) to longer-wavelength BChl a pigments that exhibit PB/SE in the 820 nm region. The latter BChl a pigments have orientations that differ considerably from those of the 800 nm pigments. Whether the 800 nm absorbing pigments are BChl a or long wavelength forms of BChl c is not yet clear.