The excited-state structure and energy-transfer dynamics,
including their dependence on temperature and
redox conditions, were studied in chlorosomes of the green sulfur
bacterium Chlorobium tepidum at low
temperatures by two independent methods: spectral hole burning in
absorption and fluorescence spectra and
isotropic one-color pump−probe spectroscopy with ∼100 fs
resolution. Hole-burning experiments show that
the lowest excited state (LES) of BChl c aggregates is
distributed within approximately 760−800 nm, while
higher excitonic states of BChl c (with absorption maximum
at 750 nm) possess the main oscillator strength.
The excited-state lifetime determined from hole-burning
experiments at anaerobic conditions was 5.75 ps
and most likely reflects energy transfer between BChl c
clusters. Isotropic one-color absorption difference
signals were measured from 720 to 790 nm at temperatures ranging from 5
to 65 K, revealing BChl c
photobleaching and stimulated emission kinetics with four major
components, with lifetimes of 200−300 fs,
1.7−1.8 ps, 5.4−5.9 ps, and 30−40 ps at anaerobic conditions.
The lifetimes are attributed to different
relaxation processes of BChl c, taking into account their
different spectral distributions as well as limitations
arising from results of hole burning. Evidence for at least two
spectral forms of BChl c in chlorosome is
reported. There is a striking similarity between the spectrum and
kinetics of the 5.4−5.9 ps component with
those of the LES determined from hole burning. A pronounced change
of isotropic decays was observed at
around 50 K. The temperature dependence of the isotropic decays is
correlated with temperature-dependent
changes of BChl c fluorescence emission. Further, the
temperature decrease leads to an increase in the relative
amplitude of the 200−300 fs component. At aerobic conditions,
both hole burning and pump−probe
spectroscopy show that the lifetime of the LES shortens to ∼2.6 ps,
as a result of excitation quenching by a
mechanism presumably protecting the cells against superoxide-induced
damage. This mechanism operates
on at least two levels, the second one being characterized by a 14−16
ps lifetime.
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