Fast Energy Transfer and Exciton Dynamics in Chlorosomes of the Green Sulfur Bacterium Chlorobium tepidum

Abstract

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.