FAST ENERGY-TRANSFER AND EXCITON DYNAMICS IN CHLOROSOMES OF THE GREENSULFUR BACTERIUM CHLOROBIUM-TEPIDUM

The excited-state structure and energy-transfer dynamics, including th eir dependence on temperature and redox conditions, were studied in ch lorosomes of the green sulfur bacterium Chlorobium tepidum at low temp eratures by two independent methods: spectral hole burning in absorpti on and fluorescence spectra and isotropic one-color pump-probe spectro scopy with similar to 100 fs resolution. Hole-burning experiments show that the lowest excited state (LES) of BChl c aggregates is distribut ed within approximately 760-800 nm, while higher excitonic states of B Chl c (with absorption maximum at 750 nm) possess the main oscillator strength. The excited-state lifetime determined from hole-burning expe riments at anaerobic conditions was 5.75 ps and most likely reflects e nergy transfer between BChl c clusters. Isotropic one-color absorption difference signals were measured from 720 to 790 nm at temperatures r anging from 5 to 65 K, revealing BChl c photobleaching and stimulated emission kinetics with four major components, with lifetimes of 200-30 0 Es, 1.7-1.8 ps, 5.4-5.9 ps, and 30-40 ps at anaerobic conditions. Th e lifetimes are attributed to different relaxation processes of BChl c , taking into account their different spectral distributions as well a s limitations arising from results of hole burning. Evidence for at le ast 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 pro nounced change of isotropic decays was observed at around 50 K. The te mperature dependence of the isotropic decays is correlated with temper ature-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 an d pump-probe spectroscopy show that the lifetime of the LES shortens t o similar to 2.6 ps, as a result of excitation quenching by a mechanis m presumably protecting the cells against superoxide-induced damage. T his mechanism operates on at least two levels, the second one being ch aracterized by a 14-16 ps lifetime.