ENERGY TRAPPING IN THE PURPLE SULFUR BACTERIA CHROMATIUM-VINOSUM AND CHROMATIUM-TEPIDUM

Energy trapping in chromatophores of the purple sulfur bacteria Chroma tium vinosum and Chromatium tepidum has been examined by means of pico second transient absorption spectroscopy. In C. vinosum, time constant s for excitation quenching in the core antenna by open and closed reac tion centres were 50 and 230 ps, respectively. In C. tepidum, excitati on quenching by closed reaction centres occurred with a single time co nstant of 310 ps With open reaction centres, the kinetics of decay of excitations in the core antenna were more complicated. The decay was m ulti-exponential, with a major contribution from a 140 ps component an d minor contributions from 40 ps and 530 ps components. By comparison with the kinetics of oxidation of the primary donor, the 140 ps compon ent was identified as being due to excitation trapping by open reactio n centres. This trapping time is slow as compared with other purple ba cteria, and is most likely a consequence of the red-shift of the core antenna with respect to the reaction centre. The origin of the 40 ps c omponent is not entirely dear, but it probably represents energy trans fer to the reaction centre as well. The 530 ps component may reflect a fraction of bacteriochlorophyll a that transfers its energy to the re action centre very inefficiently, or not at all. In C. vinosum as well as in C. tepidum, no kinetic components were found that could be asso ciated with internal core antenna relaxation, which implies that equil ibration within the core antenna occurred within our time resolution o f 10-15 ps.