Energy trapping in the purple sulfur bacteria Chromatium vinosum and Chromatium tepidum

Abstract

Energy trapping in chromatophores of the purple sulfur bacteria Chromatium vinosum and Chromatium tepidum has been examined by means of picosecond transient absorption spectroscopy. In C. vinosum, time constants for excitation quenching in the core antenna by open and closed reaction centres were 50 and 230 ps, respectively. In C. tepidum, excitation quenching by closed reaction centres occurred with a single time constant of 310 ps. With open reaction centres, the kinetics of decay of excitations in the core antenna were more complicated. The decay was multi-exponential, with a major contribution from a 140 ps component and minor contributions from 40 ps and 530 ps components. By comparison with the kinetics of oxidation of the primary donor, the 140 ps component was identified as being due to excitation trapping by open reaction centres. This trapping time is slow as compared with other purple bacteria, 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 component is not entirely clear, but it probably represents energy transfer to the reaction centre as well. The 530 ps component may reflect a fraction of bacteriochlorophyll a that transfers its energy to the reaction centre very inefficiently, or not at all. In C. vinosum as well as in C. tepidum, no kinetic components were found that could be associated with internal core antenna relaxation, which implies that equilibration within the core antenna occurred within our time resolution of 10–15 ps.