ENERGY-TRANSFER IN THE INHOMOGENEOUSLY BROADENED CORE ANTENNA OF PURPLE BACTERIA - A SIMULTANEOUS FIT OF LOW-INTENSITY PICOSECOND ABSORPTION AND FLUORESCENCE KINETICS

The excited state decay kinetics of chromatophores of the purple photo synthetic bacterium Rhodospirillum rubrum have been recorded at 77 K u sing picosecond absorption difference spectroscopy under strict annihi lation free conditions. The kinetics are shown to be strongly detectio n wavelength dependent. A simultaneous kinetic modeling of these exper iments together with earlier fluorescence kinetics by numerical integr ation of the appropriate master equation is performed. This model, whi ch accounts for the spectral inhomogeneity of the core light-harvestin g antenna of photosynthetic purple bacteria, reveals three qualitative ly distinct stages of excitation transfer with different time scales. At first a fast transfer to a local energy minimum takes place (approx imate to 1 ps). This is followed by a much slower transfer between dif ferent energy minima (10-30 ps). The third component corresponds to th e excitation transfer to the reaction center, which depends on its sta te (60 and 200 ps for open and closed, respectively) and seems also to be the bottleneck in the overall trapping time. An acceptable corresp ondence between theoretical and experimental decay kinetics is achieve d at 77 K and at room temperature by assuming that the width of the in homogeneous broadening is 10-15 nm and the mean residence time of the excitation in the antenna lattice site is 2-3 ps.