Femtosecond Pump-Probe Analysis of Energy and Electron Transfer in Photosynthetic Membranes of Rhodobacter capsulatus

Low-intensity, 295 K, femtosecond pump-probe transient absorption measurements are described that have been performed to investigate energy and electron transfer in photosynthetic membranes from a Rhodobacter capsulatus strain lacking functional light harvesting antenna complex II. Spectral and kinetic similarities between the absorption changes of isolated reaction centers and those of reaction centers in membranes upon 800-nm excitation suggest that the charge separation process in both cases is very similar. An ultrafast energy relaxation process observed near 872 nm when 800-nm excitation is used is interpreted as interexcitonic relaxation within the antenna, though other interpretations, such as vibrational relaxation, are possible. On the basis of global exponential fitting analysis of the time-dependent spectral changes using 800- and 880-nm excitation wavelengths to selectively excite the reaction center and the LHI antenna, respectively, it is found that excitation energy transfer and trapping in Rb. capsulatus is limited by the overall rate of energy transfer between the antenna and the reaction center. This conclusion is supported by the observation that excitation at 800 nm, but not 880 nm, results in absorbance changes indicative of charge separation with a lifetime (3.1 ps) very close to that reported for charge separation in isolated reaction centers (3.5 ps). Thus, most reaction centers that are directly excited undergo charge separation and not backward energy transfer to the LHI antenna complexes. Both a kinetic model analysis and a direct comparison between time-resolved spectra obtained using different excitation wavelengths resulted in an energy-detrapping efficiency of about 15 ± 10%.