FEMTOSECOND PUMP-PROBE ANALYSIS OF ENERGY AND ELECTRON-TRANSFER IN PHOTOSYNTHETIC MEMBRANES OF RHODOBACTER-CAPSULATUS

Low-intensity, 295 K, femtosecond pump-probe transient absorption meas urements are described that have been performed to investigate energy and electron transfer in photosynthetic membranes from a Rhodobacter c apsulatus strain lacking functional light harvesting antenna complex I I. 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 b oth cases is very similar. An ultrafast energy relaxation process obse rved near 872 nm when 800-nm excitation is used is interpreted as inte rexcitonic 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 u sing 800- and 880-nm excitation wavelengths to selectively excite the reaction center and the LHI antenna, respectively, it is found that ex citation energy transfer and trapping in Rb. capsulatus is limited by the overall rate of energy transfer between the antenna and the reacti on center. This conclusion is supported by the observation that excita tion at 800 nm, but not 880 nm, results in absorbance changes indicati ve of charge separation with a lifetime (3.1 ps) very close to that re ported for charge separation in isolated reaction centers (3.5 ps). Th us, most reaction centers that are directly excited undergo charge sep aration and not backward energy transfer to the LHI antenna complexes. Both a kinetic model analysis and a direct comparison between time-re solved spectra obtained using different excitation wavelengths resulte d in an energy-detrapping efficiency of about 15 +/- 10%.