Excitation transfer and charge separation kinetics in purple bacteria. (1) Picosecond fluorescence of chromatophores from Rhodobacter capsulatus wild type

Abstract

The fluorescence kinetics of chromatophores from Rhodobacter capsulatus wild type have been measured with picosecond time resolution over the wavelength range from 850 to 940 nm. The data have been analyzed both by global lifetime analysis yielding lifetimes and decay-associated spectra (DAS) and by global target analysis techniques yielding rate constants and species-associated spectra (SAS). In the global lifetime analysis five lifetime components were necessary for a good fit. The lifetimes are: τ 1 = 9 ps showing a DAS with positive and negative amplitudes; τ 2 = 40 ps with a DAS maximum at ≈ 890 nm; τ 3 = 95 ps with a DAS maximum at ≈ 895 nm; τ 4 = 260 ps with a DAS similar to that of lifetime component τ 3; a fifth lifetime component of 940 ps has nearly negligible amplitude. In the global target analysis several kinetic models were tested. A homogeneous model with sequential energy transfer LHC II ↔ LHC I harr; RC does formally fit the data only if the detailed charge separation and charge recombination processes at the reaction center (RC) are taken into account explicitly. However, despite the good fit, such a model must be excluded as a valid description both on the basis of simple thermodynamic considerations as well as due to the fact that it predicts unreasonable RC rate constants. Instead a heterogeneous model, assuming a mixture of chromatophores with open and closed RCs describes the situation both formally and physically quite well. For such a model the rate constants of the RC electron transfer processes and their free energy values as well as the energy transfer rate constants and SAS of the antenna pools were obtained. The extrapolated RC kinetics for open RCs agrees well with that known from isolated purple bacterial RCs. The rate constants for energy transfer processes among antenna pools and from antenna to RCs indicate that at room temperature the exciton kinetics of the entire antenna system is limited and determined by the RC charge separation, i.e., the exciton decay is trap limited. Our data are compared with the corresponding data for Rb. sphaeroides.