Kinetics of absorbance and anisotropy upon excited state relaxation in thereaction center core complex of a green sulfur bacterium

Properties of the excited states in reaction center core (RCC) complexes of the green sulfur bacterium Prosthecochloris aestuarii were studied by mean s of femtosecond time-resolved isotropic and anisotropic absorption differe nce spectroscopy at 275 K. Selective excitation of the different transition s of the complex resulted in the rapid establishment of a thermal equilibri um. At about 1 ps after excitation, the energy was located at the lowest en ergy transition, BChl a 835. Time constants varying between 0.26 and 0.46 p s were observed for the energy transfer steps leading to this equilibrium. These transfer steps were also reflected in changes in polarization. Our me asurements indicate that downhill energy transfer towards excited BChl a 83 5 occurs via the energetically higher spectral forms BChl a 809 and BChl a 820. Low values of the anisotropy of about 0.07 were found in the `two-colo r' measurements at 820 and 835 nm upon excitation at 800 nm, whereas the 'o ne-color' kinetics showed much higher anisotropies. Charge separation occur red with a time constant varying between 20 and 30 ps.