Photoinduced transient absorbance spectra of P840/P840(+) and the FMO protein in reaction centers of Chlorobium vibrioforme

The kinetics of photoinduced absorbance changes in the 400-ns to 100-ms tim e range were studied between 770 and 1025 nm in reaction center core (RCC) complexes isolated from the green sulfur bacterium Chlorobium vibrioforme. A global, multiple stretched-exponential analysis shows the presence of two distinct but strongly overlapping spectra. The spectrum of the 70-mus comp onent consists of a broad bleaching with two minima at 810 and 825 nm and a broad positive band at wavelengths greater than 865 nm and is assigned to the decay of (3)Bchl a of the Fenna-Matthews-Olson (FMO) protein. The contr ibution of the 70-mus component correlates with the amount of FMO protein i n the isolated RCC complex. The spectrum of the 1.6-mus component has a sha rp bleaching at 835 nm, a maximum at 805 nm, a broad positive band at wavel engths higher than 865 nm, and a broad negative band at wavelengths higher than 960 nm. When the RCC is incubated with inorganic iron and sulfur, the 1.6-mus component is replaced by a component with a lifetime of similar to 40 mus, consistent with the reconstruction of the F, cluster. We propose th at the 1.6-mus component results from charge recombination between P840(+) and an intermediate electron acceptor operating between A(o) and F-x. Our s tudies in Chlorobium RCCs show that approaches that employ a single wavelen gth in the measurement of absorption changes have inherent limitations and that a global kinetic analysis at multiple wavelengths in the near-infrared is required to reliably separate absorption changes due to P840/P840(+) fr om the decay of (3)Bchl a in the FMO protein.