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

The kinetics of photoinduced absorbance change in the 400-ns to 100-ms time 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-μs component 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 ³Bchl a of the Fenna-Matthews-Olson (FMO) protein. The contribution of the 70-μs component correlates with the amount of FMO protein in the isolated RCC complex. The spectrum of the 1.6-μs component has a sharp bleaching at 835 nm, a maximum at 805 nm, a broad positive band at wavelengths 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-μs component is replaced by a component with a lifetime of ∼40 μs, consistent with the reconstruction of the F_x cluster. We propose that the 1.6-μs component results from charge recombination between P840⁺ and an intermediate electron acceptor operating between A_o and F_x. Our studies in Chlorobium RCCs show that approaches that employ a single wavelength 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⁺ from the decay of ³Bchl a in the FMO protein.