Secondary Electron Transfer Processes in Membranes of Heliobacillus mobilis

Picosecond transient absorption difference spectroscopic experiments were performed on membranes of the antenna/reaction center complex of Heliobacillus mobilis to study the electron transfer processes. Particular emphasis was placed on the blue spectral region, where the difference spectra of iron-sulfur centers and quiñones are significantly different. Spectra were measured at room temperature in the wavelength region from 400 to 470 nm and from 630 to 730 nm. Laser excitation was into the 788 nm Q_y band of the bacteriochlorophyll g of the reaction center complex. Global analysis in both wavelength regions reveals three kinetic components. A 25 ps phase originates from the decay of the excited state of antenna to form the primary charge-separated state P798⁺Ao^-; a 600 ps component is assigned to the electron transfer from the primary electron acceptor A₀ to a secondary electron acceptor; a nondecaying component on the time scale measured represents the formation of the secondary charge-separated state. When the secondary electron acceptors were reduced by adding dithionite at pH 11, the 600 ps component disappeared. Only a 25 ps component and a constant were observed in the 630-730 nm region. The 25 ps component is assigned to the excitation decay in the antenna and the formation of P798+A₀^-, just as in the nonreduced sample. In the reduced sample, the P798+A₀^- state does not decay on the time scale measured. In the 400-470 nm region, the same kinetic behavior was observed. The absorption difference spectra of the primary and the secondary electron acceptor were constructed from different charge-separated states. The A₀^- - A₀^- spectrum resembles the spectrum of the same state from photosystem I, which also contains a Chi a molecule as the primary electron acceptor. The secondary electron acceptor X has an X^- - X difference spectrum similar to Fx in photosystem I from higher plants. The spectra do not give any evidence in favor of a quinone acceptor in heliobacteria, although they do not rule out the possibility that such an acceptor is present.