Energetic Coupling in the Primary Processes of Photosynthesis in Chromatium

pH Dependence of Delayed Fluorescence, Electron Transfer and Degree of Coupling

Abstract

The effects of pH on the thermodynamic properties of the proton-translocating cyclic electron transfer system in a purple photosynthetic bacterium Chromatium vinosum were studied. Two thermodynamic parameters, the flux (J_e) and force (Δ_??__e) of the electron transfer process, were analyzed. The rate of electron transfer in the re-reduction of photooxidized reactioncenter bacteriochlorophyll was used as J_e. Δ_??__e was determined from the intensity of the delayed fluorescence from bacteriochlorophyll. Δ_??__e is composed of the redox potential difference and the electrical potential difference between two electron transfer components. In the steady state under illumination, the flux-to-force ratio is determined by the following relationship:
J_e=(1-q²)L_ee Δ_??__e
where q is the “degree of coupling” of electron transfer to proton translocation and L_ee is the value of J_e/Δ_??__e when there is no back pressure by formation of Δ_??__H⁺ (electrochemical potential difference of H⁺). The value of (1-q²)L_ee increased with increasing pH in the neutral pH range. Uncouplers and ionophores that dissipate Δ_??__H⁺ increased J_e and decreased Δ_??__e. The effects were more prominent in the lower pH range. Therefore, q must be smaller at higher pH. The coupling is probably tight when redox components are saturated with protons. The experimental results agreed with the theoretical predictions for a system where a hydrogen-translocating component functions as an electron-proton symport carrier.