Electron Transfer in the Photosynthetic Membrane Influence of PH and Surface Potential on the P-680 Reduction Kinetics

The primary electron donor P-680 of the Photosystem-II reaction center was photoxidized by a short flash given after dark adaptation of photosynthetic membranes in which oxygen evolution was inhibited. The P-680+ reduction rate was measured under different conditions of pH and salt concentration by following the recovery of the absorption change at 820 nm. As previously reported for Tris-washed chloroplasts (Conjeaud, H., and P. Mathis, 1980, Biochim. Biophys. Acta, 590:353–359) a fast phase of P-680+ reduction slows down as the bulk pH decreases. When salt concentration increases, this fast phase becomes faster for pH above 4.5–5 and slower below. A quantitative interpretation is proposed in which the P-680+ reduction kinetics by the secondary electron donor Z are controlled by the local pH. This pH, at the membrane level, can be calculated using the Gouy-Chapman theory. A good fit of the results requires to assume that the surface charge density of the inside of the membrane, near the Photosystem-II reaction center, is positive at low pH values and becomes negative as the pH increases, with a local isoelectric point ∼4.8. These results lead us to propose a functional scheme in which a pH-dependent proton release is coupled to the electron transfer between secondary and primary donors of Photosystem-II. The H+/e ratio varies from 1 at low pH to 0 at high pH, with a real pK ∼6.5 for the protonatable species