How chloride functions to enable proton conduction in photosynthetic water oxidation: Time-resolved kinetics of intermediates (S-states) in vivo and bromide substitution.

Chloride (Cl) is essential for O evolution during photosynthetic water oxidation. Two chlorides near the water-oxidizing complex (WOC) in Photosystem II (PSII) structures from Thermosynechococcus elongatus (and T. vulcanus) have been postulated to transfer protons generated from water oxidation. We monitored four criteria: primary charge separation flash yield (P* → PQ), rates of water oxidation steps (S-states), rate of proton evolution, and flash O yield oscillations by measuring chlorophyll variable fluorescence (P* quenching), pH-sensitive dye changes, and oximetry. Br-substitution slows and destabilizes cellular growth, resulting from lower light-saturated O evolution rate (-20 %) and proton release (-36 % ΔpH gradient). The latter implies less ATP production. In Br- cultures, protonogenic S-state transitions (S → S → S') slow with increasing light intensity and during O/water exchange (S' → S → S), while the non-protonogenic S → S transition is kinetically unaffected. As flash rate increases in Cl cultures, both rate and extent of acidification of the lumen increase, while charge recombination is suppressed relative to Br. The Cl advantage in rapid proton escape from the WOC to lumen is attributed to correlated ion-pair movement of HOCl in dry water channels vs. separated Br and H ion movement through different regions (>200-fold difference in Bronsted acidities). By contrast, at low flash rates a previously unreported reversal occurs that favors Br cultures for both proton evolution and less PSII charge recombination. In Br cultures, slower proton transfer rate is attributed to stronger ion-pairing of Br with AA residues lining the water channels. Both anions charge-neutralize protons and shepherd them to the lumen using dry aqueous channels.