Proton release into the internal phase of thylakoids due to photosynthetic water oxidation. On the periodicity under flashing light.

Photosynthetic electron transport in green plants causes the inwardly directed transport of protons across the thylakoid membrane. The release of protons inside is well documented [l-6] . Amongst the various indirect techniques, for measuring pH changes in the submicroscopic internal phase, the one with the permeating pH, indicating dye neutral red, is distinguished by sensitivity and time resolution for the small pH changes induced under flash excitation of chloroplasts [5]. Based on spectrophotometric studies with this dye, we reported previously that proton release inside under flashing light at low frequency (less than 0.2 Hz) is kinetically biphasic, with the rapid phase (half-rise time about 300 ~.ts) attributable to the oxidation of water by photosystem II and the slower one (about 20 ms) due to the oxidation of plastohydroquinone by photosystem I [5]. It was furthermore shown, that this rapid phase can be isolated if the electron transport between the two photosystems is interrupted by the plastoquinone antagonist DBMIB [7]. Proton release due to water oxidation under these conditions is the subject of this communication. The oxidation of water to produce oxygen follows a characteristic kinetic pattern. When dark-adapted chloroplasts or algae are excited with a series of short flashes the oxygen yield per flash reveals damped oscillation with a period of four [8,9] . Kok et al. [9] interpreted this as follows: the water oxidizing enzyme system cycles stepwise through live oxidation states (So, f . . , 5’4) with Sr most stable in the dark-adapted