Cooperation of Charges in Photosynthetic O2 Evolution–i. A Linear Four Step Mechanism

Abstract— Using isolated chloroplasts and techniques as described by Joliot and Joliot6 we studied the evolution of O 2 in weak light and light flashes to analyze the interactions between light induced O 2 precursors and their decay in darkness. The following observations and conclusions are reported: 1. Light flashes always produce the same number of oxidizing equivalents either as precursor or as O 2 . 2. The number of unstable precursor equivalents present during steady state photosynthesis is ∼ 1.2 per photochemical trapping center. 3. The cooperation of the four photochemically formed oxidizing equivalents occurs essentially in the individual reaction centers and the final O 2 evolution step is a one quantum process. 4. The data are compatible with a linear four step mechanism in which a trapping center, or an associated catalyst, ( S ) successively accumulates four + charges. The S 4+ state produces O 2 and returns to the ground state S 0 . 5. Besides S 0 also the first oxidized state S + is stable in the dark, the two higher states, S 2+ and S 3+ are not. 6. The relaxation times of some of the photooxidation steps were estimated. The fastest reaction, presumably S * 1 ← S 2 , has a (first) half time ≤ 200 μsec. The S * 2 state and probably also the S * 0 state are processed somewhat more slowly (˜ 300–400 μsec).