COOPERATION OF CHARGES IN PHOTOSYNTHETIC O2 EVOLUTION–I. A LINEAR FOUR STEP MECHANISM

Abstract

— Using isolated chloroplasts and techniques as described by Joliot and Joliot[6] we studied the evolution of O₂ in weak light and light flashes to analyze the interactions between light induced O₂ 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 O₂. 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 O₂ 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 S⁴⁺ state produces O₂ and returns to the ground state S₀. 5. Besides S₀ also the first oxidized state S⁺ is stable in the dark, the two higher states, S²⁺ and S³⁺ are not. 6. The relaxation times of some of the photooxidation steps were estimated. The fastest reaction, presumably S*₁←S₂, 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).