COOPERATION OF CHARGES IN PHOTOSYNTHETIC O2 EVOLUTION-II. DAMPING OF FLASH YIELD OSCILLATION,DEACTIVATION*

Abstract— A quantitative analysis is made of a linear 4-step model for photosynthetic O₂ evolution in which each photochemical trapping center or an associated enzyme cycles through 5 oxidation states (S₀, S₁, S₂, S₃, S₄). The overall reaction is: S₀→ S₄, S₄→ S₀+ O₂, where k_d= rate of dark reaction. Based on data obtained with isolated chloroplasts, four aspects were considered: (1) The two perturbations which damp the oscillation of the O₂ flash yield in a flash sequence given after a dark period-(a) a failure rate (α) of the trapping centers in the photochemical conversion (‘misses’) and (b) double effective excitations in a fraction (8) of the centers which are in the S₀ and S₁ states (‘double hits’). Best fit with the experimental data is obtained for α= 0.1, β=0.05. (2) The kinetics and the mechanism of deactivation—the loss of + charges (O₂ precursors) in the dark. The momentary distribution of the four oxidation states (S₀-_-S₃) in a sample can be computed from the O₂ yields of four consecutive flashes with corrections for a and β. The time course of the various states in the dark following specific preilluminations reveals that deactivation proceeds in single equivalent steps: S₃→S₂, S₂,→S₁. S₁, itself stable in the dark, is the end product of deactivation. The ground state S₀ cannot be formed by deactivation in the dark but is only formed during illumination. (3) The various ratios [S₀]/[S₁] which can be observed in a sample after deactivation following different preilluminations with flashes or continuous light. (4) The transients of the O₂ evolution rate in weak continuous light as observed after deactivation with or without flash preillumination. In all instances satisfactory agreement is found between the observations and the predictions of the model.     

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 S ⁴⁺ state produces O₂ 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, S²⁺ and S³⁺ 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).