Abstract
The overall kinetics of photosystem II was scanned by means of a double flash technique. Oxygen evolution by pea thylakoids provided with an artificial electron acceptor was measured under a regime of double flashes of variant intervals. The pH optimum in H 2O lies near p 1H 7.2 (p 1H meaning the pH in H 2O), with a first-order rate constant of 800 s −1 at 20°C. In deuterium oxide (D 2O), a plateau of maximum reaction rate was found between p 2H 6.6 and 7.8 (p 2H meaning the “pH” in D 2O), the highest rate constant being 550 s −1. The apparent kinetic deuterium isotope effect is therefore 1.45. Outside this plateau region, there seems to exist another isotope effect of 1.2 to 1.3. These effects are small but may nevertheless reflect the fact that more than one step of the photosystem II reaction sequence is involved in the splitting of a bond to hydrogen. However, the effects may also be solvent effects or located on the acceptor side of the photosystem.
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