Effects of Cl−and other anions on the rate of HILL reaction inEuglenachloroplasts were investigated. Cl−accelerated the reaction rate with ferricyanide as HILL oxidant; Br−, F−and I−were also effective; NO3−, PO42−and SO42−were less effective. Divalent cations, Ca2+and Mg2+, were also highly effective. The promoting effects of these ions were highly dependent on pH and the nature and concentration of the HILL oxidant used. Accelerating effects of the ion increased with decreasing concentrations of ferricyanide. Generally, the stimulating effect of Cl−was much more marked at pH 7–7.5, with little effect at pH 5. Thus, the pH-activity relationship in the HILL reaction is more or less markedly modified by addition of ions.Cl−, and other anions, accelerated the reaction by affecting only the dark rate-limiting portion of the HILL reaction; the light reaction constant remained uninfluenced. We inferred that some reaction step, at which ferricyanide receives electron from photosystem 2, is accelerated by Cl−and other ions. Cl−effects were rather small, or undetectable, with DPIP orp
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