NMR study of chloride ion interactions with thylakoid membranes

The role of Cl^- in photosynthetic O₂ evolution has been investigated by observing the ³⁵Cl NMR linewidth under a variety of conditions in aqueous suspensions of chloroplasts, primarily for the halophytes Avicennia germinans, Avicennia marina, and Aster tripolium but also for spinach. The line broadening shows there is weak, ionic binding of Cl^- to thylakoids, the bound Cl^- exchanging rapidly (>>10⁴ sec^-1) with free Cl^- in solution. The binding is necessary for O₂ evolution to occur. Michaelis-Menten constants obtained from the Cl^- dependence of the O₂ evolution rate are ≈15-70 mM for the halophytes compared with 0.6 mM for spinach (0.5 mM with Br^-). There appear to be two types of Cl^- binding sites in halophytes, of which the stronger is the activator, at lower Cl^-], of O₂ evolution. The ³⁵Cl line broadening includes a nonspecific interaction, which becomes apparent at high Cl^- concentrations (≥0.5 M).