Mg2+-H + Exchange in Chloroplast Membranes in Dark

This work was initially undertaken to investigate the effects of Mg2+ on light-induced proton move­ ment and formation of proton gradient across the thylakoid membranes. In course of investigation it was observed that addition of Mg2+ in a chloroplast suspension kept in the dark caused extensive ex­ trusion of protons from the membranes into the medium. This report describes the properties of Mg2+ — H+ exchange with the conclusion that most of the protons released by Mg2+ were previously bound. The pH dependence of H+ extrusion indi­ cated that the groups involved in Mg2+ — H+ ex­ change were mostly protonated at pH 6 and mostly deprotonated at pH 8.