9-Aminoacridine Binding to Chloroplast Membranes in Dark. Reversal by Mg2+

9-Aminoacridine (9AA) binds to photosynthetic membranes of unillum inated chloroplasts in low-salt media. The binding was insensitive to the uncouplers of photophosphorylation. The ap­ parent binding constant was 140 j u m . The binding isotherm as a function of 9AA concentration was sigmoid, and approximately 3 mol 9AA/mol chlorophyll was bound at saturating concentrations of 9AA. Addition of Mg2+ partially reversed the binding of 9AA in chloroplasts in the dark as observed by a Mg2+-induced increase of 9AA fluorescence as well as by spectrophotometric measurements of free 9AA. It appeared, however, that use of fluorescence techniques for measuring free 9AA intro­ duced an error in the estimation of the magnitude of binding, particularly at low concentration of 9AA ( < 7 5 jMM). This is probably due to change in fluorescence yield of membrane-bound 9AA on addition of cations. The nature of the binding of 9AA to the thylakoid membranes and the effects of Mg2+ thereupon suggest that both chemical binding of cations and screening of surface charge of the membranes should be considered in discussing the mechanism of cation action on chloroplast structure and function. Interpretation of these data with respect to heterogeneity of sites of cation action upon or within chloroplast membranes is discussed.