9-Aminoacridine fluorescence changes as a measure of surface charge density of the thylakoid membrane.

1. When suspended in a low cation-containing medium, chloroplast thylakoid membranes and carboxymethyl-cellulose particles quench the fluorescence from 9-aminoacridine (Searle, G.F.W. and Barber, J. (1978) Biochim. Biophys. Acta 502, 309--320). 2. Relief of this quenching is achieved by adding cations to the suspension medium with the order of effectiveness being C3+ greater than C2+ greater than C+, indicating that the fluorescence acts as an indicator of the surface electrical potential. 3. Using the Gouy-Chapman theory, the differential effect of divalent (methyl viologen) and monovalent (K+) cations has been used to calculate surface charge densities. 4. The calculations indicate that the surface charge density on the thylakoids significantly increases when cations are added to the low cation-containing medium. Under the same conditions the surface charge density of glutaraldehyde-fixed thylakoids and carboxymethyl-cellulose particles remained essentially constant. 5. It is argued that the 9-aminoacridine technique is able to probe localized areas on the membrane surface and that the variability of the surface charge density of untreated thylakoids may be due to redistribution of charges associated with membrane stacking as suggested by Barber and Chow (Barber, J. and Chow, W.S. (1979) FEBS Lett. 105, 5--10).