The interaction of atebrin with phospholipid vesicles.

The interaction of atebrin with phosphatidylcholine and phosphatidylcholine-phosphatidic acid vesicles has been followed by equilibrium dialysis, and by photometric, fluorimetric and NMR techniques. The presence of negative charges in the phospholipids enhances the binding of atebrin. The absorbance and NMR spectral changes and fluorescence quenching occurring with phosphatidic acid are attributed to dimerization of the dye interacting electrostatically with negative groups. The dissociation constant of the binding of the dye to phosphatidylcholine vesicles was 1.4 mM; those of binding to the negative sites of phosphatidic acid were approx. 150 and 3 muM. The dye is probably located at the interphase with the acridine ring interacting with the anionic groups of phosphatidic acid and the tail freely floating in the aqueous phase. The results are discussed also in view of the use of atebrin as a probe of the energized state in natural membranes and of the suggestion that atebrin may be used as a transmembrane pH indicator in liposomes or natural membranes.