Orientation of ruthenium dipyridophenazine complexes in liposome membranes sensitively controlled by ligand substituents

Abstract

Liposomes can be deformed to ellipsoidal shapes in a shear flow, and the orientation of chromophores in the lipid bilayers can then be examined with polarised light (linear dichroism, LD). The linear dichroism distinguishes between chromophore transition moments oriented along the lipid chains (negative LD), and parallel to the membrane surface (positive LD). A series of ruthenium(II) complexes comprising differently substituted dipyridophenazine (dppz) ligands has been examined with respect to their orientation when bound to phosphatidylcholine liposome bilayers. The polarisations, energies and absorption overlap of the electronic transitions of the ruthenium complexes, known from previous work, were used to characterise the orientation of liposome bound complexes. From luminescence lifetime studies further information about the localisation of the chromophores was obtained. Monomers with unsubstituted dppz as well as alkyl-substituted dppz ligands tend to dip the dppz ligand down into the bilayer, whereas nitrile and amide substituents contribute to the alignment of the dppz group parallel to the surface. The findings give insights into mechanisms that govern orientation of membrane solutes, thus providing useful leads, for example, for the composition of membrane molecular devices.