Ligand substituents of ruthenium dipyridophenazine complexes sensitively determine orientation in liposome membrane

A series of ruthenium(II) complexes comprising differently substituted dipyridophenazine (dppz) ligands have been examined with respect to their orientation when bound to phosphatidylcholine liposome bilayers. The liposomes were subjected to shear flow, thereby deformed into ellipsoid shapes, and the orientation of chromophores in the bilayers could then be examined with polarized light (linear dichroism, LD). After correction for turbidity dichroism, LD distinguishes between chromophore transition moments oriented along the lipid chains (negative LD), and parallel to the membrane surface (positive LD). The polarizations, energies, and absorption overlap of the electronic transitions of the ruthenium complexes, known from previous work, could be used to characterize the orientation of liposome bound complexes. From luminescence lifetime studies further information about the localization of the chromophores was obtained. Assuming that the positively charged metal moiety of the complexes remains close to the bilayer surface, monomers with unsubstituted dppz as well as alkyl substituted dppz ligands were found to dip the dppz ligand down into the bilayer, whereas nitrile and amide substituents contributed 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.