Binding of Luminescent Ruthenium(II) Molecular Probes to Vesicles

A systematic study of the binding of luminescent Ru(II) complexes with α-diimine ligands (2,2‘-bipyridine, 1,10-phenanthroline, and substituted analogues) to neutral (1-palmitoyl-2-oleoyl- sn -glycero-3-phosphocholine) and anionic (phospholipid was l -α-phosphatidylserine (Brain; sodium salt)) unilamellar vesicles is presented. Excited state lifetime measurements coupled with differential oxygen quenching of the bound and unbound complex provide a quantitative measure of binding. The roles of charge and complex hydrophobicity are examined in regard to binding strength. Binding is strongly dependent on electrostatic attraction, but hydrophobic interactions are equally important and can enhance binding by orders of magnitude. These results provide criteria and guidelines for the design of luminescent inorganic molecular probes for use in biological and, especially, membrane systems.