Luminescent ruthenium(II) polypyridine biotin complexes: Synthesis, characterization, photophysical and electrochemical properties, and avidin-binding studies

Two luminescent ruthenium(II) polypyridine complexes containing a biotin moiety [Ru(bpy)₂(L1)](PF₆)₂ (1) and [Ru-(bpy)₂(L2)](PF₆)₂ (2) (bpy = 2,2′-bipyridine; L1 = 4-(N-((2-biotinamido)ethyl)amido)-4′-methyl-2, 2′-bipyridine; L2 = 4-(N-((6-biotinamido)hexyl)amido)-4′-methyl-2, 2′-bipyridine) have been synthesized and characterized, and their photophysical and electrochemical properties have been studied. Upon photoexcitation, complexes 1 and 2 display intense and long-lived triplet metal-to-ligand charge-transfer (³MLCT) (dπ(Ru) → π*(L1 or L2)) emission in fluid solutions at 298 K and in low-temperature glass. We have studied the binding of these ruthenium(II) biotin complexes to avidin by 4′-hydroxyazobenzene-2-carboxylic acid (HABA) assays, luminescence titrations, competitive assays using native biotin, and quenching experiments using methyl viologen. On the basis of the results of these experiments, a homogeneous competitive assay for biotin has been investigated.