Ruthenium(II)−Phenanthroline−Biotin Complexes:  Synthesis and Luminescence Enhancement upon Binding to Avidin

We report the synthesis, characterization, and avidin-binding properties of two novel ruthenium complexes, [Ru(bpy)₂(phen-biotin)][PF₆]₂ 1 and [Ru(phen)₂(phen-biotin)][PF₆]₂ 2 (bpy = 2,2‘-bipyridine; phen = 1,10-phenanthroline, phen-biotin = 5-(10-amidobiotinyl)-1,10-phenanthroline)). We demonstrate that both biotinylated compounds bind to avidin through their biotin moieties with high affinity and in a 4:1 ratio. The binding of compounds 1 and 2 to avidin results in an enhancement in luminescence intensity (∼1.4×, ∼1.6×, respectively), relative to the unbound biotinylated ruthenium complexes. This behavior is markedly different from biotinylated organic dyes, whose fluorescence is quenched upon binding to avidin. Thus, ruthenium−biotin complexes 1 and 2 can form the basis of new, simplified biotin−avidin assays, which involve luminescence detection of the relevant biotinylated molecule through cross-linking with avidin.