Incorporation of a biotinylated ruthenium tris(bipyridine) [Ru(bPY)(2)(Biot-bpy)](2+) (1) in either avidin or streptavidin-(strept)avidin-can be conveniently followed by circular dichroism spectroscopy. To determine the stepwise association constants, cooperativity, and chiral discrimination properties, diastereopure (Lambda and Delta)-1 species were synthesized and incorporated in tetrameric (strept)avidin to afford (Delta-[Ru(bPY)(2)(Biot-bpy)](2+))(x)subset of avidin, (Lambda-[Ru(bPY)(2)(Biot-bpy)](2+))(x)subset of cavidin, (Delta-[Ru(bpy)(2)(Biot-bpy)](2+))(x)subset of streptavidin, and (Lambda-[Ru(bpy)(2)(Biot-bpy)](2+))(x)subset of streptavidin (x = 1-4) For these four systems, the overall stability constants are log beta(4) = 28.6, 30.3, 36.2, and 36.4, respectively. Critical analysis of the CD titrations data suggests a strong cooperativity between the first and the second binding event (x = 1, 2) and a pronounced difference in affinity between avidin and streptavidin for the dicationic guest 1 as well as modest enantiodiscrimination properties with avidin as host.