(Strept)avidin as Host for Biotinylated Coordination Complexes:  Stability, Chiral Discrimination, and Cooperativity

Incorporation of a biotinylated ruthenium tris(bipyridine) [Ru(bpy)2(Biot-bpy)]2+ (1) in either avidin or streptavidin(strept)avidincan be conveniently followed by circular dichroism spectroscopy. To determine the stepwise association constants, cooperativity, and chiral discrimination properties, diastereopure (Λ and Δ)-1 species were synthesized and incorporated in tetrameric (strept)avidin to afford (Δ-[Ru(bpy)2(Biot-bpy)]2+)x⊂avidin, (Λ-[Ru(bpy)2(Biot-bpy)]2+)x⊂avidin, (Δ-[Ru(bpy)2(Biot-bpy)]2+)x⊂streptavidin, and (Λ-[Ru(bpy)2(Biot-bpy)]2+)x⊂streptavidin (x = 1−4) For these four systems, the overall stability constants are log β4 = 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.