Synthesis and Avidin Binding of Ruthenium Complexes Functionalized with a Light-Cleavable Free Biotin Moiety.

In this work the synthesis, photochemistry, and streptavidin interaction of new Ru(tpy)(bpy)(SRR') complexes where the R' group contains a free biotin ligand, are described. Two different ligands SRR' were investigated: An asymmetric ligand where the Ru-bound thioether is a -acetylmethionine moiety linked to the free biotin fragment via a triethylene glycol spacer and a symmetrical ligand containing two identical biotin moieties. The coordination of these two ligands to the precursor [Ru(tpy)(bpy)Cl]Cl was studied in water at 80 °C. In such conditions the coordination of the asymmetric ligand occurred under thermodynamic control. After the reaction, a mononuclear and a binuclear complex were isolated. In the mononuclear complex, the ratio of methionine- {} vs. biotin-bound {} regioisomer was 5.3 and the free biotin fragment of allowed to purify it from its isomer at small scales using avidin affinity chromatography. Coordination of the symmetrical ligand afforded Ru(tpy)(bpy)() {} in synthetically useful scales (100 mg), good yield (82 %), and without traces of the binuclear impurity. In this complex, one of the biotin remains free whereas the second one is coordinated to ruthenium. Photochemical release of ligand from occurred upon blue light irradiation (465 nm) with a photosubstitution quantum yield of 0.011 that was independent of the binding of streptavidin to the free biotin ligand.