Development of next-generation photolabile copper cages with improved copper binding properties

Seven new nitrogen-donor ligands that contain a photoactive nitrophenyl group within the ligand backbone have been prepared and evaluated for their binding affinity for copper(II) and zinc(II). Among this series, the ligand 3Gcage (pyridine-2-carboxylic acid {1-(2-nitro-phenyl)-3-[(pyridin-2-ylmethyl)-amino]-propyl}-amide) has the best affinity for copper(II), with an apparent dissociation constant at pH 7.4 of 0.18 fM. Exposure of buffered aqueous solutions of 3Gcage or Cu(II)-bound 3Gcage to UV light induces bond cleavage in the ligand backbone, which reduces the denticity of the ligands. The quantum yields of photolysis for 3Gcage in the absence and presence of Cu(II) are 0.66 and 0.43, respectively. Prior to photolysis, the 3Gcage ligand inhibits copper from generating hydroxyl radicals in the presence of hydrogen peroxide and ascorbic acid; however, hydroxyl radical formation increases by more than 300% following light activation, showing that the reactivity of the copper center can be triggered by light.