Site-specific DNA oxidation by a dinuclear copper complex containing a photoisomerizable azobenzene ligand

A dinuclear copper complex possessing an azobenzene backbone (CuII2 1) was synthesized and its affinity towards DNA was investigated and found to be dependent on the trans–cis isomeric forms. Upon exposure to UV light at 365 nm the trans form of the complex (CuII2 trans-1) underwent photoisomerization into the cis-isomer (CuII2 cis-1), which reverted to the original trans-form on exposure to visible light at 420 nm. Both the trans and cis isomers exhibited moderate DNA cleavage activity toward plasmid duplex DNA in the presence of a reducing agent 3-mercaptopropionic acid (MPA) suggesting that the CuI species could activate molecular oxygen to form a reactive oxygen species in situ. Interestingly, the isomeric copper complexes showed different site-specificities in the oxidation of an oligodeoxynucleotide having single-strand–double-helix junctions in the sequence. The trans isomer CuI2 trans-1 induced oxidation at the consecutive deoxyguanosine site (GG) near the junction; on the other hand, such site-specificity was not observed in the case of CuI2 cis-1.