Photoinduced Intercalation and Coordination of a Dirhodium Complex to DNA: Dual DNA Binding

Two new complexes, cis-H,H-[Rh2(OCCH3NH)2(LL)(CH3CN)2]2+, where LL=bpy (2, bpy=2,2′-bipyridine) and dppz (3, dppz=dipyrido[3,2-a:2′,3′-c]phenazine), were prepared from the reaction of cis-H,H-[Rh2(OCCH3NH)2(CH3CN)6]2+ (1) with the corresponding bidentate ligand. The bpy and dppz ligands chelate to the same rhodium atom and are positioned trans to the amidato N atoms, as determined by the single crystal X-ray structure of 2. Irradiation of 2 and 3 with visible light in water results in the exchange of one CH3CNeq ligand for an H2O molecule with quantum yields, Φ400, of 0.040 and 0.044, respectively (λirr=400 nm). The identities of the photoproducts of 2 and 3 were determined to be cis-H,H-[Rh2(OCCH3NH)2(L)(H2O)(CH3CN)]2+, where L is bpy (4) and dppz (5), respectively. Mobility shift assays show that 4 crosslinks double-stranded DNA, and ESI-MS experiments indicate that both 4 and 5 form covalent adducts with single-stranded DNA. In addition, relative viscosity and 2D NMR experiments show that the dppz ligand of 5 also intercalates into DNA upon irradiation, making 3 a dual-binding agent that both intercalates and covalently binds to DNA upon the absorption of visible light.