Photoinduced interactions of two dirhodium complexes with d(GTCGAC)2 probed by 2D NOESY.

The interactions between the 6-mer duplex oligonucleotide d(GTCGAC)2 and the photoactive dirhodium complexes cis-H,H-[Rh2(HNOCCH3)2(L)(CH3CN)4](2+), where L represents bpy (1, 2,2'-bipyridine) and dppz (2, dipyrido[3,2-a:2',3'-c]phenazine), were probed using 2D (1)H-(1)H NOESY NMR spectroscopy. Complex does not interact with the duplex in the dark, but binds covalently to the terminal guanine following irradiation with visible light. Similar behavior was observed for 2, but in addition to the photoinduced covalent DNA binding, the planar dppz ligand of the complex shields the terminal cytosine protons after irradiation. The results are consistent with photoinduced guanine coordination and end-capping of the duplex through π-stacking interactions with the terminal GC base pair. These data show that in the presence of the 6-mer duplex oligonucleotide, 1 and 2 exhibit photoinduced covalent binding to DNA. In addition, the π-stacking interactions of 2 with the duplex are enhanced upon irradiation.