A study of some ruthenium polypyridyl complexes as DNA binders and photocleavage reagents

The nature of the binding of several ruthenium polypyridyl complexes containing 2, 2`-bipyridine (bipy), 4, 4`-dimethyl-2, 2`-bipyridine (DMB), 1,10-phenanthroline (phen), 4,7-diphenyl-1,10-phenanthroline(DPP), 2, 2`, 2`-terpyridine (terpy), 2, 2`-biquinoline (biq), 1,4,5,8-tetraazaphen-anthrene (TAP) and 1,4,5,8,9,12-hexaazatriphenylene (HAT), with calf thymus DNA, poly[d(A-T)] and poly[d(G-C)] were studied by absorption and emission spectroscopy, DNA melting techniques, and emission lifetime measurements. In low ionic strength phosphate buffer, spectroscopic changes and DNA stabilization depended on the polypyridyl ligands present, and indicated binding that varied from substantially electrostatic to intercalative. Ru(bipy)2(HAT)2+ and Ru(phen)32+, which bind by partial intercalation, also show a strong preference for poly[d(A-T)]. The emission quantum yields for most complexes were increased in the presence of DNA. An exception was Ru(TAP)32+ which has a markedly reduced emission quantum yield and lifetime in the presence of poly[d(G-C)] or CT-DNA, due to photoredox interaction with guanines. Emission decays of the complexes generally showed multiexponential behaviour. The ability of the ruthenium complexes to sensitise DNA cleavage was determined using pBR322 plasmid DNA. Ru(TAP)32+ is the most efficient sensitiser while uncharged complexes and complexes with very short-lived excited states do not cleave DNA.