Near-UV or visible light irradiation of Ru(tap)2(bpy)2+ (1) (tap = 1,4,5,8-tetraazaphenanthrene; bpy = 2,2‘-bipyridyl) in the presence of duplex DNA induces the formation of covalent adducts with guanine. The adduct has been isolated from the photomodified DNA as both its nucleotide and nucleobase derivatives by using a combination of enzymatic and acid hydrolytic procedures in conjunction with HPLC. Characterization by electrospray mass spectrometry and NMR spectroscopy shows that two isomeric covalent adducts are formed in which the exocyclic amino group of a guanine nucleobase is linked to the C2 or C7 position of one of the tap ligands. It is proposed that the products are generated from the reduced ruthenium complex and the guanine radical cation resulting from photoinduced electron transfer between 1 and guanine.

Resonance Raman (RR) spectroscopy has been used to probe the interaction between dipyridophenazine (dppz) complexes of ruthenium(II), [Ru(L)2(dppz)]2+ (L = 1,10-phenanthroline (1) and 2,2-bipyridyl (2)), and calf-thymus DNA. Ground electronic state RR spectra at selected probe wavelengths reveal enhancement patterns which reflect perturbation of the dppz-centered electronic transitions in the UV−vis spectra in the presence of DNA. Comparison of the RR spectra recorded of the short-lived MLCT excited states of both complexes in aqueous solution with those of the longer-lived states of the complexes in the DNA environment reveals changes to excited state modes, suggesting perturbation of electronic transitions of the dppz ligand in the excited state as a result of intercalation. The most prominent feature, at 1526 cm-1, appears in the spectra of both 1 and 2 and is a convenient marker band for intercalation. For 1, the excited state studies have been extended to the Δ and Λ enantiomers. The marker band appears at the same frequency for both but with different relative intensities. This is interpreted as reflecting the distinctive response of the enantiomers to the chiral environment of the DNA binding sites. The results, together with some analogous data for other potentially intercalating complexes, are considered in relation to the more general application of time-resolved RR spectroscopy for investigation of intercalative interactions of photoexcited metal complexes with DNA.

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