NITROXIDE-LABELED RU(II) POLYPYRIDYL COMPLEXES AS EPR PROBES TO STUDY ORGANIZED SYSTEMS .2. COMBINED PHOTOPHYSICAL AND EPR INVESTIGATIONS OF B-DNA

We report here the application of two polypyridyl complexes of ruthenium(II), Ru(phen)2(phen-T)Cl2 and Ru(bpy)2(phen-T)Cl2, to the examination of the interactions of the family of polypyridyl metal complexes with B-DNA. Phen-T is a modified 1,10-phenanthroline ligand where T is a stable nitroxide (TEMPO, 2,2,6,6-tetramethylpiperidine-N-oxyl) which is covalently attached to the phenanthroline unit via a carbamate linkage. These nitroxide-substituted ruthenium complexes are unique in that the same compound is a probe which can be monitored by two completely independent spectroscopic techniques. We report here the comparison of time-resolved luminescence measurements and electron paramagnetic resonance (EPR) spectra in the presence of B-DNA and confirm that the data obtained using both methods are mutually consistent. The EPR spectra provide independent evidence for two distinct modes of binding of these complexes with DNA: one surface and the other intercalative. The EPR spectra have been evaluated to determine the rotational correlation times of motion of the bound radicals; all experimentally recorded EPR spectra have been successfully simulated.