Nitroxide-labeled ruthenium(II)-polypyridyl complexes as EPR probes to study organized systems. 2. Combined photophysical and EPR investigations of B-DNA

Abstract

We report here the application of two polypyridyl complexes of ruthenium(II), R~(phen)~(phen-T)CI~ and Ru- (bpy),(phen-T)C12, to the examination of the interactions of the family of polypyridyl metal complexes with B-DNA. Phen-T is a modified 1,lO-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.