SPECTROSCOPIC AND PHOTOPHYSICAL STUDIES OF THE BINDING INTERACTIONS BETWEEN COPPER PHENANTHROLINE COMPLEXES AND RNA

The interactions of two Cu(I) phenanthroline complexes, Cu(bcp)2+ and Cu(dMP)2+, where bcp and dmp denote 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline and 2,9-dimethyl-1,10-phenanthroline, respectively, with several different types of RNA have been investigated by means of spectroscopic, photophysical, and viscometric methods. Surface association occurs between Cu(dMP)2+ and RNA, but binding of CU(bcP)2+ with RNA strongly perturbs the spectral properties of the metal complex in ways that depend on the nucleotide-to-copper ratio. Thus, at low ratios, an aggregated form of Cu(bcp)2+ binds to RNA. At higher ratios, the aggregates break up, but the Cu(bcP)2+ remains bound to RNA since solvent-induced quenching of the charge-transfer emission from the copper complex continues to be suppressed. Furthermore, a circular dichroism signal can be observed from Cu(bcP)2+ in the presence of high levels of poly(C).poly(I). An intimate association with the biopolymer must occur, but viscometric measurements indicate that Cu(bcP)2+ is not intercalated into the RNA. One possibility is that the local RNA structure melts out to allow internalization of the Cu(bcp)2+ ion into the macromolecule. Another possibility is that CU(bcP)2+ may be sandwiched between separate runs of double-helical RNA structure in solution.