Binding of Ru(bpy)32+ and Ru(phen)32+ to polynucleotides and DNA. Effect of added salts on the absorption and luminescence properties

The interactions of tris(2,2′-bipyridyl)ruthenium(II) chloride and tris-(1, 10-phenanthroline)ruthenium(II) chloride, Ru(bpy)3Cl2 and Ru(phen)3Cl2 respectively, with nucleic acids were studied by means of absorption spectroscopy and time-resolved and steady state luminescence techniques in unbuffered aqueous solution at room temperature as a function of added salt, oxygen and the [nucleotide]/[sensitizer] ratio (N/S). The hypochromicity of the visible absorption band of Ru(ligand)2+3 and the changes in the luminescence intensity and luminescence decay kinetics are considerably larger in the presence of double-stranded calf thymus DNA (dsDNA) than in the presence of single- stranded DNA and polynucleotides. This is suggested to be the result of partial intercalation of the ruthenium complex into the dsDNA rather than just its higher charge density with respect to ssDNA. Spectral changes in the presence of dsDNA increase with increasing N/S ratio (maximum changes reached at N/S = 10 - 12, half-value 3 - 4). This is postulated to be due to a transition from mainly electrostatic binding to a binding in which partial intercalation plays an increased role. Addition of alkali or alkaline earth salts at very low concentrations stabilizes partial intercalation whereas higher salt concentrations lead to a release of the ruthenium complex from the strand. This effect of the salt cation increases in the order Cs < Rb < K < Na < Li < Ba < Sr < Ca < Mg < Be. For Ru(bpy)2+3 the presence of 0.5 mM Mg(ClO4) 2 or 6 mM NaClO4 are sufficient to release 50% of the ruthenium complexes which are bound to the dsDNA (N/S = 10); the corresponding half-concentrations for Ru(phen)2+3 are 0.8 mM and 40 mM respectively. The half-concentrations for release increase with increasing N/S ratio and decrease with the ionic radius of the added salt.