INTERACTION OF 5-METHYLPHENAZINIUM CATION RADICAL WITH DEOXYRIBONUCLEIC ACID

The complexes of deoxyribonucleic acid with the cation 5-methylphenazinium and the 5-methylphenazinium cation radical were studied by ultravioletvisible and electron spin resonance spectrometry. The 5-methylphenazinium cation was less strongly bound than the 5-methylphenazinium cation radical (association constants were, respectively, 0.4 × 106 and 2.6 × 106 M-1) and had fewer binding sites per deoxyribonucleic acid nucleotide (0.13 vs. 0.26). In addition a weaker type of association was found for each cation, the binding by the radical again being the stronger (association constants 0.6 × 104 and 2.3 × 104 M-1). The binding was reversed by raising the ionic strength, or, more strongly, by the presence of divalent cations. These characteristics are shared by proflavine, ethidium bromide, and other cationic ligands for which an intercalative model for the strong binding has been proposed. Association of the the 5-methylphenazinium cation radical with deoxyribonucleic acid strikingly stabilizes it against air oxidation and causes a shift in the Michaelis equilibrium toward the semireduced. species. Single-stranded (heat-denatured) deoxyribonucleic acid shows very little tendency to bind either cation strongly, an observation which may be related to the weakness of these species as frame-shift mutagens. © 1969, American Chemical Society. All rights reserved.