Interactions of DNA with divalent metal ions. IV. Competitive studies of Mn 2+ binding to AT‐ and GC‐rich DNAs

It has been inferred from previous studies that Mn 2+ ions bind preferentially to G·C base pairs in DNA, and it has even been suggested that this preference for G·C pairs might be responsible for some of the Mn 2+ specific effects observed in various biochemical reactions. In this paper we investigate the AT/GC preference of Mn 2+ by direct competition studies in which AT‐rich DNA was dialyzed against GC‐rich DNA in the presence of varying amounts of Mn 2+ . Analysis of these results demonstrates that over a wide range of Mn 2+ /DNA(P) molar ratios, Mn 2+ binds to A·T and to G·C base pairs with virtually identical affinity, although in a somewhat different mode. Both the present and previous nmr, uv, CD, and melting studies are discussed in terms of the different modes of binding of Mn 2+ to single‐ and double‐stranded DNA.