Binding of manganese(II) to DNA and the competitive effects of metal ions and organic cations. An electron paramagnetic resonance study.

The binding of manganese(II) to DNA was studied by monitoring the concentration of free Mn2+ by electron paramagnetic resonance (EPR). It was found that the association constnat of the Mn-DNA complex depends upon the degree of saturation. The competitive effects of magnesium, calcium, sodium, and a number of organic cations including the antibiotic drug daunomycin were analyzed and the parameters describing the cation-DNA interaction were evaluated. It was found that the association constant as well as the parameter describing its dependence upon the degree of saturation decrease along the series Mn, Mg, Ca, Na. Differences in the extent of interaction with the base nitrogens (N-7) are suggested as the possible mechanisms leading to these observations. The EPR spectrum of the manganese-DNA complex was found to be similar to that of manganese-nucleotide complexes suggesting a similar mode of coordination. A comparison of the results of competitive and direct binding studies reveals some salient features of the small molecule-DNA interaction and leads to the conclusion that manganese binds at the major groove of the DNA helix.