The interaction of nucleic acid with Mg-Ions

Abstract

It is a commonly accepted notion that nucleic acid invariably binds divalent cations on the negatively charged sites of their phosphate groups. Evidence is presented here to support the contention that although the negatively charged sites contribute to the energy of binding they are not always the actual sites of binding. Experiments on the relative affinities of native and denatured DNA for divalent cations suggest that the divalent cations are bound much more strongly by the nitrogenous base groups. The most likely groups involved are adenine and guanine because they contain sites favourable for the formation of five-membered chelates. Added support for the involvement of adenine is given by the loss of a substantial portion of the binding in the presence of formaldehyde which is believed to combine with the adenine amino group. These findings have important implications on the interactions of divalent cations with synthetic polynucleotides and naturally occurring nucleic acids which will be dealt with elsewhere.