Interaction of Mn²⁺ with DNA as studied by proton‐relaxation enhancement of solvent water

The binding of Mn 2+ to double‐stranded DNA in solution has been studied by measuring the proton‐relaxation enhancement of solvent water. The nmr experiments were carried out on DNAs of different base composition and showed the existence of weak and tight binding sites for Mn 2+ . The former sites are related to the electrostatic binding of the divalent cation with two contiguous phosphate groups, and the latter appear to vary almost proportionally with the number of G·C base pairs in the DNAs. Our results agree, in general, with the emerging evidence for a specific interaction of Mn 2+ with the G·C residues of DNA. Competition experiments with Mg 2+ suggested that this binding of Mn 2+ in DNA also involves the phosphate groups. More definitely, our data conform with the hypothesis made by Clement et al. of Mn 2+ chelation between the phosphate and the nitrogen N7 of a deoxyguanyl unit of DNA.