15N−15N J-Coupling Across HgII: Direct Observation of HgII-Mediated T−T Base Pairs in a DNA Duplex

N−N J -coupling across a metal center ( 2 J NN ) was clearly detected in a biological macromolecule (DNA duplex) for the first time. By using 2 J NN, the base pairing mode of mercury-mediated T−T pairs (T−Hg II −T) was definitely determined. This pairing mode was found to be a novel metal ion-binding mode for DNA and RNA molecules, in which imino proton−metal exchange processes are included. Accordingly, 2 J NN is highly important for the determination of the chemical structures of metal-mediated base pairs.