¹⁵N−¹⁵N <i>J</i>-Coupling Across Hg^II:  Direct Observation of Hg^II-Mediated T−T Base Pairs in a DNA Duplex

N−N J-coupling across a metal center (2JNN) was clearly detected in a biological macromolecule (DNA duplex) for the first time. By using 2JNN, the base pairing mode of mercury-mediated T−T pairs (T−HgII−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, 2JNN is highly important for the determination of the chemical structures of metal-mediated base pairs.