The interaction between the oligonucleotide [d(CGCGAATTCGCG)](2) and Hg(II) has been studied by H-1 and natural abundance N-15 NMR spectroscopy. The titration of the dodecamer with Hg(ClO4)2 was monitored by H-1 NMR spectroscopy in aqueous solution (pH 7) at 292 K. The titration pattern is consistent with a transition to a new conformer of the dodecamer induced by Hg(II). At intermediate stages of the titration, the H-1 signals from the new conformer coexist with those of the original one, indicating slow exchange between the two forms on the NMR time scale. In the imino region of the H-1 spectrum, the thymine N3H resonances disappear upon Hg(II) addition, showing that Hg(II) interferes with the Watson-Crick hydrogen bonds of the A.T base pairs. At about five Hg(II) ions added per dodecamer duplex, all original H-1 signals have vanished, i.e., the conformational transition is completed. Addition of KCN in 6-fold excess to Hg(II) restores the original H-1 spectrum of the dodecamer. The H-1 resonance assignment of the Hg(II) form of the dodecamer has been carried out by means of two-dimensional (DQF-COSY/ NOESY) and one-dimensional (TOE) NMR techniques. Comparison of chemical shifts and NOESY cross peaks of the original and Hg(II) form of the dodecamer indicate that it remains in a B-DNA like conformation. The most prominent changes are found for the AT tract, while the H-1 spectrum of the terminal d(CGC)(2) parts of the duplex is hardly affected by the presence of Hg(II). Comparison of the rotational correlation times of the two forms rules out the possibility of a Hg(II)-induced duplex to hairpin transition. Hg(II)-induced N-15 chemical shifts and cross peak splitting patterns of the H-1-N-15 HMQC spectra confirm that Hg(II) interacts solely with the AT tract of the duplex. The data suggest that four Hg(II) ions form covalent bonds with the four A.T base pairs, involving A5/A6 NH2 through loss of protons and T7/T8 O4 on opposite strands. The insertion of Hg(II) ions produces a "bulge" in the AT region of the duplex.