Crystal Structure of Metallo DNA Duplex Containing Consecutive Watson-Crick-like T-HgII-T Base Pairs**

The metallo DNA duplex containing mercury-mediated T-T base pairs is an attractive biomacromolecular nanomaterial which can be applied to nanodevices such as ion sensors. Reported herein is the first crystal structure of a B-form DNA duplex containing two consecutive T-Hg-II-T base pairs. The Hg-II ion occupies the center between two T residues. The N3-Hg-II bond distance is 2.0 angstrom. The relatively short Hg-II-Hg-II distance (3.3 angstrom) observed in consecutive T-Hg-II-T base pairs suggests that the metallophilic attraction could exist between them and may stabilize the B-form double helix. To support this, the DNA duplex is largely distorted and adopts an unusual nonhelical conformation in the absence of Hg-II. The structure of the metallo DNA duplex itself and the Hg-II-induced structural switching from the nonhelical form to the B-form provide the basis for structure-based design of metal-conjugated nucleic acid nanomaterials.