Stable and selective DNA base pairing by metal coordination was recently demonstrated with nucleotides containing complementary pyridine-2,6-dicarboxylate (Dipic) and pyridine (Py) bases (Meggers, E.; Holland, P.L. Tolman; W.B. Romesberg, F.E.; Schultz, P.G. J. Am. Chem. Soc. 2000, 122, 10714-10715). To understand the structural consequences of introducing this novel base pair into DNA we have solved the crystal structure of a duplex containing the metallo-base pair, The structure shows that the bases pair as designed, but in a Z-DNA conformation. The structure also provides a structural explanation for the B- to Z-DNA transition in this duplex, Further solution studies demonstrate that the metallo-base pair is compatible with Z- or B-DNA conformations, depending on the duplex sequence.