The structure of the DNA duplex dodecamer, d(CCTCTGGTCTCC. GGAGACCAGAGG), containing the cisplatin d(GpG) 1,2-intrastrand cross-link at the position denoted by asterisks, was determined in solution by high-resolution 2D NMR spectroscopy and restrained molecular dynamics refinement. The cis-[Pt(NH3)2¿d(GpG-N7(1), N7(2))¿] lesion causes the adjacent guanine bases to roll toward one another by 49 degrees, leading to an overall helix bend angle of 78 degrees. These features are more exaggerated than those observed in the X-ray crystal structure determined for the same platinated duplex [Takahara et al. (1995) Nature 377, 649-652]. A common property of the solution and crystal structures is the widening and flattening of the minor groove opposite the platinum adduct, affording geometric parameters resembling those found in A-form DNA. This deformation is especially noteworthy for the solution structure because its sugar puckers are primarily those of B-form DNA. The unwinding of the helix at the site of platination is 25 degrees. The curvature and shape of the platinated duplex are remarkably similar to those observed in DNA duplexes complexed by the HMG-domain proteins SRY and LEF-1. The structure reveals how cisplatin binding alters DNA in such a manner as to facilitate HMG-domain protein recognition.