H-1 NMR spectroscopy was used to study the binding of Delta-Co(en)(3)(3+) to the dodecanucleotide d(TCGGG-GATCCCGA)(2). Intermolecular NOE cross peaks between the metal complex methylene protons and the dodecanucleotide unambiguously establish that Delta-Co(en)(3)(3+) selectively binds at GG sequences in the major groove. The NMR experiments indicated that the dodecanucleotide maintains a B-type DNA conformation upon metal complex binding.