The N7/O6 equatorial binding interactions of the antitumor active complex Rh-2(OAc)(4)(H2O)(2) (OAc-=CH3CO2-) with the DNA fragment d(GpG) have been unambiguously determined by NMR spectroscopy. Previous X-ray crystallographic determinations of the head-to-head (HH) and head-to-tail (HT) adducts of dirhodium tetraacetate with 9-ethylguanine (9-EtGH) revealed unprecedented bridging N7/O6 guanine nucleobases that span the Rh-Rh bond. The absence of N7 protonation at low pH and the notable increase in the acidity of N1-H (pK(a) approximate to 5.7 as compared to 8.5 for N7 only bound platinum adducts), suggested by the pH dependence titrations of the purine H8 H-1 NMR resonances for Rh-2(OAc)(2)(9-EtG)(2) and Rh-2(OAC)(2)-{d(GpG)}, are consistent with bidentate N7/O6 binding of the guanine nucleobases. The pKa values estimated for N1-H (de) proto nation, from the pH dependence studies of the C6 and C2 C-13 NMR resonances for the Rh2(OAc)2(9-EtG)2 isomers, concur with those derived from the H8 1H NMR resonance titrations. Comparison of the C-13 NMR resonances of C6 and C2 for the dirhodium adducts Rh-2(OAc)(2)(9-EtG)(2) and Rh-2(OAc)(2){d(GpG)} with the corresponding resonances of the unbound ligands {at pH 7.0 for 9-EtGH and pH 8.0 for d(GpG)}, shows substantial downfield shifts of Deltadelta approximate to 11.0 and 6.0 ppm for C6 and C2, respectively; the latter shifts reflect the effect of O6 binding to the dirhodium centers and the ensuing enhancement in the acidity of N1-H. Intense H8/H8 ROE cross-peaks in the 2D ROESY NMR spectrum of Rh-2(OAc)(2){d(GpG)} indicate head-to-head arrangement of the guanine bases. The Rh-2(OAc)(2){d(GpG)} adduct exhibits two major right-handed conformers, HH1 R and HH2 R, with HH1 R being three times more abundant than the unusual HH2 R. Complete characterization of both adducts revealed repuckering of the 5'-G sugar rings to C3'-endo (N-type), retention of C2'-endo (S-type) conformation for the 3'-G sugar rings, and anti orientation with respect to the glycosyl bonds. The structural features obtained for Rh-2-(OAc)2{d(GpG)} by means of NMR spectroscopy are very similar to those for cis-[Pt(NH3)(2){d(GpG)}] and corroborate molecular modeling studies.