We employ retro models, cis-PtA(2)G(2) (A(2) = a diamine, G = guanine derivative), to assess the cross-linked head-to-head (HH) form of the cisplatin-DNA d(GpG) adduct widely postulated to be responsible for the anticancer activity. Retro models are designed to have minimal dynamic motion tb overcome problems recognized in models derived from cisplatin [A(2) = (NH3)(2)]; the latter models are difficult to understand due to rapid rotation of G bases about the Pt-N7 bond in solution and the dominance of the head-to-tail (HT) form in the solid. Observation of an HH form is unusual for CiS-PtA(2)G(2) models. Recently, we found the first HH forms for a cis-PtA(2)G(2) model with A2 lacking NH groups in a study of. new Me(2)ppzPtG(2) models. (Me(2)ppz. N,N'-dimethylpiperazine, has inplane bulk which reduces dynamic motion by clashing with the G O6 as the base rotates into the coordination plane from the ground state position approximately perpendicular to this plane. G = 5'-GMP and 3'-GMP.) The finding of an HH form (albeit in a mixture with HT forms) with both G H8 signals unusually downfield encouraged us to study additional Me(2)ppzPtG(2) analogues in order to explain the unusual special features and to identify factors that influence the relative stability of HT and HH forms. Molecular modeling techniques suggest HH structures with the H8's close to the deshielding region of the z axis of the magnetically anisotropic Pt atom, explaining the atypical shift pattern. When G = 1-Me-5'-GMP, we obtained NMR evidence that the HH rotamer has a high abundance (34%) and that the three rotamers have nearly equal abundance. These findings and the observation that the relative HT distributions varied little or not at all asa function of pH when G = Guo, 1-MeGuo, or 1-Me-5'-GMP are consistent with two of our earlier proposals concerning phosphate groups in HT forms of cis-PtA(2)(GMP)(2) complexes. We proposed that a G phosphate group can form hydrogen bonds with the cia G N1H ("second-sphere" communication) and (for 5'-phosphate) A(2) NH groups. The new results with 1-Me5'-GMP led us to propose a new role for a 5'-phosphate group; it can also favor the HH form by counteracting the natural preference for the G bases to adopt an HT orientation. Finally, the HH form was also sufficiently abundant to allow observation of a distinct Pt-195 NMR signal (downfield of the resonance observed for the HT forms) for several complexes. This is the first report of an HH Pt-195 NMR signal for cis-PtA(2)G(2) complexes.