Head-to-head right-handed cross-links of the antitumor-active bis(μN,N′-di-p-tolylformamidinato)dirhodium(II,II) unit with the dinucleotides d(GpA) and d(ApG)

Reactions of m-[Rh₂(DTolF)₂(NCCH₃) ₆](BF₄)₂ with the dinucleotides d(GpA) and d(ApG) proceed to formRh₂(DTolF)₂{d(GpA)}] and [Rh ₂(DTolF)₂{d(ApG)}], respectively, with bridging purine bases spanning the Rh-Rh unit in the equatorial positions. Both dirhodium adducts exhibit head-to-head (HH) arrangement of the bases, as indicated by the presence of H8/H8 NOE cross-peaks in the 2D ROESY NMR spectra. The guanine bases bind to the dirhodium core at positions N7 and O6, a conclusion that is supported by the absence of N7 protonation at low pH values and the notable increase in the acidity of the guanine N1H sites (pK_a ≈7.4 in 4:1 CD₃CN/D₂O), inferred from the pH-dependence titrations of the guanine H8 proton resonances. In both dirhodium adducts, the adenine bases coordinate to the metal atoms through N6 and N7, which induces stabilization of the rare imino tautomer of the bases with a concomitant substantial decrease in the basicity of the N1H adenine sites (pK_a ≈ 7.0-7.1 in 4:1 CD₃CN/ D₂O), as compared to the imino form of free adenosine. The presence of the adenine bases in the rare imino form is further corroborated by the observation of DQF-COSY H2/N1H and ROE N1H/N6H cross-peaks in the 2D NMR spectra of [Rh₂(DTolF)₂{d(GpA)}] and [Rh₂(DTolF)₂{d(ApG)}] in CD₃CN at -38°C. The 2D NMR spectroscopic data and the molecular modeling results suggest the presence of righthanded variants, HH1R, in solution for both adducts (HH1R refers to the relative base canting and the direction of propagation of the phosphodiester backbone with respect to the 5' base). Complete characterization of [Rh₂(DTolF)_{d(GpA)}] and [Rh₂(DTolF) ₂{d(ApG)}] by 2DNMR spectroscopy and molecular modeling supports anti-orientation of the sugar residues for both adducts about the glycosyl bonds as well as N- and S-type conformations for the 5′- and 3′-deoxy-ribose residues, respectively.