Novel Binding Interactions of the DNA Fragment d(pGpG) Cross-Linked by the Antitumor Active Compound Tetrakis(μ-carboxylato)dirhodium(II,II)

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Abstract
Insight into the N7/O6equatorial binding interactions of the antitumor active complex Rh2(OAc)4(H2O)2 (OAc- = CH3CO2-) with the nucleotide 5‘-GMP and the DNA fragment d(pGpG) has been obtained by one- (1D) and two-dimensional (2D) NMR spectroscopy. The lack of N7 protonation at low pH values and the significant increase in the acidity of N1−H (pKa ≈ 5.6 as compared to 8.5 for N7 only bound platinum adducts), indicated by the pH dependence study of the H8 1H NMR resonance for the HT (head-to-tail) isomer of Rh2(OAc)2(5‘-GMP)2, are consistent with bidentate N7/O6 binding of the guanine. The H8 1H NMR resonance of the HH (head-to-head) Rh2(OAc)2(5‘-GMP)2 isomer, as well as the 5‘-G and 3‘-G H8 resonances of the Rh2(OAc)2{d(pGpG)} adduct exhibit pH-independent titration curves, attributable to the added effect of the 5‘-phosphate group deprotonation at a pH value similar to that of the N1 site. The enhancement in the acidity of N1−H, with respect to N7 only bound metal adducts, afforded by the O6 binding of the bases to the rhodium centers, has been corroborated by monitoring the pH dependence of the purine C6 and C2 13C NMR resonances for Rh2(OAc)2(5‘-GMP)2 and Rh2(OAc)2{d(pGpG)}. The latter studies resulted in pKa values in good agreement with those derived from the pH-dependent 1H NMR titrations of the H8 resonances. Comparison of the 13C NMR resonances of C6 and C2 for the dirhodium adducts Rh2(OAc)2(5‘-GMP)2 and Rh2(OAc)2{d(pGpG)} with the corresponding resonances of the unbound ligands at pH 8.0, showed substantial downfield shifts of Δδ ≈ 11.0 and 6.0 ppm, respectively. The HH arrangement of the bases in the Rh2(OAc)2{d(pGpG)} adduct is evidenced by intense H8/H8 ROE cross-peaks in the 2D ROESY NMR spectrum. The presence of the terminal 5‘-phosphate group in d(pGpG) results in stabilization of oneleft-handed Rh2(OAc)2{d(pGpG)} HH1 L conformer, due to the steric effect of the 5‘-group, favoring left canting in cisplatin-DNA adducts. Complete characterization of the Rh2(OAc)2{d(pGpG)} adduct revealed notable structural features that resemble those of cis-[Pt(NH3)2{d(pGpG)}]; the latter involve repuckering of the 5‘-G sugar ring to C3‘-endo (N-type) conformation, retention of C2‘-endo (S-type) 3‘-G sugar ring conformation, and anti orientation with respect to the glycosyl bonds. The superposition of the low energy Rh2(OAc)2{d(pGpG)} conformers, generated by simulated annealing calculations, with the crystal structure of cis-[Pt(NH3)2{d(pGpG)}], reveals remarkable similarities between the adducts; not only are the bases almost completely destacked upon coordination to the metal in both cases, but they are favorably poised to accommodate the bidentateN7/O6 binding to the dirhodium unit. Unexpectedly, the two metal−metal bonded rhodium centers are capable of engaging in cis binding to GG intrastrand sites by establishing N7/O6 bridges that span the Rh−Rh bond.