Unprecedented Head-to-Head Conformers of d(GpG) Bound to the Antitumor Active Compound Tetrakis (μ-carboxylato)dirhodium(II,II)

Abstract

The N7/O6 equatorial binding interactions of the antitumor active complex Rh(2)(OAc)(4)(H(2)O)(2) (OAc(-) = CH(3)CO(2)(-)) 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) approximately 5.7 as compared to 8.5 for N7 only bound platinum adducts), suggested by the pH dependence titrations of the purine H8 (1)H 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 pK(a) values estimated for N1-H (de)protonation, from the pH dependence studies of the C6 and C2 (13)C NMR resonances for the Rh(2)(OAc)(2)(9-EtG)(2) isomers, concur with those derived from the H8 (1)H NMR resonance titrations. Comparison of the (13)C 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 approximately 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(NH(3))(2))[d(GpG)]] and corroborate molecular modeling studies.

Interactions of Metal—Metal‐Bonded Antitumor Active Complexes with DNA Fragments and DNA

“…Interactions of Dinuclear Antitumor Complexes with DNA Chifotides and Dunbar handed conformers HH1R (∼75%) and HH2R (∼ 25%), which differ in the relative canting of the two bases, 44 whereas in Rh 2 (µ-O 2 CCH 3 ) 2 {d(pGpG)}, the steric effect of the terminal 5′-phosphate group results in stabilization of only one left-handed HH1L conformer as in cisplatin-DNA adducts. 45 Detailed characterization of Rh 2 -(µ-O 2 CCH 3 ) 2 {d(GpG)} and Rh 2 (µ-O 2 CCH 3 ) 2 {d(pGpG)} by 2D NMR spectroscopy reveals notable structural features that resemble those of cis-[Pt(NH 3 ) 2 {d(pGpG)}]; the latter involve repuckering of the 5′-G sugar rings to type N, retention of type S conformation for the 3′-G sugar rings and anti orientation with respect to the glycosyl bonds.…”

“…45 Detailed characterization of Rh 2 -(µ-O 2 CCH 3 ) 2 {d(GpG)} and Rh 2 (µ-O 2 CCH 3 ) 2 {d(pGpG)} by 2D NMR spectroscopy reveals notable structural features that resemble those of cis-[Pt(NH 3 ) 2 {d(pGpG)}]; the latter involve repuckering of the 5′-G sugar rings to type N, retention of type S conformation for the 3′-G sugar rings and anti orientation with respect to the glycosyl bonds. 44,45 Superposition of the low-energy Rh 2 (µ-O 2 CCH 3 ) 2 {d(pGpG)} conformer, generated by simulated annealing calculations, and the crystal structure of cis-[Pt(NH 3 ) 2 {d(pGpG)}] reveals remarkable similarities between the adducts ( Figure 14); not only are the bases almost completely destacked (interbase dihedral angle 3′-G/5′-G ≈ 80°) upon coordination to the metal in both cases, but they are favorably poised to accommodate the bidentate N7/O6 binding to the dirhodium unit. 45 Contrary to conventional wisdom, the two octahedral rhodium atoms are capable of engaging in cis binding to GG intrastrand sites by establishing N7/O6 bridges that span the Rh-Rh bond.…”

“…The substantial increase in the acidity of C NMR titrations afford a value pK a ≈ 5.7 compared to 8.5 for N7-bound only and 9.5 for the unbound purine) is attributed to the bidentate N7/O6 coordination of the base. 44 The reaction of Rh 2 (µ-O 2 CCH 3 ) 4 with guanosine-5′-monophosphate (GMP) follows similar trends, which consist in the formation of two isomers (H-H and H-T) with the guanine unit spanning the RhRh bond in a bridging fashion via N7/O6 and a substantially decreased pK a value of the N1-H group (pK a ≈ 5.6) due to N7/O6 coordination. 45 The pronounced electronic effects on the O6/N1 guanine sites, involved in base recognition and hydrogen bonding in duplex DNA and the shifting of the pK a values to the physiological pH range, suggest that metal binding severely impairs normal Watson-Crick pairing, which bears directly on metal mutagenicity and cell death.…”

“…The reactions of Rh 2 (µ-O 2 CCH 3 ) 4 with the dinucleotides d(GpG) and d(pGpG) were probed by NMR spectroscopy. 44,45 One-dimensional 1 H and 13 C NMR studies of Rh 2 (µ-O 2 CCH 3 ) 2 {d(GpG)} (X ) H) and Rh 2 (µ-O 2 CCH 3 ) 2 {d(pGpG)} (X ) HPO 3 -) (Chart 3) revealed that both compounds contain dinucleotides bridging the dirhodium core via N7/O6 with considerable increase in the acidity of the purine sites N1-H (pK a ≈ 5.7), as in the case of 9-EtGuaH. Intense H8/H8 ROE (rotating-frame nuclear Overhauser effect) cross-peaks in the 2D ROESY NMR spectra ( Figure 13) indicate a H-H arrangement of the guanine bases for both adducts.…”

“…Intense H8/H8 ROE (rotating-frame nuclear Overhauser effect) cross-peaks in the 2D ROESY NMR spectra ( Figure 13) indicate a H-H arrangement of the guanine bases for both adducts. 44,45 The Rh 2 (µ-O 2 CCH 3 ) 2 {d(GpG)} adduct exhibits two major right- …”