2D NMR study of the DNA duplex d(CTCTC*A*ACTTCC) ·d(GGAAGTTGAGAG) cross-linked by the antitumor-active dirhodium(II,II) unit at the cytosine-adenine step

The 2D NMR analysis in solution of the DNA duplex d(CTCTC *A*ACTTCC)·d(GGAAGTTGAGAG) binding to the dirhodium unit cis-[Rh₂(μ-O₂CCH₃)₂(η ¹-O₂CCH₃)]⁺ showed that an unprecedented intrastrand adduct, dsII, is formed with the dirhodium unit cross-linking in the major groove residues C5 and A6 (indicated with asterisks), also corroborated by enzyme digestion studies. Formation of the dirhodium complex dsII destabilizes significantly the duplex as indicated by the substantial decrease in its melting temperature (ΔT_m = -22.9°C). The reduced thermal stability of dsII is attributed to the decreased stacking of the bases and the complete disruption and/or weakening of the hydrogen bonds within the base pairs in the immediate vicinity of the metalation site (C5·G20 and A6·T19), but the effects due to the metal binding are more severe for the base pairs in the 5′ direction to the lesion site. The NMR spectroscopic data indicate that Watson-Crick hydrogen bonding is completely disrupted for the C5·G20 site and considerably weakened for A6·T19. In dsII, the bases C5 and A6 bind to eq positions of the dirhodium unit cis-[Rh₂(μ-O₂CCH₃) ₂(η¹-O₂CCH₃)]⁺, which retains one monodentate and two bridging acetate groups, presumably due to steric reasons. Binding of A6 takes place via N7, whereas binding of the C5 base takes place via the exocyclic N4 site, resulting in the anti-cytosine rotamer with respect to site N3 in its metal-stabilized rare iminooxo form.