Sequence‐selective metalation of double‐helical oligodeoxyribonucleotides with PtII, MnII, and ZnII Ions

Reactions of [PtCl(dien)]+ (dien=diethylenetriamine), Mn2+ and Zn2+ ions with three different double-helical oligodeoxyribonucleotides, which contain the central sequence GGXY (XY=AT, TA or CC) have been monitored by NMR spectroscopy. 2 D [1H, 15N] HSQC/HMQC NMR spectroscopy using 15N-labeled Pt(dien) shows that the rate of formation of 3'-G-N 7 and 5'-G-N 7 platinated adducts is highly sequence dependent. The relative rates of platination of 5'-G versus 3'-G are largest for the sequence -GGCC-, for which only a small fraction of the 3'-G adduct is formed; for -GGTA-, the rate of 5'-G platination is about eight times that of 3'-G, and for -GGAT- the ratio is 1.2. These values are in qualitative agreement with those obtained for G-N 7/Mn2+ selectivity as determined by paramagnetic line broadening of the adjacent G-H 8, and also G-N 7/Zn2+ selectivity as determined by G-H 8 chemical shift changes. Fluctuation in the nucleophilicity of G-N 7 may be explained by variation of the π-stacking interaction between base residues along the double helix. The reaction mixtures containing platinated 3'-G and 5'-G fractions were separated by HPLC. Since the duplexes are self-complementary, the platinated single strands were readily annealed to duplexes with twofold symmetry and analyzed by 2 D [1H, 1H] NOESY NMR spectroscopy. Unexpectedly, the 5'-G-H 8 resonance signals of both 5'-G and 3'-G platinated duplexes showed large downfield shifts in the range δ=0.3–0.6 ppm, while the 3'-G-H 8 resonance signals in both cases exhibited no, or only slight, upfield shifts. Resonance signals for several other protons in the central region undergo large chemical shift variations induced by platination, indicating that monofunctional binding to DNA leads to appreciable conformational changes.