Probing hydrogen bonding and ion-carbonyl interactions by solid-state 17O NMR spectroscopy: G-ribbon and G-quartet.

We report solid-state 17O NMR determination of the 17O NMR tensors for the keto carbonyl oxygen (O6) of guanine in two 17O-enriched guanosine derivatives:  [6-17O]guanosine (G1) and 2‘,3‘,5‘-O-triacetyl-[6-17O]guanosine (G2). In G1·2H2O, guanosine molecules form hydrogen-bonded G-ribbons where the guanine bases are linked by O6···H−N2 and N7···H−N7 hydrogen bonds in a zigzag fashion. In addition, the keto carbonyl oxygen O6 is also weakly hydrogen-bonded to two water molecules of hydration. The experimental 17O NMR tensors determined for the two independent molecules in the asymmetric unit of G1·2H2O are:  Molecule A, CQ = 7.8 ± 0.1 MHz, ηQ = 0.45 ± 0.05, δiso = 263 ± 2, δ11 = 460 ± 5, δ22 = 360 ± 5, δ33 = −30 ± 5 ppm; Molecule B, CQ = 7.7 ± 0.1 MHz, ηQ = 0.55 ± 0.05, δiso = 250 ± 2, δ11 = 440 ± 5, δ22 = 340 ± 5, δ33 = −30 ± 5 ppm. In G1/K+ gel, guanosine molecules form extensively stacking G-quartets. In each G-quartet, four guanine bases are linked together by four pairs of O6···H−N1 and N7···H−N2 hydro...