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

We report solid-state O-17 NMR determination of the O-17 NMR tensors for the keto carbonyl oxygen (O6) of guanine in two O-17-enriched guanosine derivatives: [6-O-17]guanosine (G1) and 2',3',5'-O-triacetyl-[6-O-17]guanosine (G2). In G1 center dot 2H(2)O, guanosine molecules form hydrogen-bonded G-ribbons where the guanine bases are linked by O6 center dot center dot center dot H-N2 and N7 center dot center dot center dot 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 O-17 NMR tensors determined for the two independent molecules in the asymmetric unit of G1 center dot 2H(2)O are: Molecule A, C-Q = 7.8 +/- 0.1 MHz, eta(Q) = 0.45 +/- 0.05, delta(iso) = 263 +/- 2, delta(11) = 460 +/- 5, delta(22) = 360 +/- 5, delta(33) = -30 +/- 5 ppm; Molecule B, C-Q = 7.7 +/- 0.1 MHz, eta(Q) = 0.55 +/- 0.05, delta(iso) = 250 +/- 2, delta(11) = 440 +/- 5, delta(22) = 340 +/- 5, delta(33) = -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 center dot center dot center dot H-N1 and N7 center dot center dot center dot H-N2 hydrogen bonds in a cyclic fashion. In addition, each O6 atom is simultaneously coordinated to two K+ ions. For G1/K+ gel, the experimental O-17 NMR tensors are: C-Q = 7.2 +/- 0.1 MHz, eta(Q) = 0.68 +/- 0.05, delta(iso) = 232 +/- 2, delta(11) = 400 +/- 5, delta(22) = 300 +/- 5, delta(33) = -20 +/- 5 ppm. In the presence of divalent cations such as Sr2+, Ba2+, and Pb2+, G2 molecules form discrete octamers containing two stacking G-quartets and a central metal ion, that is, (G2)(4)-M2+-(G2)(4). In this case, each O6 atom of the G-quartet is coordinated to only one metal ion. For G2/M2+ octamers, the experimental O-17 NMR parameters are: Sr2+, C-Q = 6.8 +/- 0.1 MHz, eta(Q) = 1.00 +/- 0.05, delta(iso) = 232 +/- 2 ppm; Ba2+, C-Q = 7.0 +/- 0.1 MHz, eta(Q) = 0.68 +/- 0.05, delta(iso) = 232 +/- 2 ppm; Pb2+, C-Q = 7.2 +/- 0.1 MHz, eta(Q) = 1.00 +/- 0.05, delta(iso) = 232 +/- 2 ppm. We also perform extensive quantum chemical calculations for the O-17 NMR tensors in both G-ribbons and G-quartets. Our results demonstrate that the O-17 chemical shift tensor and quadrupole coupling tensor are very sensitive to the presence of hydrogen bonding and ion-carbonyl interactions. Furthermore, the effect from ion-carbonyl interactions is several times stronger than that from hydrogen-bonding interactions. Our results establish a basis for using solid-state O-17 NMR as a probe in the study of ion binding in G-quadruplex DNA and ion channel proteins.