JACS Journal update- January 2007 -Present, continued

J. Am. Chem. Soc., 129 (8), 2398 -2407, 2007.
Probing Hydrogen Bonding and Ion-Carbonyl Interactions by Solid-State 17O NMR Spectroscopy: G-Ribbon and G-Quartet
Irene C. M. Kwan, Xin Mo, and Gang Wu*
Abstract:
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, Q = 0.45 ± 0.05, iso = 263 ± 2, 11 = 460 ± 5, 22 = 360 ± 5, 33 = -30 ± 5 ppm; Molecule B, CQ = 7.7 ± 0.1 MHz, Q = 0.55 ± 0.05, iso = 250 ± 2, 11 = 440 ± 5, 22 = 340 ± 5, 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···H-N1 and N7···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 17O NMR tensors are: CQ = 7.2 ± 0.1 MHz, Q = 0.68 ± 0.05, iso = 232 ± 2, 11 = 400 ± 5, 22 = 300 ± 5, 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 17O NMR parameters are: Sr2+, CQ = 6.8 ± 0.1 MHz, Q = 1.00 ± 0.05, iso = 232 ± 2 ppm; Ba2+, CQ = 7.0 ± 0.1 MHz, Q = 0.68 ± 0.05, iso = 232 ± 2 ppm; Pb2+, CQ = 7.2 ± 0.1 MHz, Q = 1.00 ± 0.05, iso = 232 ± 2 ppm. We also perform extensive quantum chemical calculations for the 17O NMR tensors in both G-ribbons and G-quartets. Our results demonstrate that the 17O 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 17O NMR as a probe in the study of ion binding in G-quadruplex DNA and ion channel proteins.