Friday, February 29, 2008

Hiyam's Journal update

ASAP Chem. Mater., ASAP Article, 10.1021/cm702523d Web Release Date: February 29, 2008 Copyright © 2008 American Chemical Society
Determination and Quantification of the Local Environments in Stoichiometric and Defect Jarosite by Solid-State 2H NMR Spectroscopy

Ulla Gro Nielsen,†‡ Juraj Majzlan,§ and Clare P. Grey*†

The nature and concentrations of the local environments in a series of deuterated jarosite (nominally AFe3(SO4)2(OD)6 with A = K, Na, and D3O) samples with different levels of iron and cation vacancies have been determined by 2H MAS NMR spectroscopy at ambient temperatures. Three different local deuteron environments, Fe2OD, FeOD2, and D2O/D3O+, can be separated based on their very different Fermi contact shifts of δ ≈ 237, 70, and 0 ppm, respectively. The FeOD2 group arises from the charge compensation of the Fe3+ vacancies, allowing the concentrations of the vacancies to be readily determined. Analysis of the 2H quadrupole interaction indicates that the FeOD2 groups are mobile, undergoing rapid 180° flips on the NMR time scale; the D2O/D3O+ species (located on the A sites) undergo close to isotropic motion, whereas the Fe2OD groups are rigid and are hydrogen-bonded to nearby sulfate O atoms, with a (Fe)OD−O(S) distance of 2.79(4) Å. No evidence for the intrinsic protonation reaction Fe2OH + H3O+ → Fe2OH2 + H2O is found in the hydronium jarosite, suggesting that this mechanism is not the cause of the anomalous magnetic behavior of this material. The results illustrate that 2H MAS NMR spectroscopy is an excellent probe of the local environments and defects, on the atomic/molecular level, providing information that is complementary to diffraction techniques and that will help to rationalize the magnetic properties of these materials.

J. Am. Chem. Soc., ASAP Article 10.1021/ja709975z S0002-7863(70)09975-4 Web Release Date: February 23, 2008 Copyright © 2008 American Chemical Society
Direct NMR Detection of Alkali Metal Ions Bound to G-Quadruplex DNA
Ramsey Ida and Gang Wu*

We describe a general multinuclear (1H, 23Na, 87Rb) NMR approach for direct detection of alkali metal ions bound to G-quadruplex DNA. This study is motivated by our recent discovery that alkali metal ions (Na+, K+, Rb+) tightly bound to G-quadruplex DNA are actually "NMR visible" in solution (Wong, A.; Ida, R.; Wu, G. Biochem. Biophys. Res. Commun. 2005, 337, 363). Here solution and solid-state NMR methods are developed for studying ion binding to the classic G-quadruplex structures formed by three DNA oligomers: d(TG4T), d(G4T3G4), and d(G4T4G4). The present study yields the following major findings. (1) Alkali metal ions tightly bound to G-quadruplex DNA can be directly observed by NMR in solution. (2) Competitive ion binding to the G-quadruplex channel site can be directly monitored by simultaneous NMR detection of the two competing ions. (3) Na+ ions are found to locate in the diagonal T4 loop region of the G-quadruplex formed by two strands of d(G4T4G4). This is the first time that direct NMR evidence has been found for alkali metal ion binding to the diagonal T4 loop in solution. We propose that the loop Na+ ion is located above the terminal G-quartet, coordinating to four guanine O6 atoms from the terminal G-quartet and one O2 atom from a loop thymine base and one water molecule. This Na+ ion coordination is supported by quantum chemical calculations on 23Na chemical shifts. Variable-temperature 23Na NMR results have revealed that the channel and loop Na+ ions in d(G4T4G4) exhibit very different ion mobilities. The loop Na+ ions have a residence lifetime of 220 s at 15 C, whereas the residence lifetime of Na+ ions residing inside the G-quadruplex channel is 2 orders of magnitude longer. (4) We have found direct 23Na NMR evidence that mixed K+ and Na+ ions occupy the d(G4T4G4) G-quadruplex channel when both Na+ and K+ ions are present in solution. (5) The high spectral resolution observed in this study is unprecedented in solution 23Na NMR studies of biological macromolecules. Our results strongly suggest that multinuclear NMR is a viable technique for studying ion binding to G-quadruplex DNA.

No comments: