Probing Heteronuclear 15N−17O and 13C−17O Connectivities and Proximities by Solid-State NMR Spectroscopy
Ivan Hung†, Anne-Christine Uldry, Johanna Becker-Baldus, Amy L. Webber†, Alan Wong, Mark E. Smith, Siân A. Joyce, Jonathan R. Yates, Chris J. Pickard, Ray Dupree and Steven P. Brown
Heteronuclear solid-state magic-angle spinning (MAS) NMR experiments for probing 15N−17O dipolar and J couplings are presented for [2H(NH3),1-13C,15N,17O2]glycine·2HCl and [15N2,17O2]uracil. Two-dimensional 15N−17O correlation spectra are obtained using the R3-HMQC experiment; for glycine·2HCl, the intensity of the resolved peaks for the CO and C−O2H 17O resonances corresponds to the relative magnitude of the respective 15N−17O dipolar couplings. 17O−15N REDOR curves are presented for glycine·2HCl; fits of the initial buildup (ΔS/S < 0.2) yield effective dipolar couplings in agreement with (±20%) the root-sum-squared dipolar couplings determined from the crystal structure. Experimental 15N−17O REAPDOR curves for the 15N resonances in glycine·2HCl and uracil fit well to the universal curve presented by Goldbourt et al. (J. Am. Chem. Soc. 2003, 125, 11194). Heteronuclear 13C−17O and 15N−17O J couplings were experimentally determined from fits of the quotient of the integrated intensity obtained in a heteronuclear and a homonuclear spin−echo experiment, SQ(τ) = SHET(τ)/SHOM(τ). For glycine·2HCl, 1JCO was determined as 24.7 ± 0.2 and 25.3 ± 0.3 Hz for the CO and C−O2H resonances, respectively, while for uracil, the average of the two NH···O hydrogen-bond-mediated J couplings was determined as 5.1 ± 0.6 Hz. In addition, two-bond intramolecular J couplings, 2JOO = 8.8 ± 0.9 Hz and 2JN1,N3 = 2.7 ± 0.1 Hz, were determined for glycine·2HCl and uracil, respectively. Excellent agreement was found with J couplings calculated using the CASTEP code using geometrically optimized crystal structures for glycine·HCl [1JCO(CO) = 24.9 Hz, 1JCO(COH) = 27.5 Hz, 2JOO = 7.9 Hz] and uracil [2hJN1,O4 = 6.1 Hz, 2hJN3,O4 = 4.6 Hz, 2JN1,N3 = 2.7 Hz].