Probing heteronuclear N-15-O-17 and C-13-O-17 connectivities and proximities by solid-state NMR spectroscopy

Heteronuclear solid-state magic-angle spinning (MAS) NMR experiments for probing N-15-O-17 dipolar and J couplings are presented for [H-2(NH3),1-C-13,N-15,O-17(2)]glycine center dot(HCl)-H-2 and [N-15(2), O-17(2)]uracil. Two-dimensional N-15-O-17 correlation spectra are obtained using the R-3-HMQC experiment; for glycine center dot(HCl)-H-2, the intensity of the resolved peaks for the C=O and C-(OH)-H-2 O-17 resonances corresponds to the relative magnitude of the respective N-15-O-17 dipolar couplings. O-17-N-15 REDOR curves are presented for glycine center dot(HCl)-H-2; fits of the initial buildup (Delta S/S < 0.2) yield effective dipolar couplings in agreement with (+/- 20%) the root-sum-squared dipolar couplings determined from the crystal structure. Experimental N-15-O-17 REAPDOR curves for the N-15 resonances in glycine center dot(HCl)-H-2 and uracil fit well to the universal curve presented by Goldbourt et al. (J. Am. Chem. Soc. 2003, 125,11194). Heteronuclear C-13-O-17 and N-15-O-17 J couplings were experimentally determined from fits of the quotient of the integrated intensity obtained in a heteronuclear and a homonuclear spin-echo experiment, S-Q(tau) = S-HET(tau)/S-HOM(tau). For glycine center dot(HCl)-H-2, (1)J(CO) was determined as 24.7 +/- 0.2 and 25.3 +/- 0.3 Hz for the C=O and C-(OH)-H-2 resonances, respectively, while for uracil, the average of the two NH center dot center dot center dot O hydrogen-bond-mediated J couplings was determined as 5.1 +/- 0.6 Hz. In addition, two-bond intramolecular J couplings, (2)J(OO) = 8.8 +/- 0.9 Hz and (2)J(N1,N3) = 2.7 +/- 0.1 Hz, were determined for glycine center dot(HCl)-H-2 and uracil, respectively. Excellent agreement was found with J couplings calculated using the CASTEP code using geometrically optimized crystal structures for glycine center dot HCl [(1)J(CO)(C=O) = 24.9 Hz, (1)J(CO)(C-OH) = 27.5 H Z, (2)J(OO) = 7.9 Hz] and uracil [(2h)J(N1,O4) = 6.1 Hz, (2h)J(N3,O4) = 4.6 Hz, (2)J(N1,N3) = 2.7 Hz].