Eric Ye

Variable-temperature 17O NMR studies allow quantitative evaluation of molecular dynamics in organic solids

We report a comprehensive variable-temperature solid-state (17)O NMR study of three (17)O-labeled crystalline sulfonic acids: 2-aminoethane-1-sulfonic acid (taurine, T), 3-aminopropane-1-sulfonic acid (homotaurine, HT), and 4-aminobutane-1-sulfonic acid (ABSA). In the solid state, all three compounds exist as zwitterionic structures, NH(3)(+)-R-SO(3)(-), in which the SO(3)(-) group is involved in various degrees of O···H-N hydrogen bonding. High-quality (17)O NMR spectra have been obtained for all three compounds under both static and magic angle spinning (MAS) conditions at 21.1 T, allowing the complete set of (17)O NMR tensor parameters to be measured. Assignment of the observed (17)O NMR parameters to the correct oxygen sites in the crystal lattice was achieved with the aid of DFT calculations. By modeling the temperature dependence of (17)O NMR powder line shapes, we have not only confirmed that the SO(3)(-) groups in these compounds undergo a 3-fold rotational jump mechanism but also extracted the corresponding jump rates (10(2)-10(5) s(-1)) and the associated activation energies (E(a)) for this process (E(a) = 48 ± 7, 42 ± 3, and 45 ± 1 kJ mol(-1) for T, HT, and ABSA, respectively). This is the first time that SO(3)(-) rotational dynamics have been directly probed by solid-state (17)O NMR. Using the experimental activation energies for SO(3)(-) rotation, we were able to evaluate quantitatively the total hydrogen bond energy that each SO(3)(-) group is involved in within the crystal lattice. The activation energies also correlate with calculated rotational energy barriers. This work provides a clear illustration of the utility of solid-state (17)O NMR in quantifying dynamic processes occurring in organic solids. Similar studies applied to selectively (17)O-labeled biomolecules would appear to be very feasible.

Solid-State 17O NMR Spectroscopy of Large Protein–Ligand Complexes†

Solid-state O NMR spectroscopy has attracted considerable attention because of its potential as a new probe of biological structures. 2] One prerequisite for such applications is that the sensitivity of currently available methods for solid-state O NMR spectroscopy is sufficient to allow direct detection of weak O (I = 5/2) NMR signals from a biological macromolecule of significant size. Nearly 20 years ago, Oldfield et al. reported the first set of solid-state O NMR spectra for proteins, [O2]hemoglobin and [ O2]myoglobin, under nonspinning (stationary) conditions in a moderate magnetic field of 8.45 T. However, as the authors noted, the poor signalto-noise ratios obtainable at the time did not permit any detailed spectral analysis. Later, Oldfield and co-workers successfully obtained solid-state O NMR spectra for [CO]myoglobin (16.7 kDa per ligand) under conditions of magic-angle spinning (MAS) at 11.7 T. Several groups have since reported solid-state O NMR spectroscopic studies of membrane-bound peptides. Herein we report the first comprehensive solid-state O MAS NMR spectroscopic study of large protein–ligand complexes with an emphasis on addressing the sensitivity issue. In particular, we used two robust protein–ligand complexes, egg-white avidin–[O2]biotin (64 kDa) and ovotransferrin–Al–[O4]oxalate (80 kDa), as benchmark cases to test the detection limit at a high magnetic field of 21.14 T. We discovered that the O spin–lattice relaxation times (T1) in these solid protein–ligand complexes are typically on the order of several milliseconds; therefore, very rapid data collection is possible. Furthermore, we found that several sensitivity-enhancement methods uniquely suited for halfinteger quadrupolar nuclei, such as double-frequency sweep (DFS), rotor-assisted population transfer (RAPT), and hyperbolic secant (HS) pulses, can be used to obtain highquality solid-state O NMR spectra for large protein–ligand complexes. Avidin is a glycoprotein isolated from hen egg white that forms a tetramer with a total molecular weight of about 64 kDa and can bind biotin molecules with extremely high affinity (Kd = 10 m). The O MAS NMR spectrum of the avidin–[O2]biotin complex exhibits a typical line shape arising from second-order quadrupole interactions (Figure 1).

A 115In solid-state NMR study of low oxidation-state indium complexes

115In solid-state NMR (SSNMR) spectroscopy is applied to characterise a variety of low oxidation-state indium(I) compounds. 115In static wideline SSNMR spectra of several In(I) complexes were acquired with moderate and ultra-high field NMR spectrometers (9.4 and 21.1 T, respectively). 115In MAS NMR spectra were obtained with moderate and ultra-fast (>60 kHz) spinning speeds at 21.1 T. In certain cases, variable-temperature (VT) 115In SSNMR experiments were performed to study dynamic behaviour and phase transitions. The indium electric field gradient (EFG) and chemical shift (CS) tensor parameters were determined from the experimental spectra. With the aid of first principles calculations, the tensor parameters and orientations are correlated to the structure and symmetry of the local indium environments. In addition, calculations aid in proposing structural models for samples where single crystal X-ray structures could not be obtained. The rapidity with which high quality 115In SSNMR spectra can be acquired at 21.1 T and the sensitivity of the 115In NMR parameters to the indium environment suggest that 115In SSNMR is a powerful probe of the local chemical environments of indium sites. This work demonstrates that 115In NMR can be applied to a wide range of important materials for the purpose of increasing our understanding of structures and dynamics at the molecular/atomic level, especially for the characterisation of disordered, microcrystalline and/or multi-valence solids for which crystal structures are unavailable.

Solid-State 87Sr NMR Spectroscopy at Natural Abundance and High Magnetic Field Strength

Twenty-five strontium-containing solids were characterized via (87)Sr NMR spectroscopy at natural abundance and high magnetic field strength (B0 = 21.14 T). Strontium nuclear quadrupole coupling constants in these compounds are sensitive to the strontium site symmetry and range from 0 to 50.5 MHz. An experimental (87)Sr chemical shift scale is proposed, and available data indicate a chemical shift range of approximately 550 ppm, from -200 to +350 ppm relative to Sr(2+)(aq). In general, magnetic shielding increased with strontium coordination number. Experimentally measured chemical shift anisotropy is reported for stationary samples of solid powdered SrCl2·6H2O, SrBr2·6H2O, and SrCO3, with δaniso((87)Sr) values of +28, +26, and -65 ppm, respectively. NMR parameters were calculated using CASTEP, a gauge including projector augmented wave (GIPAW) DFT-based program, which addresses the periodic nature of solids using plane-wave basis sets. Calculated NMR parameters are in good agreement with those measured.

Aluminum environments in synthetic Ca-Tschermak clinopyroxene (CaAlAlSiO6) from Rietveld refinement, 27Al NMR, and first-principles calculations

Abstract 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR) and 27Al triple quantum (3Q) MAS NMR spectroscopy have been performed at 21.1 Tesla (T), as a direct probe of environment around Al in synthetic Ca-Tschermak’s clinopyroxene, CaAlAlSiO6, henceforward referred to as CaTs. For comparison, 27Al MAS NMR of CaTs has also been performed at 14.1 T. The 27Al 3QMAS NMR spectrum of CaTs has revealed various local environments around octahedral and tetrahedral Al, both ordered and disordered, symmetrical, and distorted. Rietveld refinement of powder X‑ray diffraction data has confirmed the high-temperature, long-range disordered C2/c structure for this sample. The 27Al MAS NMR spectra of CaTs look broadly similar at 14.1 and 21.1 T. Both spectra exhibit two distinct peaks at the octahedral site and an irregularly shaped, broad tetrahedral site. The line widths are significantly broader at lower field and the 27Al 3QMAS NMR spectrum at 21.1 T exhibits several additional peaks. At least three peaks were resolved at the octahedral site (in both the MAS spectrum along F2 and the 3Q spectrum along F1), whereas two peaks are clearly resolved at the tetrahedral site at 21.1 T. One tetrahedral peak, observed at both fields, is broad in F1 and narrow in F2, spread along the isotropic shift diagonal, indicating a highly disordered but relatively symmetric environment. The second peak, newly observed at 21.1 T, is narrow in F1 but very broad in F2, indicating an ordered but highly distorted environment. This peak has not been observed at lower magnetic field strengths. Octahedral peak assignments have been made according to the number of Al atoms in the six tetrahedral sites around M1, where an increasing number of NNN tetrahedral Al (2Al, 3Al, and 4Al) is correlated with displacement of the chemical shift to higher frequency. Assigned sites are similar to the local tetrahedral environments around M1 in ordered P21/n or C2 structures, suggesting the presence of ordered domains in this otherwise disordered C2/c structure. To aid in the interpretation of these tetrahedral Al environments, density functional theory (DFT) calculations have been performed using two approximations of the CaTs crystal structure: fully ordered tetrahedral chains or fully disordered tetrahedral chains. These calculations suggest that the tetrahedral Al site is a sensitive indicator of orderdisorder in CaTs. Tetrahedral Al sites in ordered tetrahedral chains (Si-O-Al-O-Si…) are predicted to have only large CQ, whereas tetrahedral Al sites in disordered systems (Al-O-Al-O-Al…) are predicted to have only small CQ. Both environments appear to exist in the synthetic CaTs sample in this study. Cation order-disorder has implications for thermobarometry based on CaTs-containing pyroxenes. The discovery of the new highly distorted tetrahedral site at ultrahigh magnetic field suggests that highly distorted Al sites in silicate minerals may be NMR-invisible in 27Al 3QMAS NMR spectra acquired at lower fields, and these will have been systematically overlooked. This underscores the necessity to collect 27Al NMR spectra of silicates at the highest available magnetic field strength.

Spin-lattice relaxation in aluminum-doped semiconducting 4H and 6H polytypes of silicon carbide

NMR spin-lattice relaxation efficiency is similar at all carbon and silicon sites in aluminum-doped 4H- and 6H-polytype silicon carbide samples, indicating that the valence band edge (the top of the valence band), where the holes are located in p-doped materials, has similar charge densities at all atomic sites. This is in marked contrast to nitrogen-doped samples of the same polytypes where huge site-specific differences in relaxation efficiency indicate that the conduction band edge (the bottom of the conduction band), where the mobile electrons are located in n-doped materials, has very different charge densities at the different sites. An attempt was made to observe (27)Al NMR signals directly, but they are too broad, due to paramagnetic line broadening, to provide useful information about aluminum doping.