Multinuclear Magic-angle Spinning NMR Research Articles

A novel multinuclear solid-state NMR approach for the characterization of kidney stones.

The spectroscopic study of pathological calcifications (including kidney stones) is extremely rich and helps to improve the understanding of the physical and chemical processes associated with their formation. While Fourier transform infrared (FTIR) imaging and optical/electron microscopies are routine techniques in hospitals, there has been a dearth of solid-state NMR studies introduced into this area of medical research, probably due to the scarcity of this analytical technique in hospital facilities. This work introduces effective multinuclear and multidimensional solid-state NMR methodologies to study the complex chemical and structural properties characterizing kidney stone composition. As a basis for comparison, three hydrates (, 2 and 3) of calcium oxalate are examined along with nine representative kidney stones. The multinuclear magic angle spinning (MAS) NMR approach adopted investigates the , , and nuclei, with the and MAS NMR data able to be readily deconvoluted into the constituent elements associated with the different oxalates and organics present. For the first time, the full interpretation of highly resolved NMR spectra is presented for the three hydrates, based on the structure and local dynamics. The corresponding MAS NMR data indicates the presence of low-level inorganic phosphate species; however, the complexity of these data make the precise identification of the phases difficult to assign. This work provides physicians, urologists and nephrologists with additional avenues of spectroscopic investigation to interrogate this complex medical dilemma that requires real, multitechnique approaches to generate effective outcomes.

Investigation of structure and dynamics in a photochromic molecular crystal by NMR crystallography.

A photochromic anil, N-(3,5-di-t-butylsalicylidene)-4-amino-pyridine, has been studied by single-crystal X-ray diffraction, multinuclear magic-angle spinning NMR, and first-principles density functional theory (DFT) calculations. Interpretation of the solid-state NMR data on the basis of calculated chemical shifts confirms the structure is primarily composed of molecules in the ground-state enol tautomer, whereas thermally activated cis-keto and photoisomerised trans-keto states exist as low-level defects with populations that are too low to detect experimentally. Variable temperature 13 C NMR data reveal evidence for solid-state dynamics, which is found to be associated with fast rotational motion of t-butyl groups and 180° flips of the pyridine ring, contrasting the time-averaged structure obtained by X-ray diffraction. Comparison of calculated chemical shifts for the full crystal structure and an isolated molecule also reveals evidence for an intermolecular hydrogen bond involving the pyridine ring and an adjacent imine carbon, which facilitates the flipping motion. The DFT calculations also reveal that the molecular conformation in the crystal structure is very close to the energetic minimum for an isolated molecule, indicating that the ring dynamics arise as a result of considerable steric freedom of the pyridine ring and which also allows the molecule to adopt a favourable conformation for photochromism.

Multinuclear Solid-State NMR Investigation of Hexaniobate and Hexatantalate Compounds.

This work determines the potential of solid-state NMR techniques to probe proton, alkali, and niobium environments in Lindqvist salts. Na7HNb6O19·15H2O (1), K8Nb6O19·16H2O (2), and Na8Ta6O19·24.5H2O (3) have been studied by solid-state static and magic angle spinning (MAS) NMR at high and ultrahigh magnetic field (16.4 and 19.9 T). (1)H MAS NMR was found to be a convenient and straightforward tool to discriminate between protonated and nonprotonated clusters AxH8-xM6O19·nH2O (A = alkali ion; M = Nb, Ta). (93)Nb MAS NMR studies at different fields and MAS rotation frequencies have been performed on 1. For the first time, the contributions of NbO5Oμ2H sites were clearly distinguished from those assigned to NbO6 sites in the hexaniobate cluster. The strong broadening of the resonances obtained under MAS was interpreted by combining chemical shift anisotropy (CSA) with quadrupolar effects and by using extensive fitting of the line shapes. In order to obtain the highest accuracy for all NMR parameters (CSA and quadrupolar), (93)Nb WURST QCPMG spectra in the static mode were recorded at 16.4 T for sample 1. The (93)Nb NMR spectra were interpreted in connection with the XRD data available in the literature (i.e., fractional occupancies of the NbO5Oμ2H sites). 1D (23)Na MAS and 2D (23)Na 3QMAS NMR studies of 1 revealed several distinct sodium sites. The multiplicity of the sites was again compared to structural details previously obtained by single-crystal X-ray diffraction (XRD) studies. The (23)Na MAS NMR study of 3 confirmed the presence of a much larger distribution of sodium sites in accordance with the 10 sodium sites predicted by XRD. Finally, the effect of Nb/Ta substitutions in 1 was also probed by multinuclear MAS NMR ((1)H, (23)Na, and (93)Nb).

NMR crystallography of an oxovanadium(V) complex by an approach combining multinuclear magic angle spinning NMR, DFT, and spin dynamics simulations.

Bioinorganic vanadium(V) solids are often challenging for structural analysis. Here, we explore an NMR crystallography approach involving multinuclear (13) C/(51) V solid-state NMR spectroscopy, density functional theory (DFT), and spin dynamics numerical simulations, for the spectral assignment and the 3D structural analysis of an isotopically unmodified oxovanadium(V) complex, containing 17 crystallographically inequivalent (13) C sites. In particular, we report the first NMR determination of C-V distances. So far, the NMR observation of (13) C-(51) V proximities has been precluded by the specification of commercial NMR probes, which cannot be tuned simultaneously to the close Larmor frequencies of these isotopes (100.6 and 105.2 MHz for (13) C and (51) V, respectively, at 9.4 T). By combining DFT calculations and (13) C-(51) V NMR experiments, we propose a complete assignment of the (13) C spectrum of this oxovanadium(V) complex. Furthermore, we show how (13) C-(51) V distances can be quantitatively estimated.

Study of the cytotoxicity and particle action in human cancer cells of titanocene-functionalized materials with potential application against tumors

Titanocene dichloride [Ti(η 5-C 5H 5) 2Cl 2] ( 1), has been grafted onto dehydrated hydroxyapatite (HAP), Al 2O 3 and two mesoporous silicas MSU-2 (Michigan State University Silica type 2) and HMS (Hexagonal Mesoporous Silica), to give the novel materials HAP/[Ti(η 5-C 5H 5) 2Cl 2] ( S1) (1.01 wt.% Ti), Al 2O 3/[Ti(η 5-C 5H 5) 2Cl 2] ( S2) (2.36 wt.% Ti), HMS/[Ti(η 5-C 5H 5) 2Cl 2] ( S3) (0.75 wt.% Ti) and MSU-2/[Ti(η 5-C 5H 5) 2Cl 2] ( S4) (0.74 wt.% Ti), which have been characterized by powder X-ray diffraction, X-ray fluorescence, nitrogen gas sorption, multinuclear magic angle spinning NMR spectroscopy, IR spectroscopy, thermogravimetry analysis, UV spectroscopy, scanning electronic microscopy and transmission electronic microscopy. The cytotoxicity of the titanocene-functionalized materials toward human cancer cell lines from five different histogenic origins: 8505 C (anaplastic thyroid cancer), A253 (head and neck cancer), A549 (lung carcinoma), A2780 (ovarian cancer) and DLD-1 (colon cancer) has been determined. M 50 values (quantity of material needed to inhibit normal cell growth by 50%) and Ti-M 50 values (quantity of anchored titanium needed to inhibit normal cell growth by 50%) indicate that the activity of S1– S4 against studied human cancer cells depended on the surface type as well as on the cell line. In addition, studies on the titanocene release and the interaction of the materials S1– S4 with DNA show that the cytotoxic activity may be due to particle action, because no release of titanium complexes has been observed in physiological conditions, while electrostatic interactions of titanocene-functionalized particles with DNA have been observed.

Structure of (NH4)3GaF6 Investigated by Multinuclear Magic-Angle Spinning NMR Spectroscopy in Comparison with Rietveld Refinement

The structure of ammonium gallium cryolite (NH(4))(3)GaF(6) was investigated by (19)F and (69,71)Ga magic-angle spinning (MAS) NMR in comparison with X-ray powder diffraction followed by Rietveld refinement. In agreement with previous thermodynamic measurements, NMR experiments on (NH(4))(3)GaF(6) support the model of rigid GaF(6) octahedra. At high spinning speeds (30 kHz), the scalar coupling between the six equivalent (19)F nuclei and (69,71)Ga can be directly observed in the powder spectra. The coupling constants are J(19)F(69)Ga = 197 Hz and J(19)F(71)Ga = 264 Hz. To explain the (71)Ga spectra recorded at 3 kHz a small distribution of quadrupolar frequencies has to be included. The spread of the spinning sidebands hints to a largest nu(Q) value of 28 kHz for (71)Ga. This can be explained by the occurrence of highly symmetric GaF(6) octahedra, which are tilted against the surrounding atoms. In addition, the incomplete motional excitation does not average out the quadrupolar effects. NMR findings are in discrepancy to those of Rietveld refinement. As result it appears that X-ray diffraction is not sensitive enough to deliver proper results.

Structural and Catalytic Studies of a Trimethyltin Vanadate Coordination Polymer

The organotin vanadate [Me3SnVO3] (1) has been prepared and characterised in the solid state by powder X-ray diffraction (XRD), thermogravimetric analysis, multinuclear magic-angle spinning (MAS) NMR, IR and Raman spectroscopy. The phase purity and structure of microcrystalline 1 were confirmed by carrying out a full Rietveld structural refinement at ambient temperature and from conventional powder XRD. 51V and 119Sn MAS NMR data for compound 1 were in agreement with the predicted structure, showing two equally-abundant, nonequivalent Me3Sn groups and two equally-abundant, nonequivalent vanadium atoms. The compound was applied as a catalyst for the liquid-phase epoxidation of olefins at 55 °C using tert-butyl hydroperoxide (tBuOOH) as the oxidant. The reaction rate for the different substrates followed the order cis-cyclooctene > (R)-(+)-limonene ≅ trans-2-octene > cyclododecene > styrene > 1-octene; the corresponding epoxides were the only observed products. Leaching tests indicated that the catalytic epoxidation of cyclooctene was mainly heterogeneous in nature.

Short-Range Order and Site Connectivities in Sodium Aluminoborate Glasses: I. Quantification of Local Environments by High-Resolution 11B, 23Na, and 27Al Solid-State NMR

The local structure of glasses in the system Na2O−B2O3−Al2O3 is studied by multinuclear magic-angle spinning (MAS) solid-state NMR spectroscopy at 7.0 and 11.7 T. The 23Na isotropic chemical shifts vary linearly with Na2O concentration, indicating that the sodium ions are homogeneously distributed over the network and not clustered. The 11B MAS NMR spectra reveal the presence of trigonal BO3/2 units, tetrahedral BO4/2- sites, and three-coordinate BO2/2O- species containing nonbridging oxygen. The quantitative contributions of these three types of sites are obtained by detailed computer simulations of these spectra. High-resolution 27Al satellite transition spectroscopy indicates that aluminum is predominantly present as AlO4/2- sites. The structure of these glasses is discussed in terms of various melt reaction schemes, in which the network former oxides B2O3 and Al2O3 react with O2- introduced by the network modifier Na2O. While the data suggest that the formation of AlO4/2- units takes priority over bor...

Solid-State 27Al and 29Si NMR and H CRAMPS Studies of the Thermal Transformations of the 2:1 Phyllosilicate Pyrophyllite

Solid-state multinuclear magic-angle spinning (MAS) NMR studies of the thermal transformations of the 2:1 phyllosilicate mineral, pyrophyllite, over the temperature range 150−1350 °C are reported. 27Al and 29Si NMR and 1H CRAMPS techniques have been used to follow the progress of dehydroxylation between 150 and 550 °C. At 550 °C, pyrophyllite is completely dehydroxylated in 7 days to pyrophyllite dehydroxylate, an aluminosilicate intermediate containing 5-coordinate aluminum, on the basis of the MAS 27Al NMR measurements at 14 T. MAS 27Al and CP/MAS (cross-polarization) and SP/MAS (single-pulse) 29Si NMR results indicate that the dehydroxylate is formed prior to the separation of the silica−alumina layer. At 950 °C, the thermally induced transformation of pyrophyllite anhydride results in separation of the silica−alumina layer. A transition-alumina-type phase, containing 4- and 6-coordinate aluminum, is formed between 950 and 1050 °C. In addition, a high content of amorphous silica glass and a small amount of a poorly ordered Si/Al-containing mullite phase forms between 950 and 1050 °C. At 1250−1350 °C, the 29Si NMR shows that this glassy silica is converted to cristobalite, while the 27Al NMR indicates that this process is accompanied by conversion of octahedral aluminums to tetrahedral aluminums, possibly by incorporation of aluminums into an amorphous Si/Al-containing phase. The mechanism of dehydroxylation and of the higher temperature transformations of pyrophyllite dehydroxylate are discussed in light of these multinuclear solid-state MAS NMR results.

Cytochrome c interactions with cardiolipin in bilayers: a multinuclear magic-angle spinning NMR study.

The influence of cytochrome c binding to cardiolipin bilayers on the motional characteristics of each component has been analyzed by magic-angle spinning (MAS) NMR. Observations were made by NMR of natural abundance 31P, 13C, and 1H nuclei in the lipid as well as sites enriched with 13C in the protein. Analysis of methyl carbons enriched in ([epsilon-13CH3]methionine)cytochrome c at residues 65 and 80 reveal quite different behavior for these sites when the protein was bound at a 1:15 molar ratio with hydrated cardiolipin. Cross-polarization (CP) shows a single broad resonance downfield in the methyl region which corresponds to the spectral characteristics of methionine 65 in the solution protein when subjected to moderate thermal perturbations. These observations suggest that although methionine 65 remains motionally restricted when the protein binds to the lipid bilayers, this residue becomes less shielded and exposed to more chemically distinct environments than in the native state of the protein. In contrast to its behavior in native oxidized protein, the methionine 80 methyl could be detected following direct pi/2 pulse excitation, and this residue is assumed to be released from the axial ligand site on the heme iron to become more exposed and highly mobile in the protein-lipid complex. An analysis of the CP response for natural abundance 13C nuclei in the lipid reveals a general increase in motions with slower rates (tens of kilohertz) on binding with cytochrome c, except for sites within the region of fatty acyl chain unsaturation which appear to be selectively mobilized in the complex with protein. It is concluded that, aside from effects on the unsaturated segments, the bound protein induces new modes of slow motions in the lipid assemblies rather than restricting the overall reorientation freedom of the lipid. The strong paramagnetic effects observed previously on the relaxation of phosphorus in protein-bound lipid [Spooner, P.J.R., & Watts, A. (1991) Biochemistry 30, 3880-3885] were not extended to any carbon and proton sites observable by MAS NMR in the lipid, and this infers a specific interaction of lipid phosphate groups with the heme. However, when protein was bound to cardiolipin mixed at a 1:4 mole ratio with dioleoylphosphatidylcholine in bilayers, no direct interaction with the heme was apparent from the phosphorus NMR relaxation behavior in this component, resolved by MAS. Instead, the spectral anisotropy of cardiolipin phosphorus was determined to be reduced, indicating that, on binding with cytochrome c, the headgroup organization was perturbed in this component.(ABSTRACT TRUNCATED AT 400 WORDS)