Magic-angle Spinning 13C NMR Spectra Research Articles

Influence of Air Humidity Level on the Structure and Mechanical Properties of Thermoplastic Starch-Montmorillonite Nanocomposite during Storage.

Thermoplastic starch (TPS) consisting of corn starch and glycerol as a plasticizer, and TPS-montmorillonite (MMT) nanocomposite were stored at room temperature in the air with relative humidities (RH) of 11, 55 and 85% for seven weeks. Mechanical testing and dynamic mechanical thermal analysis (DMTA) were performed to detect changes in their mechanical properties. Solid-state NMR spectroscopy monitoring the changes in molecular mobility in the samples provided an insight into relations between mechanical properties and local structure. The results of mechanical testing indicated that the addition of MMT results in the increase in the tensile strength and Young's modulus while elongation at break decreased, indicating the reinforcing effect of MMT. DMTA experiments revealed a decrease in glass transition temperature of starch-rich phase below room temperature for samples stored at higher RH (55 and 85%). This indicates that absorbed water molecules had additional plasticizing effect on starch resulting in higher mobility of starch chain segments. Recrystallization in these samples was deduced from the shape of cross-polarization magic-angle spinning 13C NMR spectra. The shape of broad-line 1H NMR spectra reflected changes in molecular mobility in the studied samples during seven weeks of storage and revealed that a high amount of water molecules impacts the starch intermolecular hydrogen bond density.

Structural Studies of the Polymorphs of Carbamazepine, Its Dihydrate, and Two Solvates

High-field cross-polarisation magic-angle spinning 13C NMR spectra are presented for the four known polymorphs of anhydrous carbamazepine, for a dihydrate, and for two solvates. These are all distinctive, despite relatively low spectral dispersion, and give immediate information about the crystallographic asymmetric unit. The results for the trigonal and the two monoclinic forms are consistent with the published crystal structures. That of the triclinic form was found to contain four molecules in the crystallographic asymmetric unit, which has recently been confirmed by an X-ray diffraction study. NMR shows that the dihydrate has one molecule in the asymmetric unit, and the full crystal structure derived from single-crystal X-ray diffraction work is reported herein. It is found to be ordered and monoclinic, in contrast to the reported disordered orthorhombic structure. The discrepancy is attributed to the common occurrence of multiple micro-twinning. Shielding computations using a method which takes explicit account of the repetition inherent in a crystal lattice are reported for the P-monoclinic form and are compared to the experimental chemical shifts. The NMR data of all the forms are discussed in relation to variations in the molecular geometry of the hydrogen-bonded dimers (except in the case of two solvates). Chemical shift variations are explored as a function of the amide torsion using the Gaussian computer program.

The Effect of Bulk Magnetic Susceptibility on Solid State NMR Spectra of Paramagnetic Compounds

The effect of bulk magnetic susceptibility (BMS) on solid state NMR spectra of paramagnetic compounds was investigated theoretically and experimentally. The BMS shift was calculated for cylindrical and spherocylinderical containers with some ratios of the lengthLand the diameterD. The results show the best resolution can be obtained by using a long cylindrical sample container withL/D> 10 and by exciting only the region near the center of the container. The effect of the random orientations and distributions of crystallites in a powder sample was also calculated according to a model proposed by Schwerket al.[J. Magn. Reson. A119, 157 (1996)] with removing the Fermi contact term from their model. Static and the magic-angle spinning13C NMR spectra were recorded on two paramagnetic compounds of Ln(C2D5SO4)3· 8H2O where Ln = Pr, Yb. The modified theory predicts the BMS broadening of the experimental spectra very well.

A Cross-Polarization Magic-Angle Spinning 13C NMR Characterization of the Stable Solid-State Forms of Cimetidine

□ Proton decoupled, cross-polarization magic-angle spinning 13C NMR spectra of four polymorphic forms (A, B, C, and D) and a monohydrate form (M1) of the histamine H2 antagonist cimetidine were obtained, and the chemical shifts of the various forms were tabulated. A modified polarization inversion pulse sequence was used to distinguish quaternary, methine, methylene, and methyl carbon resonances and thereby assist spectral assignment. It is also shown that the solid-state form of cimetidine in a commercial formulation can be reliably ascertained by NMR, despite the presence in the spectrum of signals from organic excipients that are much more intense than those from the compound.

Comparison of black coral skeleton and insect cuticle by a combination of carbon-13 NMR and chemical analyses

Cross-polarization, magic-angle spinning 13C NMR spectra of skeletal components of individual colonies of the New Zealand black coral, Antipathes fiordensis, have a marked similarity to spectra of the sclerotized exoskeleton of the adult tobacco hornworm, Manduca sexta. NMR analysis estimates the organic content of the loadbearing skeletal base of A. fiordensis as 70% protein, 10% chitin, 15% diphenol, and 5% lipid by weight, and that of M. Sexta moth cuticle as 60% protein, 20% chitin, 15% diphenol, and 5% lipid. The younger pinnules or tips of A. fiordensis are less than 3% diphenol by weight. The only diphenols extracted from coral skeleton by hydrochloric acid are 3-(3,4-dihydroxyphenyl)- dl-alanine (DOPA) and 3,4-dihydroxybenzaldehyde (DOBAL), while the predominant diphenols in acid extracts of insect cuticles are N-acyldopamines. More DOPA is found in the base than in the tips of A. fiordensis and it appears to be a peptidyl component of coral skeletal protein. The oxidation of DOPA and DOBAL to quinones may provide mechanical stabilization of the coral skeleton by crosslinking of structural proteins to other proteins or to chitin.

High-Resolution Solid-State Cross-Polarization Magic-Angle Spinning 13C NMR of Metal-Free Phthalocyanines

Abstract The high-resolution solid-state cross-polarization magic-angle spinning 13C NMR spectra of α, β, τ and X-form metal-free phthalocyanines (H2pc) were investigated. Chemical shifts differed according to crystal forms. As a result of the temperature dependence of chemical shifts, we could observe the change in the spectra because of the inner protons.

Dielectric and13C NMR studies of the carbon monoxide clathrate hydrate

The structure I clathrate hydrate of carbon monoxide has been studied using dielectric measurements and13C NMR spectroscopy. Broad, weak dielectric absorption curves with maxima at 2.2–3.8 K yieldEa = 0.14 kJ mol−1 for the average Arrhenius activation energy associated with the reorientation of the low polarity guest. Except for H2S this represents the fastest reorienting polar guest known among the clathrate hydrates. The low temperature dielectric absorption curves can best be fitted with a Cole-Davidson asymmetric distribution of relaxation times and activation energies (withθ = 0.06 at 4 × 106 Hz), which at 107 Hz has been resolved into a double symmetric distribution of discrete relaxation times for CO in the small and large cages. The cross-polarization magic angle spinning13C NMR spectra indicate identical chemical shifts for CO in the small and large cages, in contrast to other hydrates. The static spectra show that the CO molecules undergo anisotropic reorientation in the large cages and that there is still considerable mobility at 77 K. One possible model for the anisotropic motion has the CO rapidly moving among sites over each of the 14 faces of the cage with the CO axis orientated towards the cage centre. The cage occupancy ratio at 220 K,θs/θL = 1.11, indicates slightly greater preference of CO for the small cage.

Effects of unsaturation on 2H-NMR quadrupole splittings and 13C-NMR relaxation in phospholipid bilayers

Motional order and motional rates in unsonicated phospholipid bilayers were assessed as a function of unsaturation of the phospholipid. A measurement sensitive to motional order was obtained using 2H-NMR of 18:1, 18:1-phosphatidylcholine labelled at positions 9 and 10 with deuterium and included as a probe in phospholipid bilayers of interest at 10 mole percent. Spin lattice relaxation times from magic angle spinning 13C-NMR spectra of phospholipid dispersions of interest were used as a measure of motional rates. Measurements were made of phospholipid bilayers containing from 0 to 8 double bonds per molecule. No large effect of an increase in unsaturation was noted for the 2H-NMR quadrupole splittings or for the 13C-NMR spin lattice relaxation rate.

Direct measurement of poly(beta-hydroxybutyrate) in a pseudomonad by solid-state 13C NMR.

Four narrow lines are observed in the high-resolution cross-polarization magic-angle spinning 13C NMR spectra of intact, lyophilized samples of Pseudomonas sp. LBr, in addition to the broader lines normally associated with bacterial cellular material. These narrow lines arise from poly(beta-hydroxybutyrate). The cellular carbon contained in this storage material can be measured quantitatively and nondestructively from the solid-state NMR spectra. We find that cells starved for phosphorus store up to 50% of their total carbon as poly(beta-hydroxybutyrate). When such cells are used to inoculate medium containing a source of phosphorus, all of the poly(beta-hydroxybutyrate) is metabolized by the time the culture has reached midlogarithmic growth phase.

Suppression of spinning sidebands in magic-angle-spinning NMR spectroscopy

In our group magic-angle-spinning (MAS) i3C NMR spectroscopy has been used to study tobacco mosaic virus (TMV), a well-known virus with a molecular weight of 42 X 106, that consists of 2200 identical coat protein subunits and a single RNA chain of 6600 nucleotides. Well-resolved MAS 13C NMR spectra have been obtained in which several carbon resonances of amino acids of the coat protein can be assigned, which are not observable in conventional high-resolution NMR spectroscopy (I). The appearance of spinning sidebands in MAS 13C NMR spectra of this large biomolecule is a severe problem since interesting resonances can be obscured by the spinning sidebands. This is especially the case at high NMR measuring frequencies (i.e., 75 MHz), even at the maximal achievable spinning frequency (i.e., ~4.5 kHz). In Fig. 1 a comparison is given of the MAS 13C NMR spectra of TMV at 45 and 75 MHz. Note that at 75 MHz the spinning sidebands of the carbonyl resonances strongly overlap with the aromatic resonances and vice versa. Also observe a dramatic increase of the intensity of the spinning sidebands in the 75-MHz spectrum as compared to the 45-MHz one. To make full use of the advantages of MAS NMR at high measuring frequencies (i.e., an increase of signal-to-noise ratio and resolution), a suitable technique for removing or suppressing the spinning sidebands would be extremely valuable to properly analyze complicated spectra as shown in Fig. 1. A number of methods have been proposed that identify or suppress spinning sidebands (2-6). For MAS NMR spectra of biosystems in which resonances appear over a broad range of chemical shift values, a method that shifts the phase of the spinning sidebands in combination with coaddition of spectra (6) seems to be the most promising one. Dixon (6) utilized a pulse sequence of four 180” pulses to alter the phase of the spinning sidebands. In this communication, we introduce a simple alternative in which a single 180” pulse is used to alter the phase of the spinning sidebands. In addition we demonstrate that the effects of a 180” pulse on spinning sidebands can easily be observed in a model system consisting of a spinning sample in a conventional high-resolution NMR spectrometer. We also show some calculations explaining these effects.

Cross-polarization magic-angle spinning 13C NMR spectra of coals of varying rank

Cross-polarization, magic-angle spinning 13C nuclear magnetic resonance spectra have been obtained on a series of coals that span the ASTM rank classification. Aromaticities thus obtained follow the classification reasonably well, increasing as rank increases NMR parameters were extensively varied to determine optimum conditions for the cross polarization contact time and pulse repetition rate. Good correlations were obtained between both the fixed carbon and the volatile matter obtained from the proximate analysis, and the aromatic carbon obtained from NMR. Generally the NMR spectra of lignites and low rank coals exhibit varying degrees of fine structure.

Cross polarization magic-angle spinning 13C NMR spectra of oil shales

Cross plarization magic-angle spinning 13C NMR spectra have been obtained on oil shales representing a variety of geologic ages, origins, depositional environments, and source locations. The spectra show variations in the aliphatic and aromatic carbon distributions of the oil shales and reveal correlations between aliphatic carbon contents and potential shale oil yields. Hints of additional fine structure are present in the spectra of some samples, and examples are given of the spectral resolution that may be obtainable on other solid samples of geochemical interest.