Cross-polarization Magic-angle Sample Spinning Research Articles

Recent progress on solid-state cross-polarization (CP)/magic angle spinning (MAS) NMR measurements: a tool for investigating structural changes in stimuli-responsive organic compounds

Abstract Over the years, cross-polarization/magic angle sample spinning (CP/MAS) NMR has proven to be valuable for obtaining detailed structural information in solid-state materials. A notable advantage of this technique is that it does not require a well-organized arrangement in the sample such as single crystals. Consequently, even materials with low crystallinity such as amorphous substances can be analyzed to obtain structural information. In recent years, research on externally stimuli-responsive molecules utilizing crystal phase transitions has attracted attention. Certain aryl disilane and oligosilane compounds exhibit external stimulus responsiveness even in the crystalline state due to the hindered rotation of the Si–Si σ bond and sparse crystal packing. Understanding the structural alternations linked to changes in physical properties is crucial for investigation of the responsiveness. High-resolution CP/MAS NMR spectra are particularly suitable for analyzing crystalline polymorphs, crystal phase transitions, and solid-state conformations. In this report, we present the results of natural abundance 13C, 29Si, and 15N CP/MAS NMR experiments conducted on externally stimuli-responsive disilane-bridged molecules. These measurements provide information on the structure before and after crystal phase transitions, as well as the amorphous state. A comprehensive analysis of our findings is essential for elucidating alternations in the structure and arrangement of crystalline materials under external stimuli.

Changes to the Contour Length, Molecular Chain Length, and Solid-State Structures of Nanocellulose Resulting from Sonication in Water.

Sonication in water reduced the average contour lengths of nanocellulose prepared from wood cellulose fiber and microcrystalline cellulose. Most of the kinks in the wood cellulose nanofibrils were formed during the initial 10 min of sonication. Fragmentation occurred at the kinks and rigid segments associated with depolymerization during subsequent sonication for 10-120 min, resulting in the formation of cellulose nanocrystals with low aspect ratios. Solid-state cross-polarization magic angle sample spinning 13C-nuclear magnetic resonance revealed that the original crystalline regions of the cellulose were partly transformed to fibril surfaces or disordered regions by both pretreatment and the subsequent fragmentation of molecular chains during sonication. The nanocellulose prepared from microcrystalline cellulose had different fragmentation behavior with regard to molecular chain length following sonication. The results indicated that on average the hexagonal 36 cellulose chain structure formed the cross-section of each wood cellulose microfibril.

Comparison of the Structural Properties and Nutritional Fraction of Corn Starch Treated with Thermophilic GH13 and GH57 α-Glucan Branching Enzymes.

Two thermophilic 1,4-α-glucan branching enzymes (GBEs), CbGBE from Caldicellulosiruptor bescii and PhGBE from Pyrococcus horikoshii, which belong to the glycoside hydrolase family 13 and 57 respectively, were cloned and expressed in Escherichia coli. Two GBEs were identified to have α-1,6 branching activity against various substrates, but substrate specificity was distinct. Starch was modified by two GBEs and their in vitro digestibility and structural properties were investigated. Short-branched A chains with a degree of polymerization (DP) of 6–12 increased with CbGBE-modified starch, increasing the proportion of slow digestible and resistant starch (RS) fractions. PhGBE-modified starch resulted in an increase in the RS fraction only by a slight increase in part of A chains (DP, 6–9). Compared to the proportion of control not treated with GBE, the proportion of α-1,6 linkages in CbGBE- and PhGBE-modified starch increased by 3.1 and 1.6 times. 13C cross polarization/magic angle sample spinning (CP/MAS) NMR and XRD pattern analysis described that GBE-modified starches reconstructed double helices but not the crystalline structure. Taken together, CbGBE and PhGBE showed distinct branching activities, resulting in different α-1,6 branching ratios and chain length distribution, and double helices amount of starch, ultimately affecting starch digestibility. Therefore, these GBEs can be used to produce customized starches with controlled digestion rates.

13C CP/MAS NMR study of accommodation of various organic molecules in (±)-[Co(en) 3]Cl 3 ionic crystal and their local structure

Accommodation of various organic molecules into a one-dimensional nanochannel of tris(ethylenediamine) cobalt(III) chloride, [Co(en) 3]Cl 3, anhydrated racemic crystal was examined using weight increment measurements, thermogravimetric–differential thermal analysis (TG–DTA), powder X-ray diffraction (PXRD) and 13C cross-polarization/magic angle sample spinning (CP/MAS) NMR techniques. Results showed that the (±)-[Co(en) 3]Cl 3 ionic crystal accommodates n-alkanes, n-alkylalcohols, n-alkylamines, and acetonitrile in the 1D nanochannel without decomposition of the crystal lattice. The guest molecules are in all-trans conformation in the nanochannel. Furthermore, the activation energies of uni-axial molecular reorientation of n-alkanes and n-alkylalcohols in the nanochannel were determined using temperature dependence of the 1H spin–lattice relaxation time in the rotating frame ( T 1ρ). Results demonstrated that the activation energy per CH 2 unit (6.5 kJ mol −1) in n-alkylalcohols was twice that in n-alkanes (3.3 kJ mol −1), implying the possibility of dimer formation of n-alkylalcohols in the nanochannel by hydrogen bonding through OH group. These findings strongly suggest that the (±)-[Co(en) 3]Cl 3 ionic crystal has adsorption ability to various organic molecules. It will open new avenues to explore new types of zeolites that include ionic crystals.

Cocrystallization and phase segregation of blends of poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

The competition between cocrystallization and phase segregation in blends of poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB–HV) containing 9 and 15% HV has been studied by 13C cross-polarization magic-angle sample spinning (CPMAS) NMR and DSC techniques. The PHB homopolyester samples with selectively 13C-enriched methylene carbon (PHB∗) were used. Assuming a simple two-phase model, the 13C resonances were resolved into two peaks arising from the crystalline and amorphous phases. Owing to the 13C-enrichment of PHB∗, the relative areas of the crystalline peaks for the methylene and the methine carbon resonances on 13C CPMAS NMR spectra change depending on the PHB∗ content in the crystalline phase. By comparing the ratio of these areas for the blends with those for pure PHB∗ and PHB–HV, the composition in the crystalline phase of the blends was determined. For PHB∗/PHB–9%HV blends, the composition in the crystalline phase is very similar to that in the blend. This means that almost perfect cocrystallization of PHB and PHB–9%HV occurs in these blends. For PHB∗/PHB–15%HV blends, the PHB content in the crystalline phase is larger than that in the whole blend. The phase segregation precedes the crystallization in these blends. Therefore, the degree of phase segregation (or the percentage of PHB–HV that segregates from the growth front of crystals) changes depending on the HV content of PHB–HV. As the HV content increases, the copolymer content in the crystalline phase decreases.

13CP/MAS NMR Study of Motion and Local Structure of Phenethyl- ammonium Ion in [C6H5CH2CH2 NH3]2PbX4(X = Cl, Br, I)

Abstract Ionic motion and local structure of phenethylammonium ion (C6H5CH2CH2NH3+) in a series of phenethylammonium lead(II) halides, [C6H6CH2CH2NH3]2PbX4 (X = Cl, Br, I) were studied by means of 13C cross polarization/magic angle sample spinning (CP/MAS) NMR technique. Among the three salts, remarkable differences in the spectra were observed in the signals corresponding to the phenyl carbons. The peaks from C2, C3, C5, and C6 in the phenyl group were split into three for the chloride and into two for the bromide, although in the iodide those were observed as a single peak. Coalescence of those peaks, as well as line broadening were observed on heating. This indicated that peak split brings about disorder of the orientation in the phenyl group around the C1-C4 axis, suggesting to have three and two orientations for the chloride and the bromide, respectively. Above room temperature the phenyl group undergoes chemical exchange among these orientations, and at higher temperature, reorientation with a large amplitude takes place around its axis. The apparent activation energies of the reorientation of the phenyl group for the chloride and bromide were estimated from the temperature dependence of the linewidth of the resonance peaks to be about 24 kJ mol-1 , 25 kJ mol-1 , respectively, which is similar to in the iodide (25.0 kJ mol-1). Ab initio molecular orbital energy calculation was carried out to evaluate the potential barrier of the internal rotation of the phenyl group in a free phenethylammonium ion. The intramolecular interaction was evaluated to be 13.9 kJ mol-1 from the calculation, and the intermolecular interaction results to be 10 kJ mol-1 .

New class of cellulose fiber spun from the novel solution of cellulose by wet spinning method

AbstractA novel cellulose solution, prepared by dissolving an alkali‐soluble cellulose, which was obtained by the steam explosion treatment on almost pure natural cellulose (soft wood pulp), into the aqueous sodium hydroxide solution with specific concentration (9.1 wt %) was employed for the first time to prepare a new class of multifilament‐type cellulose fiber. For this purpose a wet spinning system with acid coagulation bath was applied. The mechanical properties and structural characteristics of the resulting cellulose fibers were compared with those of regenerated cellulose fibers such as viscose rayon and cuprammonium rayon commercially available. X‐ray analysis shows that the new cellulose fiber is crystallographically cellulose II, and its crystallinity is higher but its crystalline orientation is slightly lower than those of other commercial regenerated fibers. The degree of breakdown of intramolecular hydrogen bond at C3[Xam(C3)] of the cellulose fiber, as determined by solid‐state cross‐polarization magic‐angle sample spinning (CP/MAS) 13C NMR, is much lower than other, and the NMR spectra of its dry and wet state were significantly different from each other, indicating that cellulose molecules in the new cellulose fiber are quite mobile when wet. This phenomenon has not been reported for so‐called regenerated cellulose fibers.

Silicon‐29 solid‐state nuclear magnetic resonance spectroscopy of composite interfaces

AbstractThe types of structures and bonds that are formed with silicons in the composite interface were studied using 29Si cross‐polarization/magic angle sample spinning (CP/MAS) nuclear magnetic resonance spectroscopy (NMR). The change in mobility of silane coupling agent bonded to silica, as compared with bulk hydrolyzed silane coupling agent, can be monitored by the change in line width and the shift of resonances to higher fields, as well as by the change in the silicon‐proton cross‐polarization time TSiH. In the silane coupling agent‐matrix resin interface, the TSiH values reflect the change in mobility as a function of the concentration and degree of hydrolysis of the silane coupling agent. It has been demonstrated that quantitative measurements of TSiH can be used to investigate relative mobilities.

Physiological and 15N-NMR analysis of molecular nitrogen fixation by Methanococcus thermolithotrophicus, Methanobacterium bryantii and Methanospirillum hungatei

Two mesophilic methanogenic bacteria, Methanobacterium bryantii strain MOH and Methanospirillum hungatei strain GP1 were demonstrated, using several different experimental approaches, to fix dinitrogen. Evidence includes (1) growth with N 2 as the sole nitrogen source; (2) incorporation of 15N 2 into cellular material (both soluble amino acid pools and insoluble cell protein and other macromolecules) detected by 15N-NMR spectroscopy; (3) acetylene reduction to ethylene by the cells, and inhibition of this reaction by bromoethanesulfonic acid (BES), a methanogen inhibitor. High-resolution 15N-NMR analysis of ethanol extracts of these organisms and cross-polarization magic-angle sample spinning analysis of the solid debris from these extracts are compared to labeled material from Methanococcus thermolithotrophicus, a methanogen previously determined to fix dinitrogen.

Suppression of signals from the probe in bloch decay spectra

An approach to suppressing spurious resonances due to the probe in NMR is presented which utilizes DEPTH (I) pulse sequences to select homogeneous regions of the radiofrequency field. This method is well suited to eliminating background resonances from the probe in Bloch decay spectra in solids and the glass resonance in solution 29Si NMR. Bloch decay spectra obtained in probes that are designed primarily for cross-polarization magic-angle sample spinning, CP/MAS, studies often contain resonances from materials which are close to the coil. These materials are needed for, among other reasons, insulating the coil and holding it in place, as stators and bearings in the MAS assembly, as insulation in quarter wave stubs, and for thermal insulation where variable temperature capabilities are desired. The actual materials, Teflon, Kel-F, boron nitride, Macor, and aluminum oxide, are generally chosen since they contain no protons and, therefore, do not yield a cross-polarization spectrum. In Bloch decay spectra, however, these materials do display broad background signals which makes the observation of dilute samples (e.g., surface species) difficult. Since these extraneous signals originate from well outside the coil, they are in a region of inhomogeneous and lower RF field than the sample of interest; therefore pulse sequences which are currently used in surface coil studies to selectively excite regions of homogeneous RF fields may be employed to suppress these signals. A similar case exists for 29Si NMR of liquids in which the glass resonance from Dewars, the sample tube and the insert on which the coil is wound, interferes. Figure I shows for various pulse widths the Bloch decay spectra of methylamine adsorbed onto a high-surface-area eta-alumina. The eta-alumina is a Bayerite with a surface area of 400 m*/g, donated by Alcoa. It was treated in vucuo for 2 h at 400°C with subsequent adsorption of methylamine at room temperature in a traditional gas manifold. The spectra were acquired on a Bruker MSL-400 with their broadband, variable temperature CP/MAS probe. The broad signal is from Teflon which is used to guide the spinner into the stator, as a thermal insulator which encloses the MAS spinning assembly, and as a dielectric in the transmission lines. It would be difficult to replace all of the Teflon in the probe without adding signals in the silicon, aluminum, or boron spectra for which the probe is also used. The sharp resonance at 24 ppm is assigned to methylamine species bound to Lewis acid sites. It is seen in the figure that the 90” pulse for the methylamine species is 4 ps, and that for the Teflon

Comparison of the solution and crystal structures of staphylococcal nuclease with 13C and 15N chemical shifts used as structural fingerprints.

We report high-resolution 13C and 15N NMR spectra of crystalline staphylococcal nuclease (Nase) complexed to thymidine 3',5'-diphosphate and Ca2+. High sensitivity and resolution are obtained by applying solid-state NMR techniques--high power proton decoupling and cross-polarization magic angle sample spinning (CPMASS)--to protein samples that have been efficiently synthesized and labeled by an overproducing strain of Escherichia coli. A comparison of CPMASS and solution spectra of Nase labeled with either [methyl-13C]methionine or [15N]valine shows that the chemical shifts in the crystalline and solution states are virtually identical. This result is strong evidence that the protein conformations in the solution and crystalline states are nearly the same. Because of the close correspondence of the crystal and solution chemical shifts, sequential assignments obtained in solution apply to the crystal spectra. It should therefore be possible to study the molecular structure and dynamics of many sequentially assigned atomic sites in Nase crystals. Similar experiments are applicable to the growing number of proteins that can be obtained from efficient expression systems.

The use of variable temperature and magic-angle sample spinning in studies of fulvic acids

Intensity distortions and poor signal to noise in the cross-polarization magic-angle sample spinning NMR of fulvic acids were investigated and attributed to molecular mobility in these ostensibly “solid” materials. We have shown that inefficiencies in cross polarization can be overcome by lowering the sample temperature to about −60°C. These difficulties can be generalized to many other synthetic and natural products. The use of variable temperature and cross-polarization intensity as a function of contact time can yield valuable qualitative information which can aid in the characterization of many materials.

A cross-polarization—magic-angle sample spinning n.m.r. study of several crystal forms of lactose

Five different crystalline forms of lactose were investigated by cross-polarization—magic-angle sample spinning (CP—MAS) 13C-n.m.r. spectroscopy. Both the anhydrous β-lactose and the α-lactose monohydrate structures are known, and the CP—MAS n.m.r. data are in agreement with those structures. The structure of the stable, anhydrous α-lactose has not been reported. The CP—MAS n.m.r. results indicate that the crystal must have two or more lactose molecules per unit cell. The chemical shifts measured for two mixed crystals having α:β ratios of 5:3 and 4:1 are a direct observation of the fact that both materials are real mixed crystals rather than glasses or physical mixtures of crystals of pure α- and β-lactose. The chemical shifts also indicate that the lactose molecules in both mixed crystals are in environments similar to the crystalline environment of the stable, anhydrous α-lactose.

Silicon-29 cross-polarization magic-angle sample spinning spectra in amorphous silicon–hydrogen films

Magic-angle sample spinning cross-polarization 29Si NMR spectra are reported for plasma-deposited amorphous silicon-hydrogen films. The data show (i) confirmation of the identification of polyhydride (SiH2)n species as seen by infrared spectroscopy and (ii) a large degree of chemical shift dispersion due to disorder in the silicon lattice.