C CPMAS studies of plant cell wall materials and model systems using proton relaxation-induced spectral editing techniques

The solid state ¹³C CPMAS NMR spectra of plant cell walls are often complex owing to superposition of resonances from different polysaccharides and the heterogeneity of the cell wall assembly. In this paper, we describe the application of a set of proton relaxation-induced spectral editing (PRISE) experiments which combine ¹H relaxation properties (T₁, T_1ρ, T₂) with ¹³C high resolution spectroscopy (CPMAS) to relate the dynamics of the plant cell walls and model systems to their domain structural details. With PRISE it has been found that in plant cell wall materials, cellulose is always associated with the long components of spin–lattice relaxation in both the laboratory and rotating frames whereas non-cellulose polysaccharides (pectin and hemicellulose) are associated with the short ones. For the proton T₂ relaxation, cellulose is only associated with the short component (below 20 μs), pectin contributes to both the short component and the long one.     

Molecular dynamics in solid riboflavin as studied by 1H NMR

Spin–lattice relaxation times T₁ and T_1d as well as NMR second moment were employed to study the molecular dynamics of riboflavin (vitamin B₂) in the temperature range 55–350 K. The broad and flat T₁ minimum observed at low temperatures is attributed to the motion of two nonequivalent methyl groups. The motion of the methyl groups is interpreted in terms of Haupt's theory, which takes into account the tunneling assisted relaxation. An additional mechanism of relaxation in the high temperature region is provided by the motion of a proton in one of the hydroxyl groups. The Davidson–Cole distribution of correlation times for this motion is assumed.     

Solid-state photodimerization of 4-aryl-1,4-dihydropyridines studied by C CPMAS NMR spectroscopy

¹³C CPMAS NMR spectroscopy has been applied to monitor the solid-state reaction of two different photodimerizing 4-phenyl-1,4-dihydropyridines yielding a cage dimer in one case and an anti-dimer in the other case. The spectra of the reacting monomers exhibit a magnetical inequivalence of chemically equivalent CO and C2/4 carbon atoms caused by a rotation of the pseudoaxially oriented 4-phenyl substituent out off the plane through N1, C3, C8 which could be determined by X-ray crystal structure analyses of the centrosymmetrically arranged monomers. The ¹³C CPMAS NMR monitoring of the cage dimer formation proves that the reaction takes place in two steps via a syn-dimer for which a non-symmetrical structure was derived from the spectrum. The non-symmetrical structure was confirmed by X-ray crystal structure analysis of one structurally related derivative. A centrosymmetric structure for both the finally formed cage dimer and the anti-dimer of the other monitored photoreaction was proved by their spectra with one set of signals for each half of the dimers, respectively. Thus, conformational properties of the molecules as well as the symmetry of the products can be directly derived from the ¹³C CPMAS NMR spectra.     

Molecular dynamics of poly(L-lactide) biopolymer studied by wide-line solid-state 1H and 2H NMR spectroscopy

The molecular dynamics of poly(L-lactide) (PLLA) biopolymer was characterized through analyses of 1H and 2H NMR line-shapes and spin-lattice relaxation times at different temperatures. At low temperatures (e.g. 90 K), the methyl group rotation is dominant leading to a significant reduction in the proton second moment. Fast methyl group reorientation occurs at ca. 130 K. In additional to the fast methyl group rotation, hydroxyl groups start to reorient as the temperature increases further, eventually leading to the breakdown of the segments of the biopolymer chains above its glass transition temperature Tg of 323 K. The analyses of the 2H NMR line-shapes indicate that both the methyl and hydroxyl reorientations can be described by the so-called cone model, in which the former has three equilibrium positions with theta(C-D) = 70.5 degrees and phi = 120 degrees while the latter one exhibits two equilibrium positions with theta(O-D) = 78 degrees and phi = 180 degrees .     

Molecular reorientation and phase transition in pyridinium hexafluoroantimonate

The temperature dependence of ¹H and ¹⁹F NMR second moment and spin lattice relaxation times in high (v _L = 60 MHz and low (B ₁ = 2 mT) magnetic fields allow one to determine both ion dynamics in polycrystalline pyridinium hexafluoroantimonate. The solid-solid phase transition discovered at 268 K appears to be connected with symmetrization of energy barriers for pseudohexagonal cation reorientation. The energy difference Δ characterizing the inequivalence of the potential wells can be treated as an order parameter. The effect of coupling of the rotational modes of cations and anions is found at the phase transition.     

Protein dynamics measurements by TROSY-based NMR experiments

The described TROSY-based experiments for investigating backbone dynamics of proteins make it possible to elucidate internal motions in large proteins via measurements of T(1), T(2), and NOE of backbone (15)N nuclei. In our proposed sequences, the INEPT sequence is eliminated and the PEP sequence is replaced by the ST2-PT sequence from the HSQC-based experiments. This has the benefit of shortening the pulse sequences by 5.4 ms (=1/2J) and results in an increase in the intrinsic sensitivity of the proposed TROSY-based experiments. The TROSY-based experiments are on average of 13% more sensitive than the corresponding HSQC-based experiments on a uniformly (15)N-labeled Xenopus laevis calcium-bound calmodulin sample on a 750-MHz spectrometer at 5 degrees C. The amide proton linewidths of the TROSY-based experiments are 2-13 Hz narrower than those of the HSQC experiments. More sensitivity gain and higher resolution are expected if the protein sample is deuterated.     

Contribution of proton NMR relaxation to the investigation of molecular dynamics in columnar mesophases of discotic and polycatenar molecules

We present in this work a review concerning wide frequency rangeT ₁ proton NMR relaxation studies performed in compounds exhibiting columnar mesophases, namely the Col_ho mesophase in the case of a liquid crystal of discotic molecules and the ø_h mesophase in the case of a liquid crystal of biforked molecules. These NMR relaxation studies were performed combining conventional and fast field cycling NMR techniques in a frequency range between 100 Hz and 300 MHz. The possibility of probing such a large frequency range has provided a way to effectively distinguish the influence, on theT ₁ relaxation profiles, of the different molecular movements observed in this type of mesophases. In addition, we present a comparison between the molecular dynamics in columnar (ø_h) and lamellar (SmC) mesophases exhibited by the same biforked compound.     

Solid state 13C NMR of 30-crown-10 ether and 30-crown-10.4H2O

The ¹³C NMR solution spectra of 30-crown-10 ether and its tetrahydrate show only one resonance at all accessible temperatures. In contrast, the solid state ¹³C NMR spectrum of the 30-crown-10.4H₂O shows two resonances in the ratio of 4:1, separated by 1.2 ppm. In the case of 30-crown-10 itself, six resolvable ¹³C resonances in the ratio of 4:1:1:2:1:1 are observed in the solid with an overall chemical shift dispersion of 5 ppm. The remarkably different spectral behavior of these two systems in the solid state is discussed in terms of the torsional environments of the crystallographically unique carbons and the results of GIAO calculations of isotropic ¹³C shieldings for simpler model compounds. Results of dipolar dephased ¹³C CPMAS spectra indicate that 30-crown-10 does not undergo a large amplitude molecular motion, in contrast to earlier results for 18-crown-6. Only a small amount of residual intensity is found in the dipolar dephased spectrum of 30-crown-10.4H₂O, indicating that it also is relatively rigid in the solid.     

Expanding the frequency range of the solid-state T1rho experiment for heteronuclear dipolar relaxation

Solid-state spin–lattice relaxation in the rotating frame permits the investigation of dynamic processes with correlation times in the range of microseconds. The relaxation process in organic solids is driven by the fluctuation of the local magnetic field due to the dipole–dipole interaction of the probe nuclei (¹³C,¹⁵N) with ¹H in close proximity. However, its effect is often hidden by a competing relaxation process due to the contact between the rotating frame ¹³C/¹⁵N Zeeman and ¹H dipolar reservoirs. In most cases the latter process becomes superior for the commonly applied low and moderate spin-lock fields and practically does not provide information about the molecular dynamics. To suppress this undesired process and to expand the dynamic range of T_{1 ρ} experiments, we present two approaches. The first one uses a resonance offset of the frequency of the spin-lock irradiation, which leads to a significant enhancement of the effective spin-lock frequency without the application of destructive high transmitter powers. We derive the theory and demonstrate the applicability of the method on various model compounds. The second approach utilizes heteronuclear ¹H decoupling during the ¹³C/¹⁵N spin-lock irradiation which disrupts the contact between the ¹³C/¹⁵N Zeeman and ¹H dipolar reservoirs. We demonstrate the method and discuss the results qualitatively.     

Local and global dynamics in the glass-forming di-isobutyl phthalate as studied by H NMR

Spin-lattice relaxation times T₁ and T_1ρ as well as ¹H NMR spectra have been employed to study the dynamics of the glass-forming di-isobutyl phthalate in the temperature range extending from 100 K, through the glass transition temperature T_g, up to 340 K. Below T_g NMR relaxation is governed by local dynamics and may be attributed to rotation of methyl groups at low temperatures and to motion of isobutyl groups in the intermediate temperature interval. Above T_g the main relaxation mechanism is provided by overall molecular motion. The observed relaxation behavior is explained by motional models assuming asymmetrical distributions of correlation times. The motional parameters obtained from Davidson-Cole distribution, which yields the best fit of the data at all temperatures are given.     

Proton NMR study of molecular dynamics and phase transitions in methylammonium hexachloro plumbate, [NH3CH3]2PbCl6

Proton spin lattice relaxation time (T₁), measured as a function of temperature in the range 375–77 K, shows slope changes at 333, 221 and 111 K, in addition to a first order phase transition at 150K. The observed T₁ behaviour and second moment (M₂) variation with temperature are explained on the basis of the different possible motions of CH₃ and NH₃ groups.     

Molecular dynamics of agarose gels investigated by NMR and rayleigh scattering of Mössbauer radiation

Summary NMR, X-ray and Rayleigh scattering of M?ssbauer radiation (RSMR) measurements have been performed on agarose-water systems for different levels of water content. The NMR data provided information on the relaxation rates of protons in both polysaccharide and solvent. The RSMR measurement allowed us to determine the mean square atomic displacements <u ²> as a function of the scattering vector. The <u ²> values of carbon and oxygen atoms are larger when measured at distances corresponding to the helix diameter than along the chain bonds. At low hydration levels, the water molecules closely associated to the polysaccharide chains follow the dynamics of the polymer. A marked change in the dynamics of both polymer and water has been evidenced when the water weight exceeds 20% of the total weight of the sample.

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Riassunto Sono state eseguite misure NMR, di diffrazione di raggi X e di scattering Rayleigh della radiazione M?ssbauer (RSMR) su sistemi agarosio-acqua con differenti gradi di idratazione. I dati NMR hanno fornito informazioni sulla velocità di rilassamento dei protoni, sia nel polisaccaride che nel solvente. Le misure RSMR hanno consentito di determinare lo spostamento atomico quadratico medio <u ²> in funzione del vettore di scatteringQ. I valori di <u ²> per gli atomi di carbonio e di ossigeno misurati per valori diQ corrispondenti alle distanze caratteristiche del diametro delle eliche sono maggiori di quelli relativi alle distanze tipiche dei legami intracatena. Per bassi valori di idratazione le molecole di acqua legate direttamente alle catene polisaccaridiche seguono la dinamica del polimero. Una marcata variazione nella dinamica, sia del polimero che dell'acqua, è stata osservata in campioni in cui l'acqua presente superava il 20% del peso totale del campione.

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Резюме Проводятся измерения ЯМР, рентгеновской дифракции и рэлеевского рассеяния Мёссбауэровского излучения на системах агара с водой для различных уровней содержания воды. Данные ЯМР обеспечивают информацию о скоростях релаксации протонов в полисахариде и в растворителе. Измерения рэлеевского рассеяния Мёссбауэровского излучения позволяют определить средние квадратичные смещения атомов (<u ²>), как функцию вектора рассеяния. Значения <u ²> для атомов углерода и кислорода, измеренные на расстояниях, соответствующих диаметру спирали, оказываются больше значений смещений, измеренных вдоль связей цепочек. При низких уровнях гидратации молекулы, воды, тесно связанные с цепочками полисахарида, следуют динамике поимера. Заметное изменение динамики полимера и воды обнаруживается, когда содержание воды превышает 20% от полного веса образца.

     

The 15N and 13C solid state NMR study of intramolecular hydrogen bond in some Schiff's bases

A series of 11 Schiff's bases derived from substituted salicylaldehyde and aliphatic amines has been studied in the solid state by ¹⁵N and ¹³C cross-polarization magic angle spinning (CPMAS) nuclear magnetic resonance (NMR). ¹⁵N CPMAS is especially useful for investigation of the tautomerism in the compounds considered, owing to the large difference in the nitrogen chemical shifts of OH and NH tautomers. In the solid state, three of the compounds examined were shown by ¹⁵N NMR to exist as OH tautomeric forms, and the remaining eight as the corresponding NH forms. This was confirmed by ¹³C CPMAS. The results reported were compared with those obtained in CDCl₃ solutions.     

Solid-state 13C NMR study of poly(vinyl alcohol) gels

Poly(vinyl alcohol) (PVA) with 55% and 61% syndiotacticity, and their related dry and hydrated gels obtained by two different freeze–thawing cycles have been investigated using the solid-state ¹³C CP-MAS NMR technique. From a comparative analysis of the spectra, evidence was obtained that the gelation process largely disrupts the intramolecular hydrogen-bonded network of the PVA. The addition of water to the dry gels favours their swelling, destroying intra-chain hydrogen bonds between hydroxyl groups as a function of the degree of tacticity and the gelation procedure, and promotes the formation of new networks of interchain hydrogen bonds. Information on the dynamics of the polymeric domains in the kilohertz range has been obtained from the analysis of the spin relaxation times T_1ρ(¹H) and T_1ρ(¹³C), indicating that homogeneous arrangements of the amorphous or swollen polymeric chains exist, independent of the preparation method or the tacticity of the PVA chains.     

Application of Solid-State C NMR Spectroscopy and Dipolar Dephasing Technique to Determine the Extent of Condensation in Technical Lignins

Solid-state ¹³C NMR spectroscopy and dipolar dephasing technique was used to determine the extent of condensation in various technical lignins. The accuracy of dipolar dephasing method was first investigated with the aid of some lignin model compounds and two various methods to determine the degree of condensation were compared. On the basis of the model compound experiments both methods based on dipolar dephasing technique can be applied to investigate the extent of condensation in lignin. The lignin results indicate that technical softwood lignins, as well as enzymatically isolated wood lignin, are more condensed than milled wood lignin, which is generally assumed to represent native lignin. Residual lignins isolated after oxygen delignification and peroxide bleaching stages were found more condensed than residual lignin in unbleached pulps. In studies of the spent liquor lignins of flow-through kraft pulping the extent of condensation was found to increase as the cooking proceeded.     

Low-frequency molecular dynamics studied by spin-lock field cycling imaging

Spin-lock adiabatic field cycling imaging (SLOAFI) relaxometry was shown to be a useful technique for obtaining a fast study of spin-lattice relaxation dispersion in the rotating frame. The aim of the present article is to describe some technical aspects of the experiment in more detail, while showing simple examples that can be compared with laboratory frame relaxation. We also present here a general discussion of the equations for an off-resonance experiment used to analyze low-frequency molecular dynamics.     

Sensitivity enhancement in (13)C solid-state NMR of protein microcrystals by use of paramagnetic metal ions for optimizing (1)H T(1) relaxation

We discuss a simple approach to enhance sensitivity for (13)C high-resolution solid-state NMR for proteins in microcrystals by reducing (1)H T(1) relaxation times with paramagnetic relaxation reagents. It was shown that (1)H T(1) values can be reduced from 0.4-0.8s to 60-70 ms for ubiquitin and lysozyme in D(2)O in the presence of 10 mM Cu(II)Na(2)EDTA without substantial degradation of the resolution in (13)C CPMAS spectra. Faster signal accumulation using the shorter (1)H T(1) attained by paramagnetic doping provided sensitivity enhancements of 1.4-2.9 for these proteins, reducing the experimental time for a given signal-to-noise ratio by a factor of 2.0-8.4. This approach presented here is likely to be applicable to various other proteins in order to enhance sensitivity in (13)C high-resolution solid-state NMR spectroscopy.     

Molecular dynamics in solid pregnenolone studied by H spin–lattice relaxation

Spin–lattice relaxation times T₁ in solid pregnenolone have been studied over a wide range of temperatures, from 77 up to 417 K. The dynamic processes arising from C₃ motion of the three methyl substituents are separated, and their activation parameters are determined.     

CP dynamics of heterogeneous organic material: characterization of molecular domains in coals

Simplified equations are used in common approaches to describe cross polarization (CP) dynamics of solids. The CP behavior may be modulated by several nuclei interactions and physicochemical sample properties. At high magnetic fields and spinning speeds, these modulations can obscure the results. To elucidate their impact on the CP behavior of natural organic materials variable contact time (VCT) experiments were acquired with a high temporal resolution for two coal samples. The measurements were distinctly influenced by interfering fluctuations. Conventional approaches showed insufficient flexibility in terms of degrees of freedom to calculate the CP dynamics. The use of an original fundamental equation as model resulted in sufficient flexibility for such heterogeneous systems. The best results were obtained assuming a two component system. On these conditions a differentiation between amorphous and crystalline domains within the coal samples was enabled.