Characterization of nonderivatized plant cell walls using high-resolution solution-state NMR spectroscopy

A recently described plant cell wall dissolution system has been modified to use perdeuterated solvents to allow direct in-NMR-tube dissolution and high-resolution solution-state NMR of the whole cell wall without derivatization. Finely ground cell wall material dissolves in a solvent system containing dimethylsulfoxide-d(6) and 1-methylimidazole-d(6) in a ratio of 4:1 (v/v), keeping wood component structures mainly intact in their near-native state. Two-dimensional NMR experiments, using gradient-HSQC (heteronuclear single quantum coherence) 1-bond (13)C--(1)H correlation spectroscopy, on nonderivatized cell wall material from a representative gymnosperm pinus taeda (loblolly pine), an angiosperm Populus tremuloides (quaking aspen), and a herbaceous plant Hibiscus cannabinus (kenaf) demonstrate the efficacy of the system. We describe a method to synthesize 1-methylimidazole-d(6) with a high degree of perdeuteration, thus allowing cell wall dissolution and NMR characterization of nonderivatized plant cell wall structures.     

Refinement of hydrogen atomic position in a hydrogen bond using a combination of solid-state NMR and computation

DFT computations of the proton chemical shift for the intermolecular hydrogen bond in the white form of methylnitroacetanilide, together with the experimental value obtained by high-speed magic-angle spinning NMR, enable the N-H distance to be determined as 1.03 +/- 0.02 A.     

Molecular orbital studies of the NMR hydrogen bond shift

Magnetic shielding constants are calculated for the protons in XOH and XOH…OH₂ (XH, CH₃, NH₂, OH and F) molecules using a slightly extended set of atomic functions modified by gauge factors. These results are used to determine theoretical values for the NMR hydrogen bond shifts in the XOH…OH₂ systems. Such theoretical data are consistent with the few available experimental data. An analysis of the theoretical results reveals that there are three major types of shielding contribution to the NMR hydrogen bond shift; (a) a deshielding change due to the variation of the local currents on the hydrogen bonded proton; (b) a reduction in shielding from currents localized on the oxygen atom of the proton donor; (c) a deshielding contribution from currents induced on the oxygen atom of the proton acceptor. Except for the water dimer, contributions (a), (b) and (c) are of comparable importance for changes in isotropic shielding. For (H₂O)₂ contributions (a) and (c) are somewhat more important than contribution (b). Contribution (c) is almost totally responsible for the changes in the anistropies of the shielding tensors associated with the hydrogen bonded protons. The proton shielding anisotropy changes which occur on hydrogen bond formation are generally much larger than the corresponding variations in the isotropic values of the shielding tensors. This suggests that proton magnetic shielding anisotropies may be more sensitive measures of features of hydrogen bonding than are isotropic proton shielding constants.     

NMR method for the determination of solute hydrogen bond acidity

It is shown that the difference in the 1H NMR chemical shift of a protic hydrogen in DMSO and CDCl3 solvents is directly related to the overall, or summation, hydrogen bond acidity for a wide range of solutes. This provides a new and direct method of measuring the hydrogen bond acidity. For 54 compounds, the observed shifts for 72 protic hydrogens could be correlated to the Abraham solute hydrogen bond acidity parameter, A, with a correlation coefficient squared, R2, of 0.938 and a standard deviation, SD, of 0.054 units in A. A training equation that used half the data could predict A values for the remaining data with an average error of 0.001 and a standard deviation, SD, of 0.053 units, thus demonstrating the predictive power of the method. Unlike any previous method for the determination of solute hydrogen bond acidities, the NMR method allows the determination of A values for individual protic hydrogens in multifunctional solutes.     

Thermoplastic elastomers by hydrogen bonding 4. Influence of hydrogen bonding on the temperature dependence of the viscoelastic properties

The temperature dependence of the viscoelastic properties of thermoreversible polybutadiene networks based on hydrogen bond linkages is analyzed from the logarithmic shift factors loga _T . For binary hydrogen bond complexes thermorheologically simple behavior is observed. The temperature dependence of loga _T is described by the Williams-Landel-Ferry (WLF) equation. The thermoreversible linkages cause an increase in the apparent activation enthalpy of flow which is related to the number of complexing sites in the polymer. Thermorheologically complex behavior is observed in a system with more complex association.     

The influence of potassium cation on a strong OHO hydrogen bond

The influence of the potassium cation on the OHO hydrogen bond in potassium chloromaleate was investigated. Theoretical methods commonly used for investigating hydrogen bonds, such as the analysis of electron density at the hydrogen bond critical points, and the calculation of the shape of the potential energy curves and potential energy surfaces, show that the strong OHO hydrogen bond can be modified by the potassium cation located around the chloromaleate anion.     

Characterization of protein-ligand interactions by high-resolution solid-state NMR spectroscopy

A novel approach for detection of ligand binding to a protein in solid samples is described. Hydrated precipitates of the anti-apoptotic protein Bcl-xL show well-resolved (13)C-(13)C 2D solid-state NMR spectra that allow site-specific assignment of resonances for many residues in uniformly (13)C-enriched samples. Binding of a small peptide or drug-like organic molecule leads to changes in the chemical shift of resonances from multiple residues in the protein that can be monitored to characterize binding. Differential chemical shifts can be used to distinguish between direct protein-ligand contacts and small conformational changes of the protein induced by ligand binding. The agreement with prior solution-state NMR results indicates that the binding pocket in solid and liquid samples is similar for this protein. Advantages of different labeling schemes involving selective (13)C enrichment of methyl groups of Ala, Val, Leu, and Ile (Cdelta1) for characterizing protein-ligand interactions are also discussed. It is demonstrated that high-resolution solid-state NMR spectroscopy on uniformly or extensively (13)C-enriched samples has the potential to screen proteins of moderate size ( approximately 20 kDa) for ligand binding as hydrated solids. The results presented here suggest the possibility of using solid-state NMR to study ligand binding in proteins not amenable to solution NMR.     

Synthesis and temperature dependence of physical properties of four pyridinium-based ionic liquids: Influence of the size of the cation

In this paper, the ionic liquids 1,2-diethylpyridinium ethylsulfate, EEpyESO₄; 1-methylpyridinium methylsulfate, MpyMSO₄; 1,3-dimethylpyridinium methylsulfate, MMpyMSO₄; and 2-ethyl-1-methylpyridinium methylsulfate, EMpyMSO₄; were synthesized in our laboratory, and their experimental densities, speeds of sound, dynamic viscosities, and refractive indices were studied as a function of temperature at atmospheric pressure. Thermal expansion coefficient, molar volume, and molar refraction of these ionic liquids were calculated from the experimental density and refractive index values.     

Infrared spectroscopy on new multiphase thermoplastic elastomers based on hydrogen bond complexes - temperature dependence and orientation behaviour

The structure and deformation behaviour of a new class of thermoplastic elastomers is studied by temperature dependent infrared (IR) spectroscopy and by IR - dichroism spectroscopy. The thermoplastic elastomer is based on polybutadiene with statistically distributed side groups which form an anisotropic supramolecular structure via hydrogen bonds. Changes in the IR spectra at elevated temperatures are related to the melting of the ordered structure. The uniaxial deformation behaviour is studied by linear dichroism Fourier-transform (FT)-IR spectroscopy. A deformation model is developed which accounts for the major experimental results: while the polybutadiene segments behave as flexible chains with characteristic rubberlike elasticity the polar units within the supramolecular structure show an orientation behaviour characteristic for rodlike molecules.     

Charge transfer theory and vibrational properties of the hydrogen bond

An extension of Mulliken's charge transfer theory leads to the following correlations: the change in the transition moment of the X—H stretching vibration with the enhancement of dipole moment, and the displacement of the X—H stretching frequency with the ionization of donors. Experimental data for a number of hydrogen-bonded complexes are in accord with these predictions.     

Proton and deuterium NMR of hydrogen bonds: Relationship between isotope effects and the hydrogen bond potential

The difference in chemical shift between hydrogen bonded protons and deuterons has been examined both theoretically and experimentally. It is shown that valuable information about the hydrogen bond potential can be extracted from this isotope effect on chemical shifts.     

Kinetics of unfolding the human telomeric DNA quadruplex using a PNA trap

The kinetics of opening of the DNA quadruplex formed by the human telomeric repeat have been investigated using real-time fluorescence resonance energy transfer (FRET) measurements with a peptide nucleic acid (PNA) trap. It has been found that this opening is zero-order with respect to PNA, indicating that the initial step is a rate-limiting internal rearrangement of the quadruplex. A study of the temperature dependence of the rate of quadruplex opening was performed and the activation energy of the process estimated to be 98 +/- 8 kJ mol(-1).     

Physical properties of ionic liquids based on 1-alkyl-3-methylimidazolium cation and hexafluorophosphate as anion and temperature dependence

Densities ρ, speeds of sound u, and refractive indices n_D were measured from T = (278.15 to 343.15) K. Dynamic viscosities η were measured from T = (293.15 to 323.15) K. Surface tensions σ were determined from T = (288.15 to 313.15) K. The physical properties data were measured at atmospheric pressure. The coefficients of thermal expansion α_p of the ionic liquids were calculated from the experimental values of the density at several temperatures. The Parachor method was used to predict the densities, the refractive indices, and the surface tensions of the ionic liquids, and a comparison between experimental and predictive values was made at T = 298.15 K.     

Atom promotion and bond properties in the hydrogen and the lithium molecules

In the process of forming the hydrogen molecule, the interacting atoms apparently undergo significant promotion, in the course of which there occurs contraction of each atom's electronic density distribution. Although this step in itself is energetically unfavourable, it appears to be a key factor in building up the charge density in the internuclear region. In forming the lithium molecule, the atoms apparently do not undergo promotion to any significant extent. It is suggested that the difference in the degrees of atom promotion in the formation of these two molecules is an important reason for the great disparity in the strengths of their bonds.

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Zusammenfassung Beim Prozeß der Wasserstoffmolekülbildung unterliegen die wechselwirkenden Atome offensichtlich einer deutlichen Promovierung, bei deren Verlauf eine Kontraktion der Elektronendichteverteilung von jedem der beiden Atome stattfindet. Obwohl dieser Schritt selbst energetisch ungünstig ist, scheint er ein Hauptfaktor beim Aufbau der Ladungsdichte im Gebiet zwischen den Kernen zu sein. Bei der Bildung des Lithiummoleküls unterliegen die Atome offenbar keiner irgendwie wesentlichen Promovierung. Man vermutet, daß die Verschiedenheit des Grades der Atompromovierung bei der Formation in diesen beiden Molekülen ein wichtiger Grund für die große Diskrepanz ihrer Bindungskräfte ist.

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Résumé Dans le processus de formation de la molécule d'hydrogène, les atomes qui interagissent subissent apparemment une promotion significative, au cours de laquelle il se produit une contraction de la densité électronique sur chaque atome. Quoique cette étape soit en elle même énergétiquement défavorable, elle apparaît comme un facteur clé dans la construction de la densité de charge de la région internucléaire. Lors de la formation de la molécule de lithium, les atomes ne semblent pas subir de promotion significative. On suggère que la différence dans les degrés de promotion atomique pour ces deux molécules est une des raisons importantes de la grande différence dans les énergies de liaison.