Slow Molecular Reorientation Research Articles

Induced smectic phases composed of 5CB and EBBA mixtures: structures and dielectric property

Induced smectic phases of 5CB-EBBA binary mixtures were investigated with respect to the structures, stabilities, and molecular motion. The highest stability of the ordered smectic phase was obtained when 5CB and EBBA were mixed at 1:2. At the same ratio, the transferred charges between 5CB and EBBA took the maximum, indicating the direct correlation of the induced phase to the charge-transfer interaction. Whereas, the molecular motion monotonically slowed down with increasing EBBA, which is ascribed to the decrease of the free volume. The stability and the slow molecular reorientation may be independently controlled in the induced smectics.

Deciphering the Structure of the Gramicidin A Channel in the Presence of AOT Reverse Micelles in Pentane Using Molecular Dynamics Simulations.

Structural studies of proteins and, in particular, integral membrane proteins (IMPs) using solution NMR spectroscopy approaches are challenging due to not only their inherent structural complexities but also the fact that they need to be solubilized in biomimetic environments (such as micelles), which enhances the slow molecular reorientation. To deal with these difficulties and increase the effective rate of molecular reorientation, the encapsulation of IMPs in the aqueous core of the reverse micelle (RM) dissolved in a low-viscosity solvent has been proven to be a viable approach. However, the effect of the reverse micelle (RM) environment on the IMP structure and function is little known. To gain insight into these aspects, this article presents a series of atomistic unconstrained molecular dynamics (MD) of a model ion channel (gramicidin A, gA) with RMs formed with anionic surfactant diacyl chain bis(2-ethylhexyl) sodium succinate (AOT) in pentane at a water-to-surfactant molar ratio (W0) of 6. The simulations were carried out with different protocols and starting conditions for a total of 2.4 μs and were compared with other MDs used with the gA channel inserted in models of the SDS micelle or the DMPC membrane. We show here that in the presence of AOT RMs the gA dimer did not look like the "dumbbell-like" model anticipated by experiments, where the C-terminal parts of the gA are capped with two RMs and the rest of the dimer is protected from the oil solvent by the AOT acyl chains. In contrast, the MD simulations reveal that the AOT, Na+, and water formed two well-defined and elongated RMs attached to the C-terminal ends of the gA dimer, while the rest is in direct contact with the pentane. The initial β6.3 secondary structure of the gA is well conserved and filled with 6-9 waters, as in SDS micelles or the DMPC membrane. Finally, the water movement inside the gA is strongly affected by the presence of RMs at each extremity, and no passage of water molecules through the gA channel is observed even after a long simulation period, whereas the opposite was found for gA in SDS and DMPC.

Correlations of low-field NMR and variable-field NMR parameters with osteoarthritis in human articular cartilage under load.

NMR experiments carried out at magnetic fields below 1T provide new relaxation parameters unavailable with conventional clinical scanners. Contrast of T1 generally becomes larger towards low fields, as slow molecular reorientation processes dominate relaxation at the corresponding Larmor frequencies. This advantage has to be considered in the context of lower sensitivity and frequently reduced spatial resolution. The layered structure of cartilage is one example where a particularly strong variation of T1 across the tissue occurs, being affected by degenerative diseases such as osteoarthritis (OA). Furthermore, the presence of 1 H-14 N cross-relaxation, leading to so-called quadrupolar dips in the 1 H relaxation time dispersion, provide insight into the concentration and mobility of proteoglycans and collagen in cartilage, both being affected by OA. In this study, low-field imaging and variable-field NMR relaxometry were combined for the first time for tissue samples, employing unidirectional load to probe the mechanical properties. 20 human knee cartilage samples were placed in a compression cell, and studied by determining relaxation profiles without and with applied pressure (0.6MPa) at 50μm in-plane resolution, and comparing with volume-averaged T1 dispersion. Samples were subsequently stored in formalin, prepared for histology and graded according to the Mankin score system. Quadrupolar dips and thickness change under load showed the strongest correlation with Mankin grade. Average T1 and change of maximum T1 under load, as well as its position, correlate with thickness and thickness change. Furthermore, T1 (ω) above 25 mT was found to correlate with thickness change. While volume-averaged T1 is not a suitable indicator for OA, its change due to mechanical load and its extreme values are suggested as biomarkers available in low-field MRI systems. The shape of the dispersion T1 (ω) represents a promising access to understanding and quantifying molecular dynamics in tissue, pointing toward future in vivo tissue studies.

Gd-DTPA-Dopamine-Bisphytanyl Amphiphile: Synthesis, Characterisation and Relaxation Parameters of the Nanoassemblies and Their Potential as MRI Contrast Agents.

Here, a new amphiphilic magnetic resonance imaging (MRI) contrast agent, a Gd(III)-chelated diethylenetriaminepentaacetic acid conjugated to two branched alkyl chains via a dopamine spacer, Gd-DTPA-dopamine-bisphytanyl (Gd-DTPA-Dop-Phy), which is readily capable of self-assembling into liposomal nanoassemblies upon dispersion in an aqueous solution, is reported. In vitro relaxivities of the dispersions were found to be much higher than Magnevist, a commercially available contrast agent, at 0.47 T but comparable at 9.40 T. Analysis of variable temperature (17)O NMR transverse relaxation measurements revealed the water exchange of the nanoassemblies to be faster than that previously reported for paramagnetic liposomes. Molecular reorientation dynamics were probed by (1)H NMRD profiles using a classical inner and outer sphere relaxation model and a Lipari-Szabo "model-free" approach. High payloads of Gd(III) ions in the liposomal nanoassemblies made solely from the Gd-DTPA-Dop-Phy amphiphiles, in combination with slow molecular reorientation and fast water exchange makes this novel amphiphile a suitable candidate to be investigated as an advanced MRI contrast agent.

Role of Solvents on the Thermodynamics and Kinetics of Forming Frustrated Lewis Pairs

To enhance our understanding of the role of solvents on the thermodynamics and kinetics of forming frustrated Lewis pairs (FLP), we carried out a systematic simulation study on these systems in dichloromethane and toluene solvents. These molecular systems are of particular interest due to their relevance in the catalytic hydrogenation and hydrogen storage processes. While the computed structural observables for both molecules are very similar, the slow molecular reorientation was consistent with the size of the species. The computed free-energy profiles for the FLP in both solvents show similar gross characteristics but differ in details. We observe two well-defined contact regions and a solvent-separated regions with different well depths and barrier heights to dissociation. The kinetics of solute–pair interconversion was studied using transition-state theory, comparing Kramers and Grote–Hynes treatments of the dynamic response of the solvent. These rate results were used to predict solvent effects on dy...

Slow molecular processes in a nematic liquid crystal confined to random porous network formed by aerosil particles

Frequency dispersion measurements of proton spin–lattice relaxation rates (R 1) of liquid crystal 4-propyl-4′-pentylazoxybenzene in bulk and confined samples (in random porous network of aerosil nano-particles) are reported in isotropic and nematic phases. Significant low-frequency increase in R 1 in confined samples indicates slow molecular reorientations mediated by translational displacements near the adsorbing porous surface. The resulting dispersion behavior of R 1 (∼ω − p ) reflects the nature of the random surface (p = 0.5 for equi-partition of the diffusive modes). The observed temperature-independent exponent in the isotropic phase (p = 0.34) indicates the abundance of low-wavelength surface modes. Its temperature-dependent higher values in the nematic phase (from 0.59 to 0.65 on cooling), and increased spin–lattice coupling via this mechanism, show progressive onset of longer wavelength modes. A detailed analysis shows the effect of confinement on the order director fluctuations, molecular reorientations, and translational diffusion of the molecules.

Reverse micelles in integral membrane protein structural biology by solution NMR spectroscopy.

Integral membrane proteins remain a significant challenge to structural studies by solution NMR spectroscopy. This is due not only to spectral complexity, but also because the effects of slow molecular reorientation are exacerbated by the need to solubilize the protein in aqueous detergent micelles. These assemblies can be quite large and require deuteration for optimal use of the TROSY effect. In principle, another approach is to employ reverse micelle encapsulation to solubilize the protein in a low-viscosity solvent in which the rapid tumbling of the resulting particle allows for use of standard triple-resonance methods. The preparation of such samples of membrane proteins is difficult. Using a 54 kDa construct of the homotetrameric potassium channel KcsA, we demonstrate a strategy that employs a hybrid surfactant to transfer the protein to the reverse micelle system.

Interfacial assembly of cationic peptide surfactants

Short surfactant-like peptides bearing the molecular architecture of hydrophilic and hydrophobic moieties are attractive for a wide range of biological and technological applications. However, to realize the benefits offered by them we need to understand the basic physiochemical properties arising from their interfacial self-assembly and solution aggregation. In this paper, we use a combined approach of spectroscopic ellipsometry (SE), atomic force microscopy (AFM) and neutron reflection (NR) to characterize the interfacial assembly of cationic peptide V6K2 at the hydrophilic silica–water interface. The SE measurement revealed that the peptide dynamic adsorption was characterised by a fast initial process within the first 3–5 min, followed by a slow molecular reorientation within the next 30–40 min. AFM imaging revealed the formation of a dense peptide layer incorporating defects and some large vesicles. This interfacial structural feature from AFM offered a useful starting point for fitting the neutron reflectivity profiles measured. At 0.05 wt% V6K2, the reflectivity profiles were well fitted using a two-layer model with a dense 40 A inner layer containing about 50% peptide close to the oxide surface and a loose 40 A outer layer containing some 8% peptide on the solution side. With the help of partial deuteration to the peptide (hV6hK2 and dV6hK2 and the solvent isotopic contrasts (D2O, H2O), we found that the densely packed peptide region was comprised of a sandwiched peptide bilayer with their hydrophobic tails (V6) attracted to each other and the cationic head groups (K2) projected towards the oxide surface and the bulk water. This peptide bilayer structure is similar to that formed by conventional cationic surfactants when adsorbed at the same anionic SiO2–water interface, indicating the dominant effect of hydrophobic interaction. This study has demonstrated that the combined measurements provide a useful account of structure and dynamics of interfacial peptide self-assembly.

Biaxial nematic order and phase behavior studies in an organosiloxane tetrapode using complementary deuterium NMR experiments

The biaxial nematic phase was recently observed in different thermotropic liquid crystals, namely bent-core compounds, side-chain polymers, bent-core dimers, and organosiloxane tetrapodes. In this work, a series of experiments with a nematic organosiloxane tetrapode where nuclear magnetic resonance (NMR) spectra are collected while the sample is continuously rotating around an axis perpendicular to the magnetic field, are discussed in conjunction with the analysis of a deuterium NMR experiment on the same system reported earlier. The sample used is a mixture of a deuterated probe with the tetrapode. The mixture exhibits a nematic range between -40 degrees C and 37 degrees C. The results of the two independent, but complementary deuterium NMR experiments confirm the existence of a biaxial nematic phase for temperatures below 0 degrees C with high values of the asymmetry parameter at low temperatures. The presence of slow movements of the tetrapode mesogenic units in the low-temperature regime could also be detected through the analysis of the NMR spectra. Simulations indicate that these movements are mainly slow molecular reorientations of the mesogenic units associated with the presence of collective modes in the nematic phases of this compound. In the case of tetrapodes, recent investigations attribute the origin of biaxiality to the hindering of reorientations of the laterally attached mesogenic units which constitute the tetrapode. This study relates the molecular movements with the nematic biaxial ordering of the system.

NMR spectroscopy of proteins encapsulated in a positively charged surfactant

Traditionally, large proteins, aggregation-prone proteins, and membrane proteins have been difficult to examine by modern multinuclear and multidimensional solution NMR spectroscopy. A major limitation presented by these protein systems is that their slow molecular reorientation compromises many aspects of the more powerful solution NMR methods. Several approaches have emerged to deal with the various spectroscopic difficulties arising from slow molecular reorientation. One of these takes the approach of actively seeking to increase the effective rate of molecular reorientation by encapsulating the protein of interest within the protective shell of a reverse micelle and dissolving the resulting particle in a low viscosity fluid. Since the encapsulation is largely driven by electrostatic interactions, the preparation of samples of acidic proteins suitable for NMR spectroscopy has been problematic owing to the paucity of suitable cationic surfactants. Here, it is shown that the cationic surfactant CTAB may be used to prepare samples of encapsulated anionic proteins dissolved in low viscosity solvents. In a more subtle application, it is further shown that this surfactant can be employed to encapsulate a highly basic protein, which is completely denatured upon encapsulation using an anionic surfactant.

Solid State NMR Spectroscopic Investigations of Model Compounds for Imidazole-Based Proton Conductors

The temperature dependence and the exact geometry of slow molecular reorientations in imidazolium methyl sulfonate are investigated using modern one-dimensional MAS exchange spectroscopy. Earlier high-temperature studies have evidenced a fast 180° flip motion of the imidazole ring, which is shown here to slow on cooling and is believed to be a prototypical molecular process involved in Grotthus-type proton transport in imidazole-based proton conductors intended for fuel cell applications. It is further shown that valuable information on the relative orientations of CH and NH dipolar coupling tensors with respect to the chemical shift anisotropy tensors of the respective heteronuclei can be obtained from the MAS exchange data as well as from static 13C and 15N line shapes, without the necessity of performing more involved single-crystal NMR experiments. The principal axes of the CSA tensors are found to not coincide with the CH or NH bond axes, in contrast to earlier assumptions involving similar compounds...

Role of temperature in controlling performance of photorefractive organic glasses.

We present a detailed temperature-dependence study of dielectric, birefringent, conductive, and photorefractive (PR) properties of high-performance low-molecular weight organic glasses that contain 2-dicyanomethylene-3-cyano-2,5-dihydrofuran (DCDHF) derivatives. DCDHF organic glasses sensitized with C60 exhibit high two-beam coupling gain coefficients in the red-wavelength region. However, in the best performing DCDHF glasses at room temperature the PR dynamics are limited by slow molecular reorientation in the electric field. While orientational and, therefore, PR speed can be significantly improved by increasing the temperature above the glass-transition temperature of the material, the steady-state performance may worsen. Comprehensive study of the temperature dependence of various processes, which contribute to the PR effect in DCDHF glasses, clarifies the limiting factors and allows for optimization of the overall PR performance.

Influence of cholesterol on dynamics of dimyristoylphosphatidylcholine bilayers as studied by deuterium NMR relaxation

Investigation of the deuterium (2H) nuclear magnetic resonance (NMR) relaxation rates of lipid bilayers containing cholesterol can yield new insights regarding its role in membrane function and dynamics. Spin-lattice (R1Z) and quadrupolar order (R1Q) H2 NMR relaxation rates were measured at 46.1 and 76.8 MHz for macroscopically oriented bilayers of 1,2-diperdeuteriomyristoyl-sn-glycero-3-phosphocholine (DMPC-d54) containing cholesterol (1/1 molar ratio) in the liquid-ordered phase at 40 °C. The data for various segmental positions along the DMPC-d54 acyl chain were simultaneously fitted to a composite membrane deformation model, including fast segmental motions which preaverage the coupling tensor along the lipid acyl chain, slow molecular reorientations, and small-amplitude collective fluctuations. In contrast to pure DMPC-d54 in the liquid-crystalline (Lα) phase, for the DMPC-d54:cholesterol (1/1) system a linear square-law functional dependence of the relaxation rates on the order parameter (quadrupolar splitting) does not appear evident. Moreover, for acyl segments closer to the top of the chain, the angular anisotropy of the H2 R1Z and R1Q relaxation rates is more pronounced than toward the chain terminus. The residual (preaveraged) coupling tensor has its greatest effective asymmetry parameter near the polar groups, decreasing for the groups closest to the end of the chain. The results suggest that axial rotations of the phospholipid molecules occur at a somewhat higher rate than in pure bilayers, as a consequence of the higher ordering and reduction of chain entanglement. On the other hand, the rigid cholesterol molecule appears to undergo somewhat slower axial rotation, possibly due to its noncylindrical shape. Collective motions are found to be less predominant in the case of DMPC-d54:cholesterol than for pure DMPC-d54, which may indicate an increased dynamical rigidity of lipid bilayers containing cholesterol versus pure lipid systems.

Lineshape Calculations on Spreadsheet Software

Spectral lineshapes reflecting slow molecular reorientation are calculated by a simplified numeric approach based on the stochastic Liouville equation. The relevant reorientation process is described by a stationary Markov operator which is, using a finite grid point method, represented by a matrix with a dimension equivalent to the number of orientational sites. A differential equation for the time evolution of the density matrix allows the development of the overall magnetization to be calculated in consecutive time steps. For increased convenience and flexibility, the algorithm can be installed using commercial spreadsheet software on a regular personal computer.

Direct Determination of Motional Correlation Times by 1D MAS and 2D Exchange NMR Techniques

One- and two-dimensional static and magic-angle spinning (MAS) exchange NMR experiments for quantifying slow (τc> 1 ms) molecular reorientation dynamics are analyzed, emphasizing the extent to which motional correlation times can be extracteddirectlyfrom the experimental data. The static two-dimensional (2D) exchange NMR experiment provides geometric information, as well as exchange time scales via straightforward and model-free application of Legendre-type orientational autocorrelation functions, particularly for axially symmetric interaction tensors, as often encountered in solid-state2H and13C NMR. Under conditions of MAS, increased sensitivity yields higher signal-to-noise spectra, with concomitant improvement in the precision and speed of correlation time measurements, although at the expense of reduced angular (geometric) resolution. For random jump motions, one-dimensional (1D)exchange-inducedsidebands (EIS)13C NMR and the recently developed ODESSA and time-reverse ODESSA experiments complement the static and MAS two-dimensional exchange NMR experiments by providing faster means of obtaining motional correlation times. For each of these experiments, the correlation time of a dynamic process may be obtained from a simple exponential fit to the integrated peak intensities measured as a function of mixing time. This is demonstrated on polycrystalline dimethylsulfone, where the reorientation rates from EIS, ODESSA, time-reverse ODESSA, and 2D exchange are shown to be equivalent and consistent with literature values. In the analysis, the advantages and limitations of the different methods are compared and discussed.

A simplified approach to the interpretation of nuclear spin correlations in entangled polymeric liquids

We derive closed-form expressions for a sine correlation function β(2τ,τ) which is particularly sensitive to modulations of nuclear dipolar (and quadrupolar) interactions caused by slow molecular reorientations. This function is placed in the context of traditional nuclear magnetic resonance (NMR) experiments—the Free Induction Decay R(t) and the solid echo amplitude E(τ) and families of expressions are evaluated for (i) motion governed by an exponential correlation function and (ii) macromolecular motion dominated by reptation. We introduce a simple correlation function for reptation that depends on only two parameters, M2¯, the preaveraged dipolar interaction strength and τd, the tube disengagement time. From proton NMR measurements of β in poly(dimethyl siloxane) melts we estimate M2¯=1.3×106 s−2 and τd=0.019 s at 300 K, in reasonable agreement with a calculated value for M2¯ and an independent NMR estimate of the terminal relaxation time.

A Two-Dimensional NMR Experiment for the Study of Slow Motions in Complex Chemical Systems

Abstract The use of a two-dimensional NMR experiment for the study of slow molecular reorientations in complex chemical systems is proposed. The experiment utilizes powder lineshapes arising from chemical-shift anisotropy and benefits from the fact that the effective chemical-shift anisotropy which operates during thet1period of the experiment may be “tuned” so that the powder lineshape is sensitive to the particular frequency of motion present in a sample. Theoretical analysis of the experiment is presented as well as preliminary experimental results on dimethylsulfone.

Variable-angle double rotation technique: a new two-dimensional high-resolution technique for quadrupolar nuclei

We describe a new two-dimensional high-resolution technique for nuclei with semi-integer spins subjected to strong quadrupole interactions. This variable-angle double rotation (VADOR) technique separates anisotropic spectral patterns according to the isotropic shift of each species. No sudden sample reorientation is needed for VADOR which can consequently be used regardless of the “relaxation times”. This technique can also be utilized to characterize slow molecular reorientations in two- or three-dimensional experiments. We also propose a new geometry for DOR probes ( θ e = 70.124 °, θ i = 54.736 °) which suppresses first-order interactions (chemical shift anisotropy and dipolar) more efficiently than the geometry ( θ e = 54.736 °, θ i = 30.556 °) presently employed.

Two-Dimensional Solid State NMR Studies of Ultraslow Molecular Reorientation in Solid Polymers

Abstract Slow molecular reorientations in solid polymers have been investigated by means of two-dimensional solid state 2H- and 13C-NMR.

Proton N.M.R. lineshape in nematic microdroplets

Abstract The dependence of the proton N.M.R. absorption spectrum on nematic director configuration and molecular self-diffusion in nematic submicrondroplets is analysed. The lineshape is evaluated numerically for radial and bipolar director configuration. The motional averaging is taken into account by means of a numerical simulation of the molecular diffusion which induces slow molecular reorientations due to non-uniform orientational ordering in the droplet. This diffusion process strongly affects the absorption spectra of the radial configuration, whereas spectra of the bipolar configuration are only slightly influenced. The possibility of determinating the submicrometre nematic droplet structures using the proton N.M.R. lineshape is discussed.