Transverse Nuclear Spin Relaxation Due to Director Fluctuations in Liquid Crystals A Slow-Motional Theory

Chromatographic NMR spectroscopy: the effect of hollow silica microspheres on magnetic field inhomogeneities and resonance lineshapes

We provide a description of the line broadening phenomenon observed in chromatographic NMR spectra with hollow silica microspheres.

Applications of Solid-State NMR Spectroscopy for the Study of Lipid Membranes with Polyphilic Guest (Macro)Molecules

The incorporation of polymers or smaller complex molecules into lipid membranes allows for property modifications or the introduction of new functional elements. The corresponding molecular-scale details, such as changes in dynamics or features of potential supramolecular structures, can be studied by a variety of solid-state NMR techniques. Here, we review various approaches to characterizing the structure and dynamics of the guest molecules as well as the lipid phase structure and dynamics by different high-resolution magic-angle spinning proton and 13C NMR experiments as well as static 31P NMR experiments. Special emphasis is placed upon the incorporation of novel synthetic polyphilic molecules such as shape-persistent T- and X-shaped molecules as well as di- and tri-block copolymers. Most of the systems studied feature dynamic heterogeneities, for instance those arising from the coexistence of different phases; possibilities for a quantitative assessment are of particular concern.

Slow motions in microcrystalline proteins as observed by MAS-dependent 15N rotating-frame NMR relaxation

(15)N NMR relaxation rate R1ρ measurements reveal that a substantial fraction of residues in the microcrystalline chicken alpha-spectrin SH3 domain protein undergoes dynamics in the μs-ms timescale range. On the basis of a comparison of 2D site-resolved with 1D integrated (15)N spectral intensities, we demonstrate that the significant fraction of broad signals in the 2D spectrum exhibits the most pronounced slow mobility. We show that (15)N R1ρ's in proton-diluted protein samples are practically free from the coherent spin-spin contribution even at low MAS rates, and thus can be analysed quantitatively. Moderate MAS rates (10-30 kHz) can be more advantageous in comparison with the rates >50-60 kHz when slow dynamics are to be identified and quantified by means of R1ρ experiments.

Brief overview on 2H NMR studies of polysiloxane-based side-chain nematic elastomers

This is a brief overview on recent studies on liquid crystalline elastomers (LCEs) based on polysiloxane chain, in the form of monodomain films, selectively (2)H-labeled in different parts of the LCE samples, i.e. on the crosslinker or mesogenic units. (2)H NMR spectroscopic techniques were used to measure the temperature dependence of the quadrupolar splittings, line widths and relaxation times, T(1) and T(2). From these data, several information about the orientational order parameter, S, of various LCE fragments, thermodynamic features of the isotropic-nematic transition and main motional processes could be generalized for this type of elastomers.

An integrated approach to NMR spin relaxation in flexible biomolecules: application to beta-D-glucopyranosyl-(1-->6)-alpha-D-mannopyranosyl-OMe

The description of the reorientational dynamics of flexible molecules is a challenging task, in particular when the rates of internal and global motions are comparable. The commonly used simple mode-decoupling models are based on the assumption of statistical independence between these motions. This assumption is not valid when the time scale separation between their rates is small, a situation that was found to arise in oligosaccharides in the context of certain internal motions. To make possible the interpretation of NMR spin relaxation data from such molecules, we developed a comprehensive approach generally applicable to flexible rotators with one internal degree of freedom. This approach integrates a stochastic description of coupled global tumbling and internal torsional motion, quantum chemical calculations of the local potential and the local geometry at the site of the restricted torsion, and hydrodynamics-based calculations of the diffusive properties. The method is applied to the disaccharide beta-D-Glcp-(1-->6)-alpha-D-[6-(13)C]-Manp-OMe dissolved in a DMSO-d(6)/D(2)O cryosolvent. The experimental NMR relaxation parameters, associated with the (13)CH(2) probe residing at the glycosidic linkage, include (13)C T(1) and T(2) and (13)C-{(1)H} nuclear Overhauser enhancement (NOE) as well as longitudinal and transverse dipole-dipole cross-correlated relaxation rates, acquired in the temperature range of 253-293 K. These data are predicted successfully by the new theory with only the H-C-H angle allowed to vary. Previous attempts to fit these data using mode-decoupling models failed.

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.

Slow Dynamics of Banana-Shaped Molecules: A Theoretical Approach to Analyze 2H-NMR T2 Relaxation Times

In the present work, we analyze pulsed deuterium NMR experiments performed on the isotropic and nematic phases of the banana-shaped liquid-crystalline mesogen 4-chloro-1,3-phenylene bis{4-4'-(11-undecenyloxy) benzoyloxy} benzoate (ClPbis11BB) selectively deuterated on the central ring. Starting from a previous evidence of unusual slow dynamics in the isotropic phase (Domenici V. et al., J. Phys. Chem. B 2005, 109, 769), a quantitative and model-supported analysis of the deuterium NMR data is performed here by accounting for slow-motional modulation of the magnetic anisotropies through the full solution of the stochastic Liouville equation. Focusing on the quadrupolar echo experiments performed in the nematic phase, the analysis of the transverse relaxation rate has been carried out by considering single-molecule motions and fluctuations of the local director. The main conclusions are: (a) director fluctuations are not relevant on driving the signal relaxation; (b) molecular reorientations about transverse axes control the dynamic regime of the signal relaxation and impose a full slow-motional treatment; (c) the small amplitude tumbling of the molecule within the wells of orientational potential occurs with characteristic times up to the microsecond. The outcome of our analysis has to be taken as indicative of very slow dynamics concerning out-of-plane motions of the molecules. Besides the specific application, this paper also offers the methodological tools to treat the pulsed deuterium NMR experiment in the slow-motional regime of reorientational motions and provides a detailed comparison with the usually employed fast-motional approximation.

Collective Fluctuations in Ordered Fluids Investigated by Two-Dimensional Electron−Electron Double Resonance Spectroscopy

Two-dimensional electron-electron double resonance (2D-ELDOR) is a technique that is sensitive to the dynamical processes affecting spin labels in complex fluid environments. In ordered fluids, such as membrane vesicles, the 2D-ELDOR experiment is affected by the molecular tumbling in the locally ordered environment. This motion occurs on two different time scales, the faster molecular motion relative to the local director, and the slower collective fluctuations of the director field. In the experimental study of Patyal, Crepeau, and Freed (Biophys. J. 1997, 73, 2201), it was found that the widths of the autopeaks of the 2D-ELDOR spectrum increased as a function of the mixing time. In the present work, a theory is developed for the effects of director fluctuations on the autopeaks in the 2D-ELDOR experiment by employing an analytical solution of the stochastic Liouville equation for which the director field is treated as a multidimensional Gaussian process, as previously developed by Frezzato, Kothe, and Moro (J. Phys. Chem. B 2001, 105, 1281 and J. Phys. Chem. B 2004, 108, 9505). Good agreement is found between theory and experiment, notably the only adjustable parameter is k, the bending elastic modulus of the membrane. The values of k = 11 x 10(-20) J for 1,2-dipalmitoyl-sn-glycero-phosphatidylcholine (DPPC) vesicles and k = 15 x 10(-20) J for DPPC/gramicidin A (5:1) vesicles, both at 45 degrees C, were found from the analysis and agree well with previous related measurements by other physical techniques. This establishes 2D-ELDOR as a useful technique to study the elastic properties of biological membranes.

Phase Biaxiality in Nematic Liquid Crystalline Side-Chain Polymers of Various Chemical Constitutions

In a previous deuterium NMR study conducted on a liquid crystalline (LC) polymer with laterally attached book-shaped molecules as the mesogenic moiety, we have revealed a biaxial nematic phase below the conventional uniaxial nematic phase (Phys. Rev. Lett. 2004, 92, 125501). To elucidate details of its formation, we here report on deuterium NMR experiments that have been conducted on different types of LC side-chain polymers as well as on mixtures with low-molar-mass mesogens. Different parameters that affect the formation of a biaxial nematic phase, such as the geometry of the attachment, the spacer length between the polymer backbone and the mesogenic unit, as well as the polymer dynamics, were investigated. Surprisingly, also polymers with terminally attached mesogens (end-on polymers) are capable of forming biaxial nematic phases if the flexible spacer is short and thus retains a coupling between the polymer backbone and the LC phase. Furthermore, the most important parameter for the formation of a biaxial nematic phase is the dynamics of the polymer backbone, as the addition of a small percentage of low molar mass LC to the biaxial nematic polymer from the original study served to shift both the glass transition and the appearance of detectable biaxiality in a very similar fashion. Plotting different parameters for the investigated systems as a function of T/Tg also reveals the crucial role of the dynamics of the polymer backbone and hence the glass transition.

Collective and local molecular dynamics in the lyotropic mesophases of decylammonium chloride: 1H and 2H NMR study

The collective and individual dynamics of decylammonium chloride (DACl) molecules in water environment were investigated as a function of surfactant concentration and temperature. In the presence of water the DACl forms a variety of self-assembled structures, ranging from isotropic micellar systems to lyotropic liquid crystalline phases of hexagonal, nematic, and lamellar types. In order to characterize the complex molecular dynamics that occur in the DACl-water system, we applied 1H and 2H NMR techniques that cover the whole frequency range between 1 kHz and 30 MHz. The slow molecular dynamics were studied by 1H NMR fast-field-cycling T1 measurements and pulse-frequency dependence of 2H NMR transverse relaxation time, performed by means of the Carr-Purcell-Meiboom-Gill sequence. We detected a well-expressed contribution of order director fluctuations, i.e., layer undulations, with characteristic omega(-1)(L) frequency dependence of T(-1)(1) in the lamellar phase. Its presence indicates a relatively weak impact of interactions between neighboring DACl layers. The frequency dependence of proton T(-1)(1) in the hexagonal phase exhibits a different type of frequency dispersion, T(-1)(1) approximately omega(-1.32)(L). The increase in the exponent is explained with the quasi-one-dimensional character of fluctuations in elongated cylinders. Further, the T1 and T2 relaxation times of deuterons selectively attached to the C2 and C7 segments of the hydrocarbon chains of DACl were measured at a Larmor frequency of 30.7 MHz, providing quantitative information about local molecular dynamics.