Water transbilayer diffusion in macroscopically oriented lipid bilayers as studied by pulsed field gradient NMR

Water self-diffusion in lipid bilayers macroscopically oriented on glass plates was studied by pulsed field gradient¹H nuclear magnetic resonance technique. Diffusion decays were multicomponent with a distribution of diffusion coefficients ranging from about 10⁻¹⁰ to about 10⁻¹³ m²/s. A number of measurements with variations of the sample orientation, diffusion time and the distance between the glass plates showed that the “fast” component of diffusion corresponds to water in the bilayer “cracks”. The “slow” component of diffusion corresponds to transbilayer water diffusion in the long-diffusion-time regime. For a more reliable separation of parts corresponding to fast and slow diffusion of water, a “component-resolved spectroscopy” method for the global analysis of correlated spectral data (P. Stilbs, K. Paulsen, P.C. Griffiths: J. Phys. Chem. 100, 8180, 1996) was applied.     

Structural and dynamic properties of polyoxyethylene sorbitan monooleate micelle in water dispersion studied by pulsed field gradient NMR

The diffusion phenomenon of a nonionic surfactant, polyoxyethylene sorbitan monooleate (POE-SMO), micelle in aqueous solution was investigated by pulsed field gradient nuclear magnetic resonance (PFG NMR) with a high gradient strength of 17.4 T/m at the diffusion timet _d varied from 3 to 300 ms. This high gradient strength allowed us to measure the slow self-diffusion coefficient of POE-SMO micelle, and the short diffusion time below 10 ms showed the restricted diffusion of the micelle. At the shortt _d the self-diffusion of the micelle was restricted and the restricted sizes were 1.8, 1.5, and 0.8 μm for the POE-SMO concentration of 100, 200 and 300 mM, respectively, and 0.6 μm for the POE-SMO only. The possible reason of this restriction was assumed to be the formation of a spatial network or a micellar clustering. Furthermore, a proton exchange between water molecule and surfactant OH group on the micelle surface was proposed. With respect to this proposal, the residence time of the proton at the micelle surface and the thickness of the surface were investigated from proton self-diffusion coefficients by PFG NMR.     

NMR in pulsed magnetic field

Nuclear magnetic resonance (NMR) experiments in pulsed magnetic fields up to 30.4 T focused on ¹H and ⁹³Nb nuclei are reported. Here we discuss the advantage and limitation of pulsed field NMR and why this technique is able to become a promising research tool.     

Background gradient suppression in stimulated echo NMR diffusion studies using magic pulsed field gradient ratios

By evaluating the spin echo attenuation for a generalized 13-interval PFG NMR sequence consisting of pulsed field gradients with four different effective intensities (F(p/r) and G(p/r)), magic pulsed field gradient (MPFG) ratios for the prepare (G(p)/F(p)) and the read (G(r)/F(r)) interval are derived, which suppress the cross term between background field gradients and the pulsed field gradients even in the cases where the background field gradients may change during the z-store interval of the pulse sequence. These MPFG ratios depend only on the timing of the pulsed gradients in the pulse sequence and allow a convenient experimental approach to background gradient suppression in NMR diffusion studies with heterogeneous systems, where the local properties of the (internal) background gradients are often unknown. If the pulsed field gradients are centered in the tau-intervals between the pi and pi/2 rf pulses, these two MPFG ratios coincide to eta=G(p/r)/F(p/r)=1-8/[1+(1/3)(delta/tau)(2)]. Since the width of the pulsed field gradients (delta) is bounded by 0< or =delta< or =tau, eta can only be in the range of 5< or =-eta< or =7. The predicted suppression of the unwanted cross terms is demonstrated experimentally using time-dependent external gradients which are controlled in the NMR experiment as well as spatially dependent internal background gradients generated by the magnetic properties of the sample itself. The theoretical and experimental results confirm and extend the approach of Sun et al. (J. Magn. Reson. 161 (2003) 168), who recently introduced a 13-interval type PFG NMR sequence with two asymmetric pulsed magnetic field gradients suitable to suppress unwanted cross terms with spatially dependent background field gradients.     

Ionic and molecular Self-Diffusion in Ion-Exchange materials for fuel energetics studied by pulsed field gradient NMR

Pulsed field gradient nuclear magnetic resonance technique was applied to investigate the self-diffusion mechanism of water, alcohol molecules and Li⁻ counterions in sulfocation exchangers with different structures of the polymeric matrix. It was shown that in the homogeneous perfluorinated sulfocation exchange membranes the ionic and water translation motions are controled by the hydrogen bond network forming in ionogenic channels at the high water content. At the low solvent content, the self-diffusion coefficients of methanol and ethanol are higher than the water self-diffusion coefficients. The influence of non-ion-exchange sorbed electrolyte on Li⁺ self-diffusion coefficients was observed in the heterogeneous sulfocation exchanger KU-23.     

Water self-diffusion in Chlorella sp. studied by pulse field gradient NMR

The water self-diffusion behavior in chlorella water suspension was investigated by pulsed field gradient NMR technique. Three types of water was determined, which differs according to the self-diffusion coefficients; bulk water, extracellular and intracellular water. Intracellular and extracellular water self-diffusion were restricted, and the sizes of restriction regions were 3.4 microm and 17 microm, respectively. The water molecular exchange process between these three diffusion regions was investigated. The residence time and exchange rate constant for chlorella cells were obtained. The cell wall permeability determined from the rate constant as 3 x 10(-6) m/s agreed with the permeability 10(-6) m/s obtained from time dependence of intracellular water self-diffusion coefficient. The structural cluster model of chlorella cell is estimated to describe the extracellular water self-diffusion in chlorella water suspension.     

Chemical exchange of hydroxyl protons in quercetin measured by pulsed field gradient NMR

The diffusion behavior of hydroxyl protons (OH) in quercetin in 100% DMSO-d₆ (deuterium dimethylsulfoxide) and a 90% DMSO-d₆ solution containing 10% H₂O was investigated with 600 MHz ¹H pulsed field gradient (PFG) nuclear magnetic resonance (NMR). Only resonances of the 5-hydroxyl protons (OH5) were well resolved in NMR spectra of quercetin for all solutions under study. This phenomenon is explained by the intramolecular hydrogen bonding between OH5 protons and the 4-carbonyl oxygen (CO4). During diffusion experiments, the OH5 protons showed a biexponential diffusion decay, indicating an exchange process with water. As water content in the solvents increased, the lifetime (τ _q) of the OH5 protons decreased from 96.7±10.0 ms in 100% DMSO-d₆ to 14.3±1.4 ms in the 90% DMSO-d₆ solution containing 10% H₂O, indicating an increase in the exchange rate (k _q = l/τ _q) of the OH5 protons. This study demonstrates that the diffusion approach with PFG-NMR is much faster and easier for estimating the short lifetime or fast exchange rate of hydroxyl protons in quercetin.     

Measuring pore connectivity by pulsed field gradient diffusion editing with hydrocarbon gases

Measurements of time-dependent diffusion are performed on a rock sample saturated first with water, then methane and finally ethane. The gases were selected because their increased diffusivities and relaxation times allow probing greater length scales than water and because of their practical relevance. The nuclear magnetic resonance measurements employed pulse field gradient diffusion editing pulse sequences, allowing analysis of D(t) as a function of relaxation time. Very different D(t) behaviors are observed for different relaxation times, including indications of connected pore networks at moderate relaxation times.     

Exploring the surface permeability of nanoporous particles by pulsed field gradient NMR

A new method to determine the surface permeability of nanoporous particles is proposed. It is based on the comparison of experimental data on tracer exchange and intracrystalline molecular mean square displacements as obtained by the PFG NMR tracer desorption technique with the corresponding solutions of the diffusion equation via dynamical Monte Carlo simulations. The method is found to be particularly sensitive in the "intermediate" regime, when the influence of intracrystalline diffusion and surface resistances of the nanoporous crystal on molecular transport are comparable and the conventional method fails. As an example, the surface permeabilities of two samples of zeolite NaCaA with different crystal sizes are determined with methane, as a probe molecule, at room temperature.     

Formation of p-cresol:piperazine complex in solution monitored by spin-lattice relaxation times and pulsed field gradient NMR diffusion measurements

A study of the nature of the anthelmintic p-cresol:piperazine complex in chloroform solution has been conducted using different NMR techniques: self-diffusion coefficients using DOSY; NOE, NULL, and double-selective T1 measurements to determine inter-molecular distances; and selective and non-selective T1 measurements to determine correlation times. The experimental results in solution and CP-MAS were compared to literature X-ray diffraction data using molecular modeling. It was shown that the p-cresol:piperazine complex exists in solution in a very similar manner as it does in the solid state, with one p-cresol molecule hydrogen bonded through the hydroxyl hydrogen to each nitrogen atom of piperazine. The close correspondence between the X-ray diffraction data and the inter-proton distances obtained by NULL and double selective excitation techniques indicate that those methodologies can be used to determine inter-molecular distances in solution.     

Anisotropic magnetic fluctuations in the ferromagnetic superconductor UCoGe studied by direction-dependent 59Co NMR measurements

We have carried out direction-dependent 59Co NMR experiments on a single crystal sample of the ferromagnetic superconductor UCoGe in order to study the magnetic properties in the normal state. The Knight-shift and nuclear spin-lattice relaxation rate measurements provide microscopic evidence that both static and dynamic susceptibilities are ferromagnetic with strong Ising anisotropy. We discuss that superconductivity induced by these magnetic fluctuations prefers spin-triplet pairing state.     

Microscopic displacement imaging with pulsed field gradient turbo spin-echo NMR

We present a pulse sequence that enables the accurate and spatially resolved measurements of the displacements of spins in a variety of (biological) systems. The pulse sequence combines pulsed field gradient (PFG) NMR with turbo spin-echo (TSE) imaging. It is shown here that by ensuring that the phase of the echoes within a normal spin-echo train is constant, displacement propagators can be generated on a pixel-by-pixel basis. These propagators accurately describe the distribution of displacements, while imaging time is decreased by using separate phase encoding for every echo in a TSE train. Measurements at 0.47 T on two phantoms and the stem of an intact tomato plant demonstrate the capability of the sequence to measure complete and accurate propagators, encoded with 16 PFG steps, for each pixel in a 128 x 128 image (resolution 117 x 117 x 3,000 microm) within 17 min. Dynamic displacement studies on a physiologically relevant time resolution for plants are now within reach.     

Probing lateral diffusion in lipid membranes on nanoscale by PFG NMR with high gradient strength

This work demonstrates the feasibility of noninvasive studies of diffusion on a submicrometer length scale in aligned model lipid membranes using pulsed field gradient nuclear magnetic resonance with ultrahigh (up to 35 T/m) gradient strength. Application of such gradients allows the use of sufficiently small diffusion times under conditions of narrow-pulse approximation. As a result, monitoring anomalous or restricted diffusion in lipid membranes on a length scale in the range of 100 nm becomes possible. The ability to study diffusion in lipid membranes on this length scale is very important because it is comparable with the size of biologically relevant domains (i.e., rafts), which are believed to exist in biomembranes.     

Fractal geometry of surface areas of sand grains probed by pulsed field gradient NMR

Pulsed field gradient NMR self-diffusion studies of water were used to determine surface-to-volume ratios and specific surface areas of the grains forming a glacial sand deposit. Both quantities exhibit a noninteger power-law dependence as a function of the diameters of the grains. The associated fractal dimensions of the surface area ( D(s)) and of the pore volume ( D(v)) are found to be D(s)-D(v) = -0.70+/-0.05 and D(s) = 2.20+/-0.05. The results demonstrate that NMR studies with native pore fluids are suitable to investigate the fractal nature of natural, unconsolidated porous materials.     

Anisotropic diffusion in a nematic liquid crystal- An electric field PFG NMR approach

The access to self-diffusion coefficients in anisotropic systems such as thermotropic liquid crystals by means of PFG NMR is complicated by strong dipolar interactions. Additionally, problems arise due to the immediate orientation of low-molar-mass nematic liquid crystals in an external field. The director orientation can be changed by the application of an additional electric field. This can be exploited in order to reduce the dipolar interaction to such an extent that the NMR linewidths change from a solid-state to a liquid-like situation enabling PFG NMR experiments.     

Diffusion and aggregation of Alzheimer’s Aβ1–40 peptide in aqueous trifluoroethanol solutions as studied by pulsed field gradient NMR

Pulsed field gradient nuclear magnetic resonance technique was applied to measure the self-diffusion coefficient of Aβ_1–40 peptide in trifluoroethanol (TFE) and mixed solvent TFE-water (D₂O) buffer (pD 7.8) at 293 K. The data were analyzed on the basis of the Stokes model and the hardsphere approach was used to estimate self-diffusion coefficients. It was found that the extent of the Aβ_1–40 aggregation in TFE solutions depends on the concentration of the peptide and the sample preparation protocol. After soft mixing, i.e., without any additional mechanical pretreatment of the peptide, the peptide is present in the monomeric form in TFE solutions. However, the additional water-bath sonication of the sample during the dissolution of Aβ_1–40 in TFE enforces oligomerization of the peptide with the size of aggregates ranging from tetra- to hexamers. An increase of D₂O in the mixed TFE-D₂O solvent of up to 75% leads to the aggregation of the larger part of the peptide. However, the components of self-diffusion coefficients related to low-mass Aβ_1–40 oligomers (dimers and trimers) were not observed in the diffusion decay curves. The most probable explanation is that dimers and trimers are not the principal intermediate species in the aggregation of Aβ_1–40 peptide.     

NMR relaxation and pulsed field gradient study of alginate bead porous media

Experiments are presented, which correlate molecular displacement with the multi-exponential T2 relaxation times of water flowing and diffusing through an alginate bead pack. Three systems were studied comprising beads of 3, 1 or < mm in diameter. T2-resolved propagators were obtained through a combined pulsed gradient stimulated echo (PGSTE) and Carr-Purcell-Meiboom-Gill (CPMG) experiment. Fourier transformation with respect to q produces a propagator for each echo in the CPMG train. Inverse Laplace transformation of the CPMG decays for each point (Z) in the propagator produced a two-dimensional propagator. Analysis of these two-dimensional propagators provided insight into the transport and exchange behaviour of water flowing through this system. This experiment has been simulated in a model bead structure and the resulting T2 relaxation time behaviour and T2-resolved propagators were found to be in good agreement with the experimental data. We also present a theoretical analysis of the response to the combined PGSTE/CPMG sequence in the simple model case of Pouseille flow in a cylindrical capillary, where diffusion to a surface sink is assumed to be the dominant relaxation mechanism.