Nuclear Magnetic Resonance Diffusion Measurements Research Articles

Structure and Ionic Conductivity of Li-Disordered Bismuth o-Thiophosphate Li60-3xBi16+x(PS4)36.

The structure of the first lithium-containing bismuth ortho (o)-thiophosphate was determined using a combination of powder X-ray, neutron, and electron diffraction. Li60-3xBi16+x(PS4)36 with x in the range of 4.1-6.5 possesses a complex monoclinic structure [space group C2/c (No. 15)] and a large unit cell with the lattice parameters a = 15.4866 Å, b = 10.3232 Å, c = 33.8046 Å, and β = 85.395° for Li44.4Bi21.2(PS4)36, in agreement with the structure as observed by X-ray and neutron pair distribution function analysis. The disordered distribution of lithium ions within the interstices of the dense host structure and the Li ion dynamics and diffusion pathways have been investigated by solid-state nuclear magnetic resonance (NMR) spectroscopy, pulsed field gradient NMR diffusion measurements, and bond valence sum calculations. The total lithium ion conductivities range from 2.6 × 10-7 to 2.8 × 10-6 S cm-1 at 20 °C with activation energies between 0.29 and 0.32 eV, depending on the bismuth content. Despite the highly disordered nature of lithium ions in Li60-3xBi16+x(PS4)36, the underlying dense host framework appears to limit the dimensionality of the lithium diffusion pathways and emphasizes once more the necessity of a close inspection of the structure-property relationships in solid electrolytes.

NMR relaxation and diffusion studies to probe the motional dynamics of risperidone within PLGA microsphere

The present study aims to investigate the motional dynamics of risperidone within polylactic co-glycolic acid (PLGA) microsphere by employing solution state 1H and 19F nuclear magnetic resonance (NMR) measurements. Risperidone, a second-generation fluorinated antipsychotic drug used for the treatment of schizophrenia is commercially marketed as PLGA microsphere formulation resulting in prolonged release of the drug in solution. Although the current trend in the pharmaceutical market is to develop drug formulation with long-acting release (LAR) products, complete physicochemical characterization of such formulations are scarce. Especially the effects of microsphere encapsulation on the motional properties and diffusion behavior of the drugs are not discussed adequately in any of the earlier reports. We therefore, have employed NMR relaxation and diffusion measurements to decipher the interaction of PLGA cavity water with risperidone. A detailed analysis of NMR relaxation rates confirmed the event of encapsulation and the presence of local motion in the non-fluorinated end of risperidone. Further, the relaxation data indicated a significant alteration in 19F chemical shift anisotropy (CSA) and CSA/dipole-dipole (DD) cross-correlated relaxation mechanism and decreased effect of solvent relaxation pointing out reduced water concentration within the microsphere cavity. 1H and 19F diffusion coefficients of risperidone led to the information about hydrodynamic radius of risperidone in free and encapsulated states. Measurement of hydrodynamic radius supported the presence of limited water in PLGA cavity allowing higher translational mobility of risperidone after the encapsulation.

Quantifying acid diversion efficiency through NMR tortuosity measurements

The diversion efficiency measures the acid capacity to change its flow direction to the lower permeability reservoir sections. A good acid diverter creates a tortuous (zigzagged) wormhole within the acidized core sample in a laboratory setup. Here, we studied two different acid diverters compared to the conventional hydrochloric acid (HCl) to investigate their efficiency in changing the wormhole direction inside the rock core. Nuclear magnetic resonance (NMR) can be used to measure the 3D tortuosity of a rock sample; hence, we propose it as a tool to measure acid diversion efficiency because it can be applied in the field and in the laboratory. Two acid systems were utilized: straight 15 wt.% HCl acid and gelled acid consisting of 15 wt.% HCl acid and polyacrylamide polymer (PAM). Four coreflooding experiments were conducted on 1.5 inch-diameter × 3 inch-length Indiana limestone samples, two with straight HCl acid and two with gelled HCl acid at different PAM concentrations. NMR was utilized to measure the T2 distribution of the rock samples and diffusion tortuosity in two orthogonal directions. Also, X-ray computed tomography (CT) of the acidized samples was taken to visualize the wormholes. Results showed that the polymer-based acid-created zigzagged paths, and more acid volume was consumed to create the wormholes using CT. NMR diffusion tortuosity measurements showed that the tortuosity was reduced along the wormhole direction in all experiments. Nevertheless, the gelled acid treatments showed a significant reduction of the tortuosity orthogonal to the main wormhole path. The diversion efficiency of the 50 lbm/ 1000 gal PAM-gelled acid was 35% higher compared to the straight HCl acid. On the contrary, the 30 lbm/ 1000 gal PAM-gelled acid gave only 10% improvement in the diversion. The index could screen quantitatively which diverting acid is more efficient. We introduced a new diversion index based on the NMR diffusion measurements in this study to quantify the diversion capacity of an acid system in carbonate rock.

Explicit phenomenological solutions for magnetization exposed to an arbitrary NMR diffusion steady state pulse sequence.

Explicit phenomenological solutions to recurrence relations for the bulk transverse and longitudinal magnetization found using the Torrey-Bloch equations with relaxation effects are used to investigate nuclear magnetic resonance (NMR) diffusion measurements. Of particular interest are steady state NMR (self-)diffusion measurements that reduce experimental time that can extend the techniques to quickly reacting systems. The solutions for bulk transverse and longitudinal magnetization presented here are used to investigate the average behavior of the transverse and longitudinal magnetization in forming a steady state and are used to derive new expressions for the steady state longitudinal magnetization. These solutions can be applied to a noninteracting spin 1/2 ensemble undergoing free diffusion exposed to an arbitrary NMR pulse sequence containing arbitrary magnetic field gradient waveforms. The closed algebraic form method presented here has an advantage over iterative procedures for calculating transverse and longitudinal magnetization for the analysis and development of steady state pulse sequences. Previous theoretical results for steady state diffusion measurements are also reproduced. The Mathematica code for these solutions is provided in the supplementary material.

Creation of Al‐Enriched Mesoporous ZSM‐5 Nanoboxes with High Catalytic Activity: Converting Tetrahedral Extra‐Framework Al into Framework Sites by Post Treatment

AbstractZSM‐5 zeolite nanoboxes with accessible meso‐micro‐pore architecture and strong acid sites are important in relevant heterogeneous catalysis suffering from mass transfer limitations and weak acidities. Rational design of parent zeolites with concentrated and non‐protective coordination of Al species can facilitate post‐synthetic treatment to produce mesoporous ZSM‐5 nanoboxes. In this work, a simple and effective method was developed to convert parent MFI zeolites with tetrahedral extra‐framework Al into Al‐enriched mesoporous ZSM‐5 nanoboxes with low silicon‐to‐aluminium ratios of ≈16. The parent MFI zeolite was prepared by rapid ageing of the zeolite sol gel synthesis mixture. The accessibility to the meso‐micro‐porous intra‐crystalline network was probed systematically by comparative pulsed field gradient nuclear magnetic resonance diffusion measurements, which, together with the strong acidity of nanoboxes, provided superb catalytic activity and longevity in hydrocarbon cracking for propylene production.

Creation of Al-Enriched Mesoporous ZSM-5 Nanoboxes with High Catalytic Activity: Converting Tetrahedral Extra-Framework Al into Framework Sites by Post Treatment.

ZSM‐5 zeolite nanoboxes with accessible meso‐micro‐pore architecture and strong acid sites are important in relevant heterogeneous catalysis suffering from mass transfer limitations and weak acidities. Rational design of parent zeolites with concentrated and non‐protective coordination of Al species can facilitate post‐synthetic treatment to produce mesoporous ZSM‐5 nanoboxes. In this work, a simple and effective method was developed to convert parent MFI zeolites with tetrahedral extra‐framework Al into Al‐enriched mesoporous ZSM‐5 nanoboxes with low silicon‐to‐aluminium ratios of ≈16. The parent MFI zeolite was prepared by rapid ageing of the zeolite sol gel synthesis mixture. The accessibility to the meso‐micro‐porous intra‐crystalline network was probed systematically by comparative pulsed field gradient nuclear magnetic resonance diffusion measurements, which, together with the strong acidity of nanoboxes, provided superb catalytic activity and longevity in hydrocarbon cracking for propylene production.

Diffusional Motion of Solvated Ions in Super-Concentrated Electrolytes

The evolution of solvation structure and dynamics of aqueous LiTFSI based electrolytes with varying salt concentration and competing divalent (Zn2+, Mg2+ and Ca2+) and ammonium based (Me3EtN+) cations were studied using 7Li, 19F and 1H nuclear magnetic resonance (NMR) spectroscopy and pulsed-field gradient (PFG) NMR methods. The NMR peak intensity analysis (Figure 1(a)), reveals that more than half of Li+, TFSI- and H2O molecules are invisible in liquid NMR spectra, likely due to the loss of mobility (broader NMR line width) associated with formation of bigger solvate clusters at higher salt concentrations. The 7Li, 19F and 1H chemical shift values (Figure 1(b)), were shifted to the higher field (more negative chemical shift values) with salt concentration, indicating the interaction between Li+ cations and TFSI- anions became stronger, while the hydrogen bonding network between H2O molecules became weaker resulting from the localization of water clusters in the solutions. The mobility of the remaining species were analyzed through the PFG-NMR based diffusion measurements (Figure 1(c)). It showed that Li+ cation diffuses relatively faster than TFSI- anion (D H2O>D Li>D TFSI) for these aqueous electrolytes, and differ from non-aqueous electrolyte solutions which traditionally register higher anion diffusion rates (D anion > D Li) [1-4]. The D H2O/D TFSI diffusion ratio showed that the relative mobility of water molecules increases with LiTFSI concentration and by addition of multivalent (Zn2+, Mg2+ and Ca2+) cation salt. The D Li/D TFSI diffusion ratio reaches the maximum value of ~3.4 for the 20 m LiTFSI/H2O solution, and the addition of competing cations tends to minimally affect the lithium diffusion. The different behavior between D H2O/D TFSI and D Li/D TFSI values suggest that the localized water molecules diffuse freely in the higher concentration electrolytes, while the diffusion motions of Li+ cation and TFSI- anion depend strongly on the total composition of the competing ions in the solution. The structural and diffusional properties of high concentration electrolytes will be discussed based on diffusion and chemical shift analysis. References Frömling, T., et al., Enhanced Lithium Transference Numbers in Ionic Liquid Electrolytes. J. Phys. Chem. B, 2008. 112(41): p. 12985-12990.Hayamizu, K., Temperature Dependence of Self-Diffusion Coefficients of Ions and Solvents in Ethylene Carbonate, Propylene Carbonate, and Diethyl Carbonate Single Solutions and Ethylene Carbonate + Diethyl Carbonate Binary Solutions of LiPF6 Studied by NMR. J. Chem. Eng. Data, 2012. 57(7): p. 2012-2017.Hayamizu, K., et al., Pulse-Gradient Spin-Echo 1H, 7Li, and 19F NMR Diffusion and Ionic Conductivity Measurements of 14 Organic Electrolytes Containing LiN(SO2CF3)2. J. Phys. Chem. B, 1999. 103(3): p. 519-524.Wu, T.-Y., et al., Influence of LiTFSI Addition on Conductivity, Diffusion Coefficient, Spin–Lattice Relaxation Times, and Chemical Shift of One-Dimensional NMR Spectroscopy in LiTFSI-Doped Dual-Functionalized Imidazolium-Based Ionic Liquids. J. Chem. Eng. Data, 2015. 60(3): p. 471-483. Figure 1

Multinuclear magnetic resonance investigation of cation-anion and anion-solvent interactions in carbonate electrolytes

The investigation of ion-solvent interaction in electrolytes is crucial for basic understanding of ion transport through the bulk electrolyte as well as interphase formation processes on both electrodes, which dictate performances of lithium ion batteries (LIBs). In this report, nuclear magnetic resonance (NMR) was used to study the solvation behaviors of two typical lithium salts (lithium hexafluorophosphate, or LiPF6, and lithium tetrafluoroborate, or LiBF4) dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) mixtures. With increasing salt concentration and DMC percentage in both systems, 19F NMR experiences an upfield shift along with decreasing J31P−19F and J11B−19F, which evidence the stronger interaction between the Li+ and F− in an environment with diminishing presence of high dielectric medium. The quadrupolar relaxation of 11B dominates the 19F relaxation mechanism and demonstrates that LiBF4 mainly exists as ion pairs in solution either at high salt concentration or in a medium of low polarity. 7Li, 19F, 1H NMR diffusion measurements were conducted to characterize the relative mobility of cation, anion and solvent molecules, the results of which support the conclusion above. Salt concentration and solution polarity have a much stronger effect on cation-anion aggregation and solvation in the LiBF4 system than in the LiPF6 system.

Adhesion/growth-regulatory galectins tested in combination: evidence for formation of hybrids as heterodimers.

The delineation of the physiological significance of protein (lectin)-glycan recognition and the structural analysis of individual lectins have directed our attention to studying them in combination. In this report, we tested the hypothesis of hybrid formation by using binary mixtures of homodimeric galectin-1 and -7 as well as a proteolytically truncated version of chimera-type galectin-3. Initial supportive evidence is provided by affinity chromatography using resin-presented galectin-7. Intriguingly, the extent of cell binding by cross-linking of surface counter-receptor increased significantly for monomeric galectin-3 form by the presence of galectin-1 or -7. Pulsed-field gradient NMR (nuclear magnetic resonance) diffusion measurements on these galectin mixtures indicated formation of heterodimers as opposed to larger oligomers. 15N-1H heteronuclear single quantum coherence NMR spectroscopy and molecular dynamics simulations allowed us to delineate how different galectins interact in the heterodimer. The possibility of domain exchange between galectins introduces a new concept for understanding the spectrum of their functionality, particularly when these effector molecules are spatially and temporally co-expressed as found in vivo.

Restricted lithium ion dynamics in PEO-based block copolymer electrolytes measured by high-field nuclear magnetic resonance relaxation.

The intrinsic ionic conductivity of polyethylene oxide (PEO)-based block copolymer electrolytes is often assumed to be identical to the conductivity of the PEO homopolymer. Here, we use high-field 7Li nuclear magnetic resonance (NMR) relaxation and pulsed-field-gradient (PFG) NMR diffusion measurements to probe lithium ion dynamics over nanosecond and millisecond time scales in PEO and polystyrene (PS)-b-PEO-b-PS electrolytes containing the lithium salt LiTFSI. Variable-temperature longitudinal (T1) and transverse (T2) 7Li NMR relaxation rates were acquired at three magnetic field strengths and quantitatively analyzed for the first time at such fields, enabling us to distinguish two characteristic time scales that describe fluctuations of the 7Li nuclear electric quadrupolar interaction. Fast lithium motions [up to O(ns)] are essentially identical between the two polymer electrolytes, including sub-nanosecond vibrations and local fluctuations of the coordination polyhedra between lithium and nearby oxygen atoms. However, lithium dynamics over longer time scales [O(10 ns) and greater] are slower in the block copolymer compared to the homopolymer, as manifested experimentally by their different transverse 7Li NMR relaxation rates. Restricted dynamics and altered thermodynamic behavior of PEO chains anchored near PS domains likely explain these results.

Localization and identification of thin oil layers using a slim-borehole nuclear magnetic resonance tool

In the recent years, slim-borehole nuclear magnetic resonance (NMR) tools, designed for shallow hydrogeological applications, have become available. We have developed and improved the performance of one of them to detect and characterize hydrocarbon contamination. Our objectives were (1) to generally increase the vertical resolution of NMR logs below the instrumental resolution to detect thin layers and (2) to study the feasibility of using these tools to distinguish fluids (i.e., oil and water) using NMR diffusion measurements. To increase the vertical resolution, we have jointly inverted measurements made with overlapping sensitive volumes. We have developed an example in which a thin layer of 10 cm was successfully resolved. Using this approach, the instrumental resolution of the probe (50 cm) can be increased at the cost of additional measurement time. To differentiate and quantify the water and oil phases based on their contrast in diffusion coefficient, we have used a Carr-Purcell-Meiboom-Gill pulse sequence with varying echo spacings. We have found that, due to the low magnetic gradient of the probe, diffusion coefficients [Formula: see text] were difficult to resolve. Therefore, hydrocarbons with low diffusion coefficients, e.g., crude or rapeseed oil, cannot be uniquely identified, but can be quantified and clearly distinguished from water. Hydrocarbons with higher diffusion coefficients, e.g., diesel, kerosene, and gasoline, can be unambiguously identified based on their diffusion coefficient but may be difficult to distinguish from water. We have combined both approaches, enabling NMR logging to be used for fluid identification with increased vertical resolution. We have evaluated the performance of the measurement and inversion schemes using a controlled experimental setup consisting of a rapeseed oil layer on top of water in a tank.

Dissipative particle dynamics of diffusion-NMR requires high Schmidt-numbers.

We present an efficient mesoscale model to simulate the diffusion measurement with nuclear magnetic resonance (NMR). On the level of mesoscopic thermal motion of fluid particles, we couple the Bloch equations with dissipative particle dynamics (DPD). Thereby we establish a physically consistent scaling relation between the diffusion constant measured for DPD-particles and the diffusion constant of a real fluid. The latter is based on a splitting into a centre-of-mass contribution represented by DPD, and an internal contribution which is not resolved in the DPD-level of description. As a consequence, simulating the centre-of-mass contribution with DPD requires high Schmidt numbers. After a verification for fundamental pulse sequences, we apply the NMR-DPD method to NMR diffusion measurements of anisotropic fluids, and of fluids restricted by walls of microfluidic channels. For the latter, the free diffusion and the localisation regime are considered.

Nuclear Magnetic Resonance to investigate inorganic porous materials of interest in the cultural heritage field

Nuclear Magnetic Resonance (NMR) can be successfully applied to a wide number of organic, inorganic, and hybrid materials regarding cultural heritage. High-resolution solid-state NMR provides information on the structure of materials, and portable NMR devices allow non-destructive and non-invasive in situ investigation of variably sized objects. This result is possible by combining open magnets and surface radiofrequency coils to generate a sensitive volume external to the sensor and inside the object under investigation. In this paper we focus on the application of NMR to investigate inorganic porous materials such as pottery, plasters, and stones from cultural heritage sites. 27 Al MAS and 3QMAS, and 29 Si MAS high-resolution solid-state NMR along with spectral deconvolution allowed for the investigation of the chemical structure of ancient pottery. Portable unilateral NMR was used to investigate, in a non-invasive and non-destructive way, the porous structure of pottery. The effect of protective-consolidating treatments on plaster was carefully investigated by 1 H NMR depth profiles that allowed for scanning with micrometric resolution of plaster specimens. Changes occurring in the total open porosity after treatments were also evaluated. NMR diffusion measurements provided information on the restricted geometry of the porous structure of two types of biocalcarenite and tuff. A suitable processing of collected data enabled us to define the average pore radius and pores’ interconnection in these materials.

Nuclear magnetic resonance average pore-size estimations outside the fast-diffusion regime

The pore-size distribution (PSD) of geologic materials is an important rock parameter to understand the flow of water in the subsurface. PSDs can be obtained from sieving analyses, mercury porosimetry measurements, and imaging techniques, but none of these methods is available for in situ measurements. Nuclear magnetic resonance (NMR) measurements are controlled by rock parameters such as the surface-area to pore-volume ratio. NMR is available for in situ measurements. State-of-the-art NMR relaxation time measurements need a calibration of the surface relaxivity [Formula: see text] to extract pore-size information. State-of-the-art NMR diffusion measurements avoid the calibration of [Formula: see text] but are limited to small pores. We developed an approach that estimates the average pore size without calibrating [Formula: see text] by means of incorporating higher order modes into the signal interpretation of NMR relaxation times. We conducted forward-modeling studies using an analytic solution for cylindrical tubes, 2D finite-element simulations to incorporate fractal pore spaces, and laboratory experiments on synthetic and natural samples. Our experimental data indicated that relaxation can occur outside the fast-diffusion regime not only for coarse-grained materials, but also for fine- to medium-grained unconsolidated sandy materials due to high surface relaxivities. We found that the rock-fluid interface’s roughness had a significant impact on the diffusion regime and led to an apparent increase in [Formula: see text], which may cause intermediate or slow diffusion. The methodology was limited to materials with a narrow PSD and uniform distribution of [Formula: see text] because we assumed multiexponential decay due to diffusion in single isolated pores.

Steady state effects in a two-pulse diffusion-weighted sequence.

In conventional nuclear magnetic resonance (NMR) diffusion measurements a significant amount of experimental time is used up by magnetization recovery, serving to prevent the formation of the steady state, as in the latter case the manifestation of diffusion is modulated by multiple applications of the pulse sequence and conventional diffusion coefficient inference procedures are generally not applicable. Here, an analytical expression for diffusion-related effects in a two-pulse NMR experiment (e.g., pulsed-gradient spin echo) in the steady state mode (with repetition times less than the longitudinal relaxation time of the sample) is derived by employing a Fourier series expansion within the solution of the Bloch-Torrey equations. Considerations are given for the transition conditions between the full relaxation and the steady state experiment description. The diffusion coefficient of a polymer solution (polyethylene glycol) is measured by a two-pulse sequence in the full relaxation mode and for a range of repetition times, approaching the rapid steady state experiment. The precision of the fitting employing the presented steady state solution by far exceeds that of the conventional fitting. Additionally, numerical simulations are performed yielding results strongly supporting the proposed description of the NMR diffusion measurements in the steady state.

New insights into dye chemistry and physics

A review of the academic dye research performed by the Port Sunlight group and its coworkers over the past 15 years is presented. The work is focused on three areas: (1) substrate structure; (2) dye interactions in aqueous solution and on substrates; (3) dye degradation and products. For substrates, a detailed model of the nanoenvironment experienced by chemicals within cellulose fibre is given, showing the different environments and remarkable mobility of absorbed chemicals. Advanced nuclear magnetic resonance diffusion measurements provide the complex pathways by which compounds find their way in and out of the fibre. For dye interaction, detailed theoretical and experimental studies are reported on three model dye systems, the anionic monoazo dye Orange II, the bisazo anionic dye CI Direct Blue 1, and cationic monoazo thiazolium dyes, providing a comprehensive picture of their structure. A quantitative mechanism of dye binding to cellulose is shown. Resonance Raman provides an effective forensic tool for dye identification, even from single fibres. The products and kinetics of Orange II dye degradation by one‐electron reduction in aqueous solution is given, with the identification of an indophenol dye end‐product. In cellulosic materials the reduction mechanism is similar to solution, when the higher microviscosity is accounted for. Hydrolysis of thiazolium dyes occurs at both aromatic rings of the dye but on different timescales. Measurement and calculations of the electronic structures of one‐electron‐reduced and ‐oxidised dyes are presented. The mechanism of photooxidation by sunlight of azo dyes in cotton is delineated.

Systematic bias in NMR diffusion measurements on polydisperse systems

Least-squares fitting of the Stejskal–Tanner equation is a routine process in the measurement of molecular diffusion coefficient (MDC) using Nuclear Magnetic Resonance (NMR) Spectroscopy. It is simple and elegant. However, a bias of the MDC is noticed when the system is polydispersed. This is due to improper accounts of the diffusion coefficient distribution. Eventually, it leads to a discrepancy between the observed MDC and the statistical mean value of the distribution. To reveal the discrepancy, an analytical solution is derived when the diffusion data is taken a logarithmic linearization. Computer simulation is also applied to obtain a non-linear regression result. For a Gaussian distribution of the MDCs, the bias is proportional to the square of the distribution width (linear regression), but it is also inversely proportional to the statistical mean value of the distribution (non-linear regression). This indicates that the MDC derived from Stejskal–Tanner equation only holds well for narrow distribution of MDCs. Otherwise, molecular radius derived from the Stokes–Einstein equation needs to be reconsidered due to the incorrect estimation of the MDC.

Chiroptical Phenomena in Reverse Micelles: The Case of (1R,2S)‐Dodecyl (2‐hydroxy‐1‐methyl‐2‐phenylethyl)dimethylammonium Bromide (DMEB)

(1R,2S)-Dodecyl(2-hydroxy-1-methyl-2-phenylethyl)dimethylammonium bromide (DMEB) aggregates dispersed in carbon tetrachloride have been investigated by Fourier transform infrared (FT-IR), vibrational circular dichroism (VCD) and (1)H nuclear magnetic resonance (NMR) spectroscopy at various surfactant concentration and water-to-surfactant molar ratio. Experimental data indicate that, even at the lowest investigated concentration and in absence of added water, DMEB molecules associate in supramolecular assemblies. At higher DMEB concentration the aggregates can confine water molecules, making it plausible to think that DMEB form reverse micelles and that water molecules are quite uniformly distributed among them and mainly located in the proximity of surfactant head groups. Moreover, the water state in DMEB reverse micelles has been found to be different from that in pure water, due to system-specific water/surfactant head group interactions. (1)H NMR diffusion measurements of both water and DMEB emphasize their joined translational motion characterized by a diffusion rate one order of magnitude lower than that of free molecules. Finally, VCD allowed us to show some characteristics of the association of optically active DMEB molecules as reverse micelles and water confinement inside; namely, we monitored the vibrational optical activity of deuterated hydroxyl bonds of the self-assembled DMEB molecules and their interaction with D(2)O molecules.

NMR-based diffusion pore imaging by double wave vector measurements

One main interest of nuclear magnetic resonance (NMR) diffusion experiments is the investigation of boundaries such as cell membranes hindering the diffusion process. NMR diffusion measurements allow collecting the signal from the whole sample. This mainly eliminates the problem of vanishing signal at increasing resolution. It has been a longstanding question if, in principle, the exact shape of closed pores can be determined by NMR diffusion measurements. In this work, we present a method using short diffusion gradient pulses only, which is able to reveal the shape of arbitrary closed pores without relying on a priori knowledge. In comparison to former approaches, the method has reduced demands on relaxation times due to faster convergence to the diffusion long-time limit and allows for a more flexible NMR sequence design, because, e.g., stimulated echoes can be used.

NMR study of the structure and self‐association of core peptide in aqueous solution and DPC micelles

Core peptide is a hydrophobic peptide derived from the T-cell antigen receptor-alpha chain (TCR-alpha) transmembrane region with therapeutic potential. The mechanism by which the peptide inserts into the membrane, including any requirements to change conformational or association states during the insertion, is unclear. Here, the self-association and secondary structure of Core peptide in aqueous solution and in dodecylphosphocholine (DPC) micelles were examined using various nuclear magnetic resonance (NMR) techniques. NMR diffusion measurements were performed on 0.05, 1, and 5 mM Core peptide in D2O. These samples had pH values varying from 3 to 4. A constant measured diffusion coefficient of 2 X 10(-10) m2 s(-1) was observed in these samples indicating that Core peptide was monomeric. Multidimensional NMR experiments (i.e., TOCSY and NOESY) revealed the formation of beta-strands in water at low pH and random coil in DPC micelles. The results of this study reveal that at relatively low pH, the insertion mechanism must involve Core peptide in the monomeric state but it undergoes a conformational transition during membrane insertion.