Residual Dipolar Interactions Research Articles

COMPARATIVE OBSERVATION OF THE NMR SPIN-LATTICE AND SPIN-SPIN RELAXATION IN MOLTEN AND CROSS-LINKED POLIBUTADIENE

The molten and cross-linked polybutadiene were observed by NMR methods in order to disclose the influence of the presence of the temporary and permanent junctions between the polymeric chains on the relaxation of the longitudinal and transverse magnetization. The residual dipolar interaction determined by the entanglements of the polymeric chains is observed by the pseudo-solid echo method and is associated with the existence of the temporary network.

Phospholipid bilayer surface configuration probed quantitatively by 31 P field-cycling NMR

(31)P relaxation of the diester phosphate of phospholipids in unilamellar vesicles has been studied from 0.004 to 11.7 T. Relaxation at very low fields, below 0.1 T, shows a rate increase that reflects a residual dipolar interaction with neighboring protons, probably dominated by the glycerol C3 protons. This interaction is not fully averaged by faster motion such as rotational diffusion perpendicular to the membrane surface. The remaining dipolar interaction, modulated by overall rotational diffusion of the vesicle and lateral diffusion of the lipid molecules, is responsible for the very low-field relaxation. These measurements yield a good estimate of the time-average angle between the membrane surface and the vector connecting the phosphorus to the glycerol C3 protons, based on the classic theory by Woessner. Dynamic information is also obtained. Implications for solid-state NMR and other studies are discussed.

Reduction of residual dipolar interaction in cartilage by spin‐lock technique

The influence of radiofrequency (RF) spin-lock pulse on the laminar appearance of articular cartilage in MR images was investigated. Spin-lock MRI experiments were performed on bovine cartilage plugs on a 4.7 Tesla small-bore MRI scanner, and on human knee cartilage in vivo on a 1.5 Tesla clinical scanner. When the normal to the surface of cartilage was parallel to B0, a typical laminar appearance was exhibited in T2-weighted images of cartilage plugs, but was absent in T1rho-weighted images of the same plugs. At the "magic angle" orientation (when the normal to the surface of cartilage was 54.7 degrees with respect to B0), neither the T2 nor the T1rho images demonstrated laminae. At the same time, T1rho values were greater than T2 at both orientations throughout the cartilage. T1rho dispersion (i.e., the dependence of the relaxation rate on the spin-lock frequency omega1) was observed, which reached a steady-state value of close to 2 kHz in both parallel and magic-angle orientations. These results suggest that residual dipolar interaction from motionally-restricted water and relaxation processes, such as chemical exchange, contribute to T1rho dispersion in cartilage. Further, one can reduce the laminar appearance in human articular cartilage by applying spin-lock RF pulses, which may lead to a more accurate diagnosis of degenerative changes in cartilage.

IR AND 13C-NMR SPECTRAL PROPERTIES OF THE LAYERED INCLUSION COMPOUND BIS(THIOUREA)HEXAMETHYLENETETRAMINE

The structure of the 1: 2 adduct of hexamethylenetetramine and thiourea was reanalyzed reinterpreting X-ray diffraction data and providing additional IR and NMR spectroscopic features. The product is thus described as supramolecular commensurate host-guest complex corresponding to the intercalation of hexamethylenetetramine in a layered thiourea matrix. FT-IR spectra evidence a simultaneous strengthening of both C-N and C-S bonds, that appearing as a pattern characteristic for described layered thiourea matrix. 13CP MAS-NMR spectra of the intercalated hexamethylenetetramine is essentially determined by nuclear dipolar magnetic interactions and host-guest chemical interactions arising from 13C-14N residual dipolar interaction not averaged to zero by MAS-NMR and from the magnetic non-equivalence of the guest carbon atoms caused by the hydrogen bonding interaction with the thiourea matrix

Constant time steady gradient NMR diffusometry using the secondary stimulated echo

A pulse sequence producing a second stimulated echo is suggested for the compensation of relaxation and residual dipolar interaction effects in steady gradient spin echo diffusometry. Steady field gradients of considerable strength exist in the fringe field of NMR magnets, for instance. While the absolute echo time of the second stimulated echo is kept constant throughout the experiment, the interval between the first two radiofrequency pulses is augmented leading to a modulation of the amplitude of that second stimulated echo by self-diffusion only. The unique feature of this technique is that it is of a single-scan/single-echo-signal nature. That is, no reference signals neither of the same pulse sequence nor of separate experiments are needed. The new method was tested with poly(ethylene oxide) melts and proved to provide reliable data for (time dependent) self-diffusion coefficients down to the physical limit ( D≈10 −15 m 2/s) when flip-flop spin diffusion starts to become effective.

Improvement of resolution in solid state NMR spectra with J-decoupling: an analysis of lineshape contributions in uniformly 13C-enriched amino acids and proteins

In magic angle spinning (MAS) NMR spectra of highly and uniformly 13C, 15N-enriched amino acids and proteins, homo-nuclear coupling interactions contribute significantly to the 13C linewidths, particularly for moderate applied magnetic field strengths and sample spinning frequencies. In this work, we attempted to dissect, analyze, and control the contributions of J-coupling and residual homo-nuclear dipolar coupling interactions to the linewidths of uniformly 13C, 15N-enriched crystalline alanine; these studies were carried out at 9.4 T using a range of spinning frequencies from 5 to 15 kHz. The anisotropic second-order dipolar shifts and the J-splittings are comparable in their contribution to the linewidths, but behave very differently in terms of experimental protocols for line narrowing. In contrast to the J-coupling interactions, the second-order dipolar broadening cannot be refocused using selective pulses on the passively coupled spin. We carried out experiments to remove or refocus the 13C J-coupling interactions ( ω 1 J-decoupling) using a selective DANTE pulse in the center of the indirect evolution period. Inversion profiles and bandwidths of selective DANTE pulses acting on transverse magnetization, in the regime of moderate spinning frequencies, were characterized computationally and experimentally. A dramatic improvement in the resolution of the 2D spectrum was achieved when this decoupling protocol was employed.

NMR Evidence for Residual Dipolar Interactions in SBS Systems: Influence of a Plasticizer on the Network Structure

Evidence for residual dipolar interactions in the polybutadiene domains of two poly(styrene−butadiene−styrene) block copolymers is shown at temperatures ranging from ambient to 90 °C. The two copol...

Weak alignment offers new NMR opportunities to study protein structure and dynamics.

Protein solution nuclear magnetic resonance (NMR) can be conducted in a slightly anisotropic environment, where the orientational distribution of the proteins is no longer random. In such an environment, the large one-bond internuclear dipolar interactions no longer average to zero and report on the average orientation of the corresponding vectors relative to the magnetic field. The desired very weak ordering, on the order of 10(-3), can be induced conveniently by the use of aqueous nematic liquid crystalline suspensions or by anisotropically compressed hydrogels. The resulting residual dipolar interactions are scaled down by three orders of magnitude relative to their static values, but nevertheless can be measured at high accuracy. They are very precise reporters on the average orientation of bonds relative to the molecular alignment frame, and they can be used in a variety of ways to enrich our understanding of protein structure and function. Applications to date have focused primarily on validation of structures, determined by NMR, X-ray crystallography, or homology modeling, and on refinement of structures determined by conventional NMR approaches. Although de novo structure determination on the basis of dipolar couplings suffers from a severe multiple minimum problem, related to the degeneracy of dipolar coupling relative to inversion of the internuclear vector, a number of approaches can address this problem and potentially can accelerate the NMR structure determination process considerably. In favorable cases, where large numbers of dipolar couplings can be measured, inconsistency between measured values can report on internal motions.

Residual 31P, 35,37Cl Dipolar Coupling in 31P MAS Spectra of Chlorocyclophosphazenes

In 31P MAS NMR spectra of chlorocyclophosphazenes, characteristic splittings have been observed for PCl or PCl2 groups. At different applied magnetic fields, the fine structure and total width of the patterns change in a characteristic way, demonstrating that the splittings are due to indirect spin–spin and residual dipolar interactions with the chlorine nuclei directly bonded to phosphorus. For trans-nongeminal N3P3Cl3(NMe2)3 and N3P3Cl6 as examples, the spectra have been analyzed to obtain information on chlorine nuclear quadrupole coupling constants and 35,37Cl, 31P indirect spin–spin coupling constants. Neglect of these interactions may result in misinterpretations of the multiplicity in 31P MAS spectra of chlorophosphazenes.

Application of High-Resolution-Magic-Angle-Spinning (HR-MAS) NMR Spectroscopy to Cosmetic Emulsions

Abstract HR-MAS NMR spectroscopy was utilized for the characterization of three cosmetic emul­sions. The emulsions had the same chemical composition and were prepared in the same way except the homogenization step. Magic angle spinning (M A S) of the emulsions was applied to improve signal resolution by eliminating susceptibility changes as well as residual dipolar interactions within the sample. Spin-lattice relaxation times (T1) were determined according to the inversion recovery technique in the temperature range of -10 °C to 40 °C to find out phase changes of the sample ingredients. 1H and 13C NMR signals of the individual components were assigned by use of different 1D and 2D NMR experiments. The HR-MAS NMR technique offers many possibilities to further investigate samples between the liquid and the solid state.

Strongly dipolar fluids at low densities

We propose a simple statistical mechanical theory for a strongly dipolar fluid at low densities, based on the analogy between a system of polydisperse linear chains and the equilibrium structure of these fluids as revealed by computer simulations. At low densities, both steric and dipolar interactions between long chains are weak and thus the dipolar fluid is well described as an ideal gas of polydisperse chains. We have further investigated how the residual dipolar interaction between monomers and/or additional isotropic attractions between the spheres cause the dissociation of the chains and/or the condensation of the dipolar fluid.

Comparison of residual dipolar interactions in uncross-linked and cross-linked natural rubber by high-resolution solid-state NMR spectroscopy

It is shown that local residual dipolar broadening of protons coupled to 13C is a sensitive way to observe motional constraints from networks and entanglements in cross-linked and uncross-linked rubber systems. This may be easily obtained from the indirect observation of proton spin system through 13C resonance in the conditions of slow magic-angle spinning. The method has been used to visualise the changes in the amount of physical constraints induced during mechanical treatment, which precedes the vulcanisation process.

Bis(thiourea)-1,2-diazabicyclo[2.2.2]octane. A new layered thiourea inclusion compound

Thiourea reacts with 1,4-diazabicyclo[2.2.2]octane (DABCO) in methanol, leading to the layered inclusion compound (thiourea)2·(DABCO). Single crystal X-ray diffraction analysis was carried out. The structure may be described as a supramolecular arrangement in which the DABCO molecules are intercalated in a layered network formed by two-dimensional nesting of thiourea molecules. The solid state 13C CP-MAS NMR study of the compound shows only a single line for Cα with a residual dipolar interaction between 13C and 14N of less than 80 Hz, attributed to weak guest–host interaction.

Differential broadening caused by dipolar interaction with 1H in 13C MAS NMR signals split by residual dipolar interaction with 14N

The 13C cross-polarization magic angle spinning NMR spectra of various nitroanilines and some other N-containing compounds were measured and the differential broadening between the signals split by the residual dipolar interaction with 14N was investigated. The dipolar interaction between 14N and 1H produces 13C differential broadening as well as the 13C–14N interaction does, whereas the dipolar interaction between 13C and 1H is discounted as its origin. The differential broadening caused by 1H is observed in the C*—NHR (R = H and CH3) groups at the para and ortho positions to the NO2 group in nitroanilines, C—NH2 in m-nitroaniline and 13C bonded to the amide NH group, where 14N is directly bonded to 1H and molecular motions are restricted by hydrogen bondings. At higher temperatures the differential broadening decreases in the para-type nitroanilines. The 1H-caused differential broadening is not observed for the C—NH2 group in the meta-type nitroanilines except for m-nitroaniline, C—NO2, C*—N(CH3)2 and C—NH3 groups. Molecular motions can reduce the differential broadening, which are presumably the 180° flip of the NH2 group and the rotation of the NH3 group around the C—N axis. Copyright © 1999 John Wiley & Sons, Ltd.

Differential broadening caused by dipolar interaction with1H in13C MAS NMR signals split by residual dipolar interaction with14N

Differential broadening caused by dipolar interaction with1H in13C MAS NMR signals split by residual dipolar interaction with14N

Strongly dipolar fluids: a theoretical and computational challenge

We propose a simple statistical mechanical theory for a strongly dipolar fluid at low densities, based on the analogy between a system of polydisperse polymer chains and the equilibrium structure of these fluids as shown by computer simulation. Both steric and dipolar interactions between long chains are weak at low densities and as a result the dipolar fluid, in this regime, is well described as an ideal gas of polydisperse chains. We further investigate how the residual dipolar interaction between monomers and attractive Van der Waals interactions between particles may cause dissociation of the chains and condensation of the dipolar fluid.

NMR relaxation and NMR imaging of elastomers in the course of thermal aging

A complex aging regime occurs in the course of thermal aging of elastomers. Depending on the type and the content of the rubber filler materials, temperature, chemical environment (normally air), and time, a different aging process can be observed also by nuclear magnetic resonance (NMR) [1–6]. The methods used are the common spin-echo 1H-NMR, including variable echo times and parameter-selective NMR-1H-imaging (material properties imaging). The decay of the echo-magnetization is discussed on the basis of a single-chain model with a distribution of dipolar interactions. This model is based on the influence of a very fast, but anisotropic, local motion, as well as larger and slower motions, which are able to diminish the residual dipolar interaction. Carbon-black-filled natural rubber, as well as silica and carbon-black-filled E-SBR (emulsion-polymerized styrene butadiene rubber) and S-SBR (solution-polymerized SBR) are the systems under investigation, with the results showing some characteristic features of the course of aging observable by NMR.

A Study of Dipolar Interactions and Dynamic Processes of Water Molecules in Tendon by1H and2H Homonuclear and Heteronuclear Multiple-Quantum-Filtered NMR Spectroscopy

The effect of proton exchange on the measurement of1H–1H,1H–2H, and2H–2H residual dipolar interactions in water molecules in bovine Achilles tendons was investigated using double-quantum-filtered (DQF) NMR and new pulse sequences based on heteronuclear and homonuclear multiple-quantum filtering (MQF). Derivation of theoretical expressions for these techniques allowed evaluation of the1H–1H and1H–2H residual dipolar interactions and the proton exchange rate at a temperature of 24°C and above, where no dipolar splitting is evident. The values obtained for these parameters at 24°C were 300 and 50 Hz and 3000 s−1, respectively. The results for the residual dipolar interactions were verified by repeating the above measurements at a temperature of 1.5°C, where the spectra of the H2O molecules were well resolved, so that the1H–1H dipolar interaction could be determined directly from the observed splitting. Analysis of the MQF experiments at 1.5°C, where the proton exchange was in the intermediate regime for the1H–2H dipolar interaction, confirmed the result obtained at 24°C for this interaction. A strong dependence of the intensities of the MQF signals on the proton exchange rate, in the intermediate and the fast exchange regimes, was observed and theoretically interpreted. This leads to the conclusion that the MQF techniques are mostly useful for tissues where the residual dipolar interaction is not significantly smaller than the proton exchange rate. Dependence of the relaxation times and signal intensities of the MQF experiments on the orientation of the tendon with respect to the magnetic field was observed and analyzed. One of the results of the theoretical analysis is that, in the fast exchange regime, the signal decay rates in the MQF experiments as well as in the spin echo or CPMG pulse sequences (T2) depend on the orientation as the square of the second-rank Legendre polynomial.

Differential broadening between the signals split by the residual dipolar interaction with [formula omitted] in [formula omitted] CP/MAS NMR spectra of nitroanilines

The 13 C CP/MAS NMR spectra of various nitroanilines have been measured, and the origin of the differential broadening observed between the signals split by the residual dipolar interaction with 14 N has been clarified experimentally. The dipolar interaction between 1 H and 14 N is responsible for the 13 C differential broadening, while the direct dipolar interaction with 1 H is neglected. A molecular motion can eliminate the differential broadening, which is presumably the 180° flip of the NH 2 group around the C–N axis. Therefore, the differential broadening is highly correlated to the molecular motion and the hydrogen bonding.

The use of NMR relaxation and NMR imaging in studying the aging of rubber

Nuclear magnetic resonance was used to study the aging of rubber filler materials. The methods used are the common spin-echo 'H NMR including variable echo times and parameter-selective NMR 1 H-imaging (material properties imaging). The decay of the echo-magnetization is discussed on the basis of a single chain model with a distribution of dipolar interactions. This model is based on the influence of a very fast but anisotropic local motion as well larger and slower motions, which are able to diminish the residual dipolar interaction. Carbon black filled natural rubber as well as silica and carbon black filled E(mulsion)-and S(olution)-SBR and milled tire rubber are the systems under investigation showing some characteristic features of the aging course observable by NMR.