Exchange Nuclear Magnetic Resonance Research Articles

2H nuclear magnetic resonance studies on the deuteron dynamics in betaine phosphate/phosphite mixed crystals

One- and two-dimensional 2H NMR measurements on selected single crystals of deuterated betaine phosphate/phosphite mixed crystals (DBP1-xDBPIx) in the antiferrodistortive and paraelectric phases were performed in order to study the static and dynamic behaviour of the deuterium atoms in the hydrogen bridges and in the methylene groups. In the 2H nuclear magnetic resonance (NMR) spectra of the mixed crystals, it is possible to distinguish between DBP and DBPI molecules. Furthermore, for all deuterons the electric field gradient (EFG) tensors could be determined. A characteristic dependency of the high-temperature phase transition on the phosphite concentration x was derived from the changes of the one-dimensional 2H NMR spectra. Our suggested model, which describes this dependency qualitatively, underlines the antiferrodistortive character of this transition. Furthermore, from the line-shape variations, the occurrence of processes of exchange between several deuterons, including DBP and DBPI molecules, was concluded. The presence of exchange between different molecules in the chains was confirmed by means of analysing the cross-peaks of two-dimensional 2H NMR exchange experiments. The observation of this charge-transport process along the chains from the microscopic point of view is very important for the understanding of electrical conductivity processes in these crystals.

Phenyl ring flips in crystals of bis-(4-chlorophenyl)-sulphone and a binary criterion for distinguishing chemical exchange from spin diffusion in deuteron exchange nuclear magnetic resonance

Phenyl ring flips in crystals of bis-(4-chlorophenyl)-sulphone and a binary criterion for distinguishing chemical exchange from spin diffusion in deuteron exchange nuclear magnetic resonance

Exchange rates of ethanol with water in water-saturated cement pastes probed by NMR

Diffusion of ethanol into water-saturated white cement pastes has been investigated by carbon and proton nuclear magnetic resonance (NMR). The diffusion of ethanol was shown to be Fickian, assuming one-dimensional diffusion under perfect sink boundary conditions. Derived diffusion coefficients were found to increase with increasing water/cement (w/c) ratio from (2.7 ± 0.5) 10 −8 cm 2 /s at w/c = 0.30 to (59 ± 5) 10 −8 cm 2 /s at w/c = 1.0. At the end of the exchange process, only a fraction of the total volume of water is exchanged with ethanol, varying from 60% for samples containing mainly micro- and mesopores to about 80% for samples where additional capillary pores are present. Time needed to reach 90% and 95% exchange of the total intrudable amount of ethanol in cylindrical samples with diameter of 5.5 mm varied from 1 day to nearly 3 weeks. This has importance for exchange in larger samples with typical diameters of 10 mm or more (as used in mercury intrusion porosimetry), which may require on the order of months for 90% exchange to take place. The mole fraction of ethanol and water in the pore system was determined from sampled carbon and proton NMR spectra vs. exchange time by comparing H 2 O-saturated and D 2 O-saturated samples. At the end of the exchange process, water was found to occupy the remaining volume not accessible to ethanol. In the tested w/c ratio range, the water content in all samples is below the value where damage to the pore structure normally occurs due to internal tension when exposed to drying. An empirical relationship between chemical shift of the CH 3 CH 2 OH/H 2 O peak and mole fraction of ethanol is derived, enabling the mole fraction of ethanol from the NMR peak to be estimated.

Three-dimensional nuclear magnetic resonance exchange spectroscopy with rotary resonance in rotating solids: Application to tropolone dynamics

A three-dimensional nuclear magnetic resonance (NMR) experiment is described for the investigation of molecular rearrangements in the course of chemical exchange processes. The experiment relies on the two-dimensional correlation of the chemical shift tensors before and after exchange. The chemical shift tensors are retrieved under fast magic angle spinning by rotary resonance induced by a radio-frequency field whose magnitude matches the sample spinning frequency. The third dimension serves for the separation of the two-dimensional chemical shift patterns by the isotropic chemical shifts. The three-dimensional rotary-resonance C13-exchange experiment is demonstrated with an investigation of hydrogen-transfer and molecular diffusion processes in a sample of solid tropolone.

Two-dimensional nuclear magnetic resonance study of a hydrated porous medium: An application to white cement

Structure and dynamics of a hydrated white cement were investigated by the T1-weighted lineshape, two-dimensional (2D) exchange, and 2D separation of inhomogeneous and homogeneous lineshapes nuclear magnetic resonance (NMR) techniques. T1 weighting of the proton spectrum eliminates the strong bulk water signal from the pores so that the weaker spectrum of the solid cement matrix becomes observable. The proton spectrum of the solid cement fraction exhibits a characteristic Pake doublet shape and shows a close similarity to the powder spectra of pure calcium hydroxide and the crystalline water of gypsum. The 2D exchange NMR spectrum of white cement demonstrates the existence of slow chemical exchange processes of protons in the solid matrix on a sub-kHz frequency scale. This exchange is orders of magnitude slower than the exchange between the surface and bulk water. The 2D NMR separation of inhomogeneous and homogeneous lineshapes technique demonstrates that the absorption lines of a set white cement are inhomogeneously broadened due to the existence of a distribution of static local magnetic fields. The major contribution to these fields comes from the dipolar fields of the paramagnetic electrons whereas the magnetic susceptibility differences of the liquid and solid fractions add a small part at the solid-liquid boundary where the direction of the external magnetic field is changed.

Local structure and connectivity in lithium phosphate glasses: a solid-state 31P MAS NMR and 2D exchange investigation

High resolution solid-state 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the local structure and connectivity for three xLi 2O·(1 - x)P 2O 5 glasses. The principal components of the 31P chemical shift anisotropy (CSA) tensors for the different phosphate tetrahedron sites (Q n ) and the relative populations of each Q n species were determined from simulation of the MAS spectra. The medium range structure and connectivity between similar or different Q n units in these lithium phosphate glasses were probed using novel two-dimensional (2D) MAS exchange experiments. Radio-frequency dipolar recoupling (RFDR) techniques allowed the reintroduction of the through space 31P- 31P dipolar-dipolar interaction even in the presence of fast MAS. For these binary phosphate glasses the connectivity and medium range structure for different Q n species is described well by a statistical distribution, and is consistent with a model of random depolymerization.

Local protein instability predictive of helper T-cell epitopes

Although seletivity at the levels of peptide binding to major histocompatibility complex (MHC) class II and recognition by T cells may partially account for immunodominance patterns, it is clear that differential antigen processing also exerts a strong effect. Here, Sam Landry correlates immunodominant epitopes with nearby structurally unstable segments, as identified by hydrogen-deuterium exchange nuclear magnetic resonance (NMR), and suggests that epitope presentation is directed by preferential proteolytic cleavage at the unstable sites.

Investigation of molecular structure in solids by two-dimensional NMR exchange spectroscopy with magic angle spinning

An approach to the investigation of molecular structures in disordered solids, using two-dimensional (2D) nuclear magnetic resonance (NMR) exchange spectroscopy with magic angle spinning (MAS), is described. This approach permits the determination of the relative orientation of two isotopically labeled chemical groups within a molecule in an unoriented sample, thus placing strong constraints on the molecular conformation. Structural information is contained in the amplitudes of crosspeaks in rotor-synchronized 2D MAS exchange spectra that connect spinning sideband lines of the two labeled sites. The theory for calculating the amplitudes of spinning sideband crosspeaks in 2D MAS exchange spectra, in the limit of complete magnetization exchange between the labeled sites, is presented in detail. A new technique that enhances the sensitivity of 2D MAS exchange spectra to molecular structure, called orientationally weighted 2D MAS exchange spectroscopy, is introduced. Symmetry principles that underlie the construction of pulse sequences for orientationally weighted 2D MAS exchange spectroscopy are explained. Experimental demonstrations of the utility of 2D MAS exchange spectroscopy in structural investigations of peptide and protein backbone conformations are carried out on a model 13C-labeled tripeptide, L-alanylglycylglycine. The dihedral angles φ and ψ that characterize the peptide backbone conformation at Gly-2 are obtained accurately from the orientationally weighted and unweighted 2D 13C NMR exchange spectra.

1H NMR Saturation Transfer Studies of Exogenous Ligands Binding to Metmyoglobin

Abstract Two-dimensional(2D) 1H. nuclear magnetic resonance exchange spectroscopy, 1D saturation transfer have been utilized to study the binding of imidazole(lm), 1-Methylimidazole(Melm), 1-Ethylimidazole(Etlm) to the heme iron of metmyoglobin. Some heme peripheral proton resonance's of these complexes have been first time assigned. The rates and equilibrium constants for Im, Melrn, Etlm binding to metMb are calculated from the 2D peak amplitudes. Analysis of the heme methyl shifts indicates that the orientations of the binding Im, Melm, Etlm are very similar. The steric effects and hydrogen bonding between the distal histidine and bound ligand are important factors regulating affinity.

2D exchange NMR investigations of phosphate glass ceramic

Abstract31P MAS and Exchange nuclear magnetic resonance (NMR) measurements were used to investigate the structure of P2O5‐CaO‐Na2O‐{Al2O3} glasses and glass ceramics. The progress of two‐dimensional (2D) Exchange NMR techniques is demonstrated.

1H NMR studies of pyridine binding to metmyoglobin

Two-dimensional 1H nuclear magnetic resonance exchange spectroscopy has been applied to the study of the pyridine-ligated complex of equine metmyoglobin (Py-metMb). With the known resonance assignment of metMb, some hyperfine shifted resonances of the Py-metMb complex have been assigned for the first time. Based on a matrix formalism, the kinetic and equilibrium data for pyridine binding to metMb have been evaluated from the 2D peak amplitudes. A qualitative comparison of the methyl shift pattern in metMbCN −, metMbN 3 − and imidazole metMb (metMbIm) with Py-metMb shows a reverse methyl shift between pyrrole I and IV, and the reason for this is discussed.

Rotational diffusion measurements of suspended colloidal particles using two-dimensional exchange nuclear magnetic resonance

We present here an experimental and theoretical study of the application of two-dimensional exchange nuclear magnetic resonance spectroscopy (NMR) to the investigation of the rotational diffusion of colloidal particles. The theoretical discussion includes the nature of the NMR frequency time-correlation function where the NMR interaction is represented by the chemical shift anisotropy (CSA). Time-correlation functions for the isotropic rotational diffusion of a suspension of colloidal particles containing single and multiple sites are derived in addition to time-correlation functions for the rotational diffusion of a suspension of symmetric top particles containing an isotropic distribution of a single CSA interaction. Simulations of two-dimensional exchange spectra for particles undergoing isotropic rotational diffusion are presented. We performed two-dimensional exchange NMR experiments on a colloidal suspension of spherical poly(methyl methacrylate) (PMMA) particles which were synthesized with a 20% enrichment in 13C at the carbonyl site. Rotational diffusion time-correlation functions determined from the experimental exchange spectra are consistent with the composition of the colloidal suspension. Detailed explanations of the syntheses of the enriched methyl 13C-(carbonyl)-methacrylate monomer and the small quantities of 20% enriched 13C-(carbonyl)-poly(methyl methacrylate) microspheres used for this study are presented.

Deuterium NMR Study of the Glassy Crystal Pentachlorotoluene. Hadamard Quadrupole-Order Exchange NMR

Pentachloro[α,α‘,α‘‘-2H3]toluene was studied by deuterium nuclear magnetic resonance (NMR) experiments. Three doublets were observed below 230 K, while a single doublet was observed above 300 K in a single crystal with its long axis perpendicular to the static magnetic field. The three doublets are attributable to the three disordered orientations of the molecule around its pseudo-6-fold axis. Populations in these three orientations were determined from the area intensities of the doublets. Below 150 K, the populations were found to be temperature-independent, indicating that the reorientational motions around the pseudo-6-fold axis are frozen into orientational disorder around this temperature and that the system is brought into a glassy crystalline state. A Hadamard quadrupole-order exchange NMR experiment was developed to determine the rates of the pseudo-6-fold reorientations by one-dimensional NMR. The cross and diagonal peak intensities of the exchange NMR spectra between 170 and 212 K were fitted w...

Multidimensional 2H NMR studies of the non-exponential chain relaxation of polystyrene above the glass transition

Multidimensional exchange nuclear magnetic resonance (NMR) of chain-deuterated amorphous polystyrene has been applied to elucidate the molecular dynamics above the glass transition. In addition to the diffusive aspect of the chain motion, 3D exchange NMR spectra show clear evidence of a jump-type motion, probably due to conformational changes. When the jump has taken place, no indication of long term orientational memory, i.e., back-jumps to the initial position, can be found. With a 4D exchange NMR experiment, the relaxation function of a selected sub-ensemble has been measured for motions on the 100 Hz range. This experiment indicates a broad distribution of correlation times due to parallel processes, for which different spatial regions differ in their correlation times. Another relaxation experiment proves that those spatial regions where the α-relaxation is slow also contribute with slow components to the non-exponential T 1 relaxation which is sensitive to motions with spectral densities in the range of the 2H Larmor frequency of 45 MHz ( β s process).

Three-dimensional variable-angle nuclear magnetic resonance exchange spectroscopy without rotor axis hopping

Slow, large-amplitude chain motions play an important role in determining the macroscopic mechanical properties of polymers. Although such motions have been studied quantitatively by two-dimensional (2D) nuclear magnetic resonance (NMR) exchange experiments, overlapping anisotropic patterns hamper spectral analysis, and limit applications. Variable angle correlation spectroscopy (VACSY) has proven useful in resolving such problems for rapidly spinning samples by separating anisotropic spectral patterns according to isotropic chemical shifts. In a previous study [J. Am. Chem. Soc. 115, 4825 (1993)], we described a three-dimensional (3D) NMR experiment that incorporates the VACSY method and a hop of the rotor axis to correlate the isotropic chemical shifts to 2D anisotropic exchange patterns. The hop of the rotor axis, however, presents experimental difficulties and limits the range of motional rates that may be studied. We present in this paper a new 3D VACSY exchange experiment that obtains the same correlations without the need for the rotor axis hop. A series of 2D exchange spectra are recorded with the sample spinning at different rotation axis angles. Then using the scaling of the anisotropic frequency at the different angles, we construct the data onto a 3D matrix so that a Fourier transformation directly yields the desired correlations. The technique is applied to 13C exchange NMR to study the slow molecular motion of ordered isotactic polypropylene.

Proton two-dimensional exchange nuclear magnetic resonance study of a p-xylene-d6zeolite inclusion compound

Proton two-dimensional (2D) exchange nuclear magnetic resonance (NMR) experiments and dipolar spin-lattice relaxation rate, T 1D, measurements were performed using magic-angle spinning (MAS) on a zeolite Na-ZSM-5/p-xylene α - α′-d 6 inclusion compound. The value of T 1D was found to be 3·2 ms, which is much longer than the T 1D values reported for rigid solids when the sample is spun perpendicular to the static magnetic field. This long T 1D can be interpreted as arising from the weak intermolecular 1H-1H dipolar interactions, because of the significant separation of the molecules in the cavities of the host. A satisfactory theoretical treatment which describes the 2D spectra, was developed. At short mixing times the 2D spinning-sideband intensities observed depended on the flip-angles of the second and third pulses. We show that the off-diagonal 2D spinning sidebands could be suppressed by the appropriate choice of the flip-angles of the second and the third pulses, since in the present sample the includ...

The pathways of the combined Cope rearrangement — molecular reorientation process in solid bullvalene: a deuterium 2D exchange NMR study on a single crystal

Deuterium 2D exchange nuclear magnetic resonance (NMR) spectroscopy was used to elucidate the detailed mechanisms of the dynamic processes in solid bullvalene. The measurements were performed in the temperature range of -73 to 10°C on a deuterated single crystal with the magnetic field parallel to the monoclinic plane where the four molecules in the unit cell are magnetically equivalent. In agreement with earlier carbon-13 and deuterium NMR studies, two independent processes were identified: symmetric threefold jump and valence bond isomerization (Cope rearrangement) combined with molecular reorientation. The latter process can proceed in the crystal by nine different pathways, characterized by specific cross peak patterns in the 2D spectrum. Due to the low symmetry at the crystallographic sites of the bullvalene molecules the Cope rearrangement/reorientation process may proceed with different rates along the different pathways. Analyses of the experimental results show that only four pathways are actuall...

Variable-angle three-dimensional exchange nuclear magnetic resonance spectroscopy for the study of molecular motion in complex solids

Although molecular motions are responsible for many of the macroscopic properties observed in solids, especially in polymers, methods for studying these processes in all but the simplest systems are scarce. In the present study we introduce a three-dimensional nuclear magnetic resonance experiment for characterizing ultraslow molecular motions in complex solid systems. The technique extracts dynamic information by resolving the two-dimensional exchange distributions that can be observed in spectra of static samples, according to the isotropic chemical shifts of individual molecular sites. These three-dimensional correlations are achieved by processing signals arising from a fast-spinning solid sample using two independent macroscopic axes of rotation as extraction parameters, an approach which becomes practical due to the simple scaling behavior of anisotropic chemical shifts with respect to the axis of sample rotation. The principles involved in this new spectroscopic technique are discussed, and the method is illustrated with an application to the analysis of motions in isotactic polypropylene.

Deuterium two-dimensional exchange nuclear magnetic resonance by rotor-synchronized magic angle spinning

The method of two-dimensional exchange spectroscopy under condition of magic angle sample spinning (MAS) and synchronization of the mixing time with the rotor period is extended to spin I=1 nuclei. Theoretical equations are derived for the cross peak intensities as a function of the magnetic and kinetic parameters of the system and the method is demonstrated on a number of deuterated compounds. Dimethylmalonic acid–d6 is first used to illustrate the effect of rotor synchronization by the complete absence of cross peaks when no exchange takes place. The method is then applied to two dynamic systems, viz. dimethylsulfone–d6 and thiourea–C6D12 inclusion compound. The experimental results are compared with simulations as well as with analogous experiments on nonspinning samples. Since chemical shift effects are often negligible in deuterium NMR the time domain sampling can in principle be reduced to the number of the desired spinning sidebands, resulting in considerable time savings. The main advantage of the method is the gain in sensitivity at the expense of the characteristic ridge pattern of static two-dimensional exchange experiments. The gain in sensitivity may be sufficient for performing such experiments on deuterium in isotopically normal compounds. The feasibility of such experiments is demonstrated by the recording of a one-dimensional deuterium MAS spectrum of a nonenriched sample.

Two-dimensional exchange nuclear magnetic resonance of powder samples. IV. Distribution of correlation times and line shapes in the intermediate dynamic range

The two-dimensional (2D) exchange nuclear magnetic resonance (NMR) experiment is applied to study ultraslow as well as faster chain motions in amorphous polymers in the glass transition range. Acquisition of the time domain data with a four-pulse sequence leads to new characteristics in the corresponding 2H 2D line shapes if the correlation times of the motion are in the intermediate dynamic range. From the asymmetric 2D line shapes, the width of the correlation time distribution can be determined with higher accuracy than through conventional 1D NMR methods. Experimental data are presented on two amorphous polymers—atactic polypropylene and cis-1,4-polyisoprene—and are analyzed in terms of isotropic rotational diffusion. Deviations from this simple model due to the presence of conformational transitions within the polymer backbone are detected. Close to Tg, the mean correlation times extracted from 2D exchange NMR exhibit strongly nonArrhenius behavior usually described by the Williams–Landel–Ferry (WLF) equation. In addition, the width of the correlation time distribution is found to decrease with increasing temperature.