Low-resolution NMR Research Articles

Stable reconstruction of the T(2) distribution by low-resolution NMR measurements and the classical markov and hausdorf momentum problem

Assuming that an original distribution is a probabilistic measure and the Laplace transforms are known only for a finite number of points that are affected by errors, we develop a method for reconstructing weak-sense mean values obtained by integrating smooth functions with the measure. Our method is useful in NMR if the NMR signal can be represented as a superposition of exponential terms. In these circumstances, we show how the data processing can be related to the classical Hausdorf momentum problem. First, we clarify the meaning of stable spectrum reconstruction, and then develop stable filtering and a mean value reconstruction algorithm. Our method has been tested on both simulated and real sets of spin-spin relaxation curves with noise. In view of this, our method could provide an efficient and accurate reconstruction of spin-spin relaxation data. For any reader interested in applications, a "practical recipe" that is almost self-consistent has been included. Copyright 2000 Academic Press.

Characterization of water mobility and distribution in low- and intermediate-moisture food systems

The mechanism of water uptake in low moisture cereals and cookies has been studied by NMR relaxometry and solid imaging technology implemented on a low-resolution benchtop NMR spectrometer. A comparison between classical MRI and SPRITE imaging are also presented to highlight the benefits of each technology. The spin lattice (T 1) and spin spin (T 2) relaxation times, the 1D and 2D SPRITE imaging, were determined on Smacks™, corn flakes, chocolate chips cookies, soft caramel candies with a chocolate crème filler, and corn starch/water systems. The Smacks™ and corn flakes were studied based on the soaking time in milk, and the results showed that T 1 and T 2 decreased in the first 20 sec of soaking and then increased with the soaking time. For Smacks™ stored at different relative humidity, T 1 decreased during the first day of storage and then was relatively constant over storage time indicating that the system reached an equilibrium. 1D SPRITE profiles indicated an increase in signal intensity over storage time for cookies in 58% RH. However, the moisture uptake was insignificant indicating that the water mobility (and not the amount of water) changed due to various chemical interactions in the system (hydrogen bonding, starch retrogradation, glassy/rubbery equilibrium). The T 1 and T 2 of corn starch/water systems decreased as the concentration in starch increased and temperature increased from 30°C to 60°C. However, for temperatures higher than 60°C, the relaxation times increased showing more mobility and flexibility of the polymer chains during gelatinization.

Physical gelation of melamine formaldehyde resin solutions. II. A combined light-scattering and low-resolution relaxation proton NMR study

Physical gelation of melamine formaldehyde resin solutions. II. A combined light-scattering and low-resolution relaxation proton NMR study

EPDM−Carbon Black Interactions and the Reinforcement Mechanisms, As Studied by Low-Resolution1H NMR

Proton, low-resolution T2 NMR relaxation experiments are used to study the adsorption of EPDM to the surface of carbon blacks and the structure of the physical EPDM/carbon black network in cured and uncured compounds. It is shown that a layer of immobilized EPDM is formed at the carbon black surface after mixing. The estimated thickness of the interfacial EPDM is in the range of one to two diameters of the monomer unit (≈1 nm). At temperatures of up to 160 °C, the mobility of EPDM chain units in the interface is strongly hindered and is comparable to that in an unfilled EPDM at temperatures 10−15 deg above Tg. It is suggested that the sites of the interaction between the carbon black and the EPDM cause physical adsorption network junctions in the rubber matrix. The average molar mass of EPDM chains between the adjacent adsorption junctions in bound EPDM rubber is about 1800−2500 g/mol and depends on the content and the type of the filler. The mean end-to-end distance between the adsorption junctions is comparable to the average distance between the adjacent carbon black aggregates. This suggests that the carbon black aggregates are interconnected by EPDM bridging chains, and a continuous EPDM/carbon black physical network is formed in the bound rubber fraction of the compound. The results obtained for uncured filled rubbers provide strong evidence of a “bimodal” structure of the physical network. The two types of EPDM chains and/or chain fragments, which have strongly different densities of EPDM−carbon black adsorption junctions, are present in the elastomer matrix outside of the EPDM−carbon black interface. There is an EPDM fraction that is loosely bound to the carbon black by adsorption interactions. This loosely bound rubber has numerous adsorption network junctions, similar to those of the bound rubber. The other fraction of EPDM, the extractable (unbound) rubber, has a relatively low number of adsorption network junctions and can apparently be extracted from the compound. The fraction of loosely bound EPDM chains, as measured by NMR, increases as the maximum possible EPDM−carbon black contact area per unit volume of the elastomer increases, regardless of the type of carbon black, and is relatively close to the content of the bound rubber. Results of mechanical property tests on the carbon black filled vulcanisates reveal that several factors contribute to the reinforcing effect of the filler. Besides the hydrodynamic effects, the occluded EPDM, the chemical cross-links, and the formation of the physical EPDM−carbon black network, the filler−filler interactions provide a significant contribution to the modulus in the low-strain region, while they are broken at high strains.

Determination of Water in Zeolite a Using Low-Resolution NMR

Determination of Water in Zeolite a Using Low-Resolution NMR

Poly-1-oxy-2-phenyltrimethylene as studied by 1H pulsed low resolution NMR: a possible oxygen scavenger

Aromatic polymers can adsorb a large amount of oxygen on their aromatic rings. In semicrystalline aromatic polymers, the amount of adsorbed oxygen is much larger in the amorphous phase than in the crystalline counterpart. As a consequence, fully amorphous aromatic polymers are more suitable as oxygen scavengers than semicrystalline polymers. A pulsed low resolution 1H-NMR relaxation study on semicrystalline and fully amorphous poly-1-oxy-2-phenyltrimethylene (St–CO), deuteriated and not deuteriated on the backbone, is reported. T 1 relaxation values were measured at 30 and 57 MHz and compared with the values of two aromatic polymers previously studied, syndiotactic polystyrene (s-PS) and polyphenyleneoxide (PPO), both patented as oxygen scavengers. For all these polymers, using a set of equations, at each temperature, the amount of adsorbed oxygen was calculated. Very short T 1 values are observed at the low temperature point of PPO which is the best oxygen scavenger at low temperature, while St–CO adsorbs oxygen efficiently at room temperature. Thus, St–CO might be suitable to be used as an oxygen scavenger. In St–CO, a 1H T 1 ρ relaxation study on the rotating frame has also been performed. In the atactic copolymer, in the temperature range 150–160 K, a sharp transition was observed only in the presence of oxygen. In agreement with a previously given interpretation for analogous data, the observed transition might be related to low frequency motions present in low molecular weight components.

The structure in solution of the b domain of protein disulfide isomerase.

Protein disulfide isomerase (PDI) is a multifunctional protein of the endoplasmic reticulum, which catalyzes the formation, breakage and rearrangement of disulfide bonds during protein folding. It consists of four domains designated a, b, b and a. Both a and a domains contains an active site with the sequence motif -Cys-Gly-His-Cys-involved directly in thiol-disulfide exchange reactions. As expected these domains have structures very similar to the ubiquitous redox protein thioredoxin. A low-resolution NMR structure of the b domain revealed that this domain adopts a fold similar to the PDI a domain and thioredoxin [Kemmink, J., Darby, N.J., Dijkstra, K., Nilges, M. and Creighton, T.E. (1997) Curr. Biol. 7, 239-245]. A refined ensemble of solution structures based on the input of 1865 structural restraints shows that the structure of PDI b is well defined throughout the complete protein except for about 10 residues at the C-terminus of the sequence. 15N relaxation data show that these residues are disordered and not part of this structural domain. Therefore the domain boundaries of PDI can now be fixed with reasonable precision. Structural comparison of the PDI b domain with thioredoxin and PDI a reveals several features important for thiol-disulfide exchange activity.

High-resolution solution structure of the retinoid X receptor DNA-binding domain

The retinoid X receptor (RXR) is a member of the nuclear hormone receptor superfamily of transcriptional regulators and plays a central role in the retinoid and, through its ability to heterodimerize with other nuclear hormone receptors, non-steroid signaling pathways. The DNA-binding and recognition functions of RXR are located in a conserved 83 amino acid residue domain that recognizes the consensus sequence AGGTCA. In order to provide a detailed picture of its structure, we have calculated a high-resolution solution structure of the C195A RXRα DNA-binding domain. Structures were calculated using 1131 distance and dihedral angle constraints derived from 1H, 13C and 15N NMR spectra. The structures reveal a perpendicularly packed, “loop-helix” fold similar to other nuclear hormone receptor DNA-binding domains and confirm the existence of the C-terminal helix, which was first observed in the low-resolution NMR structure. The C-terminal helix is well formed and is stabilized by packing interactions with residues in the hydrophobic core. The solution structure of RXR is very similar to that determined by X-ray crystallographic studies of the RXR-TR heterodimer complex with DNA, except that in the latter case no electron density was observed for residues corresponding to the C-terminal helix. Other differences between the X-ray and NMR structures occur in the second zinc-binding loop, which is disordered in solution. Heteronuclear 15N NOE measurements suggest that this loop has enhanced flexibility in the free protein.

Use of Pulsed Nmr Spectroscopy to Measure the Amount of Solid Deposits As a Function of Temperature in Waxy Crudes

Paraffin crystallization in crude oils and in gas condensates is a major problem for petroleum exploitation. Thermodynamic models are used to evaluate the risk. With them, one calculates the Wax Appearance Temperature (WAT) as well as the crystallized fraction as a function of temperature, for temperatures below the WAT. During development, these models are tested against experimental data. Although different methods for measuring the WAT are available in the literature and at petroleum companies, such methods unfortunately do not exist for measuring the crystallized fraction as a function of temperature. In this work, we have developed a low resolution NMR method for measuring the crystallized fraction as a function of temperature. The method was first evaluated using pure samples and was then applied to crude oils. The results were compared to those obtained both from differential calorimetry and from the model.

Fluorescence resonance energy transfer in studies of inter-chromophoric distances in biomolecules.

Fluorescence resonance energy transfer (FRET) is a technique widely used in studies of interchromophoric distances in biomolecules such as peptides, proteins and nucleic acids. FRET is especially useful in determination of conformational changes caused by a solvent, presence of denaturing agents, diffusion and other external factors. Precision of interchromophoric distances obtained using the FRET technique is comparable with that of low-resolution X-ray diffraction and NMR data. Comparison of FRET results with the crystal structure for several proteins is reviewed. Moreover, the effect of the orientation factor kappa2 value on FRET results and determinants of kappa2 are discussed.

Gastric carcinogenesis: 2-chloro-4-methylthiobutanoic acid, a novel mutagen in salted, pickled Sanma hiraki fish, or similarly treated methionine.

The customary salting and pickling of fish in high risk gastric cancer regions were modeled to explore the relevant causative chemicals. The fish Sanma hiraki was treated with sodium chloride and sodium nitrite at pH 3. Previously, it had been found that an extract of the treated fish was mutagenic in Salmonella typhimurium TA 1535 without S9 and also that it induced glandular stomach cancer upon gavage to rats. We now demonstrate that the mutagenicity was enhanced by preincubation of the raw meat for several days before salt-nitrite treatment. HPLC techniques showed that three mutagens were present in the fish extract. One of the mutagens was found to be stable over the pH range of 1.0-9.0. This mutagen was purified by silica gel solid phase extraction, followed by a series of reverse phase HPLC steps, and was characterized by low and high resolution MS, NMR, and FT-IR. While N-nitroso compounds were generally believed to be associated with gastric carcinogenesis, it was unexpectedly found that the mutagen has the novel structure 2-chloro-4-methylthiobutanoic acid (CMBA). Based on the structure, it seemed likely that methionine might be the precursor, and this was, indeed, proven. Both salt and nitrite are essential factors for forming this mutagen. The yield of CMBA was linear for chloride concentrations from 0 to 800 mM NaCl. Of 20 amino acids reacted with nitrite and chloride at pH 3, only methionine generated a mutagen for S. typhimurium TA 1535. Tryptophan gave a product mutagenic in S. typhimurium TA 100 and TA 98, but not TA 1535, and in the case of tyrosine, the mutagen was active only for TA 100. These results suggest an important role for salt in gastric carcinogenesis and provide new approaches for exploring the formation of mutagens/carcinogens for specific target organs.

Synergistic effect of zeolite in an intumescence process: study of the carbonaceous structures using solid-state NMR

This work deals with the chemical effect of zeolite 4A added to the intumescent ammonium polyphosphate (APP)–pentaerythritol (PER) system [fire retardant (FR) polyethylene-based formulation]. The study of the thermal behaviour of the system using thermogravimetric (TG) analysis shows that the presence of the zeolite leads to an increase in the stability of the material at high temperature (T > 500 °C). Chemical analysis and cross-polarisation dipolar-decoupled magic-angle spinning (CP–DD–MAS)13C NMR reveals that the materials resulting from the thermal treatment of the APP–PER and APP–PER/4A systems are formed by carbonaceous and phosphocarbonaceous species and that the zeolite enhances the stability of the phosphocarbonaceous species. DD–MAS 31P NMR indicates that this stability may be due to an absence of pyrophosphate species in the heat-treated APP–PER/4A system. Micro-Raman spectroscopy and 1H NMR of the solid state provides information on the structure of the carbonaceous material. Thermal treatment led to the formation of a ‘turbostatic carbon’ with a local structure of this ‘carbon species’, with the zeolite permitting retention of the comparatively unorganised carbon species in the high-temperature range. Moreover, a low-resolution solid-state 1H NMR study of the spin–lattice and spin–spin relaxations of the samples allows an evaluation of the size of the slow relaxation domains using the Goldman–Shen procedure. It is shown that the zeolite hinders the formation of small molecules in the polyaromatic network. Finally, the relationship between the formation of a coherent macromolecular network and the improved FR performance of the APP–PER/4A system is proposed.

Temperature dependence of Water Self-Diffusion in the Gel phase of a Potassium Palmitate System

The temperature dependence of water self-diffusion in the gel and liquid crystalline phases of the potassium palmitate/water system was examined using pulsed field gradient spin-echo (PGSE) low resolution NMR experiments. The temperature dependence of the water self-diffusion in lamellar and hexagonal phases was characterised by trends parallel to pure water. The average ”two-site” self- diffusion model was used to analyse the composition dependence of the self-diffusion coefficients. Temperature dependence of water self-diffusion in the gel phase presented different and interesting behaviour. The slopes of the Arrhenius self-diffusion were strongly composition dependent. The results were interpreted as a dependence on T of the bound water fraction, while the self-diffusion obstruction factor was considered constant.

The Use of Low Resolution NMR to Monitor the Extent of Cure in an Epoxy Resin System

The solidification of samples of an epoxy resin has been monitored by low field NMR. The shape of the free induction decay curve and the initial signal intensity are shown to depend upon the state of cure and results from this method are compared to those from mechanical measurements.

Evaluation of Starch Modifications from Thermo-Mechanical Processes by NMR

Low resolution NMR technique was used to measure the effects of different process conditions on the transverse relaxation signal of different industrial products, sauces, cake batters and corn flakes. In sauces, the increase of the global relaxation time was related to cooking time and temperature. The cooking effect of batters was associated to a significant increase of the liquid-like starch protons and a transfer of sugared water in the starch partially gelatinized. The transverse relaxation behaviour of corn was very sensitive to the flaking process: a proton population tended to disappear with the increase of the flaking intensity.

Low-Field Nuclear Magnetic Resonance Relaxation Study of Stored or Processed Cod

Proton transverse relaxation has been measured in heated, pressure-treated, and frozen stored cod by pulse low-resolution NMR spectroscopy. These treatments resulted in the appearance of an additional component in the proton transverse relaxation which can be explained in terms of water exudation. The transverse relaxation time associated with this supplemental component increased with the temperature of the thermal treatment or with the temperature and duration of the frozen storage. For cod, these variations in NMR relaxation parameters can be used to detect nonreversible deterioration due to frozen storage or high hydrostatic pressure treatment.

Determination of Water Droplet Size Distributions by Low Resolution PFG-NMR: II. "Solid" Emulsions

In this work, it is shown that pulsed field gradient low resolution NMR allows us to determine water droplet size distribution in "solid" emulsions such as butters and light butters. For the characterization of such samples, the potential use of a mathematical model, formed from a term of restricted and unrestricted diffusion, is confirmed. Actually, they can be made of aqueous feeders due to an incomplete emulsification. NMR results have been compared to those obtained by image analysis made on pictures of 5-6 μm thick cuts.

Proton Low‐Field NMR Measurements on Crackers

ABSTRACTWe have shown that the signal obtained for crackers (3.5% water content) by low resolution NMR method using the CPMG sequence, described the relaxation of the protons of fat. For water content up to 7.1% the water signal is not seen by a CPMG sequence, the water has therefore a very short spin‐spin relaxation time (T2 < 0.5 ms). However, that water modifies the mobility of protons of fat at solid‐liquid interfaces. Protons exchangeable by deuterium affect neither the relaxation time of the short component (about 1 ms), nor its intensity. Between 7.1% and 14.8%, the water had a spin‐spin relaxation time of about 1 msec and its signal was added to the short component of the fat. Thus, low resolution NMR could be used to follow the crystallization of fat in such samples after cooking.

Determination of Water Droplet Size Distributions by Low Resolution PFG-NMR: I. "Liquid" Emulsions

The aim of this work is to show that a low resolution NMR spectrometer in combination with a pulsed field gradient unit can be used for the determination of the water droplet size distributions of water-in-oil emulsions. To improve the treatment of NMR measurements, we added an unrestricted diffusion component to the restricted one used up to now. The purpose of this second term is either (i) to improve the fitting of the experimental values to take into account that the distribution of the droplet sizes is not perfectly lognormal, or (ii) to show a significant unrestricted diffusion component due to poor emulsification of the sample. The NMR results are compared with those of laser diffraction. These results are the first step in a study of "solid" water-in-oil emulsions such as butter or margarine.

Structure and Molecular Motions in Crosslinked Isotactic Polypropylene

The structure and the molecular motions of chemically crosslinked iPP samples have been analysed. The samples differ in gel content (i.e. degree of crosslinking). The X-ray analysis shows that two crystalline phases (α and γ) are present, their relative amount depending on the gel content. The thermal analysis suggests that the crosslinked samples are characterized by smaller and more defective crystals. The sample structure (in terms of crystallinity, phase composition and crystal defects) affects the molecular motion phenomena, analysed using the low resolution NMR spectroscopy. In particular, the analysis of the FID (free induction decay) signal shows a complex relationship between temperature, molecular mobility, and phase composition, which agrees with the thermal data.