NMR Relaxation Times T1 Research Articles

Solid state 13C and 1H MAS NMR investigations of C60(ferrocene‒d10)2 complex

The article presents NMR investigations of a molecular crystalline material C60(ferrocene‒d10)2, which consists of two highly mobile molecules. 13C and 1H NMR line shapes and T1 relaxation times are discussed and compared with results obtained for the undeuteriated compound. The mobilities of C60 and ferrocene molecules were found to increase in the complex in comparison with the two components as crystalline materials.

Solid state 1H NMR study, humidity sensitivity and protonic conduction of gel derived phosphosilicate glasses

1H Magic Angle Spinning (MAS) NMR and 1H T1 and T1ρ relaxation times measurements were used to investigate structural aspects of gel derived glasses of composition 10P2O5·90SiO2 (10P) and 30P2O5·70SiO2 (30P), with the aim of understanding the different humidity sensitive behaviour observed for films of the same composition. The experimental evidence suggests that 10P and 30P gel samples heat treated at 100 °C have a similar POH environment while further heating causes a different structural evolution of POH units. In particular, the 30P sample heat treated at 400 °C shows a single 1H resonance while a number of resonances are seen in the 1H spectrum of the 10P sample heat treated at 400 °C. Further evidence of the different structure of 10P and 30P samples was given by the NMR relaxation times measurements, the gels were found to be heterogeneous on a nanometric scale. Domains, about 60 nm in size, related to 1H resonance in POH were evenly distributed in the matrix of both gels, only in the 10P gel also larger domains (about 700 nm) were found that can be related to 1H resonance in SiOH groups. Finally, MAS in combination with variable temperature static NMR spectra provided detailed information on the network structural features present in the 30P sample heat treated at 300 °C as well as in the 10P sample heat treated at 400 °C and allowed investigation of the dynamic processes taking place in these samples.

Molecular Dynamics Simulations of Octyl Glucoside Micelles: Dynamic Properties

Dynamic properties of octyl glucoside (OG) micelles were explored using molecular dynamics simulations. Systems studied included individual β-OG micelles containing 10, 20, 27, 34, 50, and 75 lipids; two 20 lipid β-OG micelles; a disperse solution of 27 β-OG; and four molecules of glucose. Calculated 13C NMR T1 relaxation times for the tail carbons of micelle aggregation numbers between 34 and 75 agreed well with experiment; these results are consistent with estimates of the micelle size based on translational diffusion. However, T1's for the headgroup carbons, which couple strongly with the solvent, were too large. This was primarily due to the low viscosity of the TIP3P water model, and subsequent scaling of the relaxation times led to agreement with experiment for the carbons in the glucose ring, but not the exocyclic carbon; the likely reason for the latter discrepancy is a torsional potential barrier that is slightly too high. A detailed analysis of the micelle dynamics revealed shape changes on the time scale of tens to hundreds of picoseconds, while rotation and lipid diffusion within the micelle occur over nanoseconds. The primary components of NMR T1 relaxation are lipid wobble and chain isomerization, as well as slower concerted motions on the time scale of the shape changes. Lipid lateral diffusion and overall micelle tumbling do not contribute significantly to NMR relaxation. Micelle self-assembly on the nanosecond time scale was also demonstrated. The two 20 lipid micelles merged and the 27 dispersed lipids aggregated, highlighting a new range of behavior accessible to molecular dynamics simulation.

Poly(N-vinyl-2-pyrrolidone) and 1-Octyl-2-pyrrolidinone Modified Ionic Microemulsions

The influence of the nonionic polymer poly(N-vinyl-2-pyrrolidone) (PVP) in comparison to the surfactant 1-octyl-2-pyrrolidinone (OP) on the phase behavior of the system SDS/pentanol/xylene/water was studied. In both modified systems a strong increase in the water solubilization capacity was found, accompanied by a change in the spontaneous curvature toward zero. In the polymer-modified system an isotropic phase channel is formed with increasing polymer content that connects the L1 and the L2 phase. The lamellar liquid crystalline phase is destabilized in both cases. In the L1 phase the adsorption of PVP at the surface of the microemulsion droplets and the formation of a cluster-like structure is proven by several methods like 13C NMR T1 relaxation time measurments, zeta potential measurements, and rheology. In the L2 phase a modification of the interface of the inverse droplets is detected by a shift in the percolation boundary (conductivity) and 13C NMR T1 relaxation measurements. The formation of a cluster-like structure can be assumed on the basis of our rheological measurements.

The relationship between radiation-induced chemical processes andtransverse relaxation times in polymer gel dosimeters

The effects of ionizing radiation in different compositions of polymer geldosimeters are investigated using FT-Raman spectroscopy and NMR T2relaxation times. The dosimeters are manufactured from differentconcentrations of comonomers (acrylamide andN,N'-methylene-bis-acrylamide) dispersed in different concentrations ofan aqueous gelatin matrix. Results are analysed using a model of fastexchange of magnetization between three proton pools. The fraction ofprotons in each pool is determined using the known chemical composition ofthe dosimeter and FT-Raman spectroscopy. Based on these results, thephysical and chemical processes in interplay in the dosimeters are examinedin view of their effect on the changes in T2. The precipitation ofgrowing macroradicals and the scavenging of free radicals by gelatin areused to explain the rate of polymerization. The model describes the changesin T2 as a function of the absorbed dose up to 50 Gy for the differentcompositions. This is expected to aid the theoretical design of new, moreefficient dosimeters, since it was demonstrated that the optimum dosimeter(i.e, with the lowest dose resolution) must have a range of relaxationtimes which match the range of T2 values which can be determined withthe lowest uncertainty using an MRI scanner.

Complexes of EDTA in aqueous solutions. Structural aspects from a 13C NMR relaxation study

13 C NMR relaxation times T1 of the carboxylate groups in EDTA and its complexes with Mg(II), Ca(II), Zn(II) and Al(III) ions in aqueous solutions were measured at different magnetic field strengths, allowing the determination of the contribution of the chemical shift anisotropy mechanism to the total relaxation rate and, consequently, the shielding tensor (chemical shift) anisotropy of these nuclei. A discussion of these values in relation to the possible structure of these species in solution and a comparison with the values obtained in the solid state were performed.

On the investigation of homeopathic potencies using low resolution NMR T2 relaxation times: an experimental and critical survey of the work of Roland Conte et al

On the investigation of homeopathic potencies using low resolution NMR T2 relaxation times: an experimental and critical survey of the work of Roland Conte et al

13C NMR Studies of Hyaluronan. 2. Dependence of Conformational Dynamics on Chain Length and Solvent

The conformational dynamics of low molecular weight hyaluronan (HA) polymer and oligosaccharides have been assayed by 13C NMR T1 relaxation time measurements. For polymeric HA in aqueous solution, the average relaxation time for ring carbons was not significantly affected by changing the ionic strength or by changing counterion type. The average relaxation time of HA was found to be similar to that for chondroitin 4-sulfate or chondroitin 6-sulfate in aqueous solution. As has been observed previously for other polysaccharides, the segmental motions of HA chains studied at room temperature in aqueous solution appear to be dominated by viscous damping of the chain motions. Within a chain structure, there are variations in observed T1 that correspond to differences in relative mobility. The hydroxymethyl substituent group shows a difference in relaxation rate relative to the ring to which it is attached, depending on the ring configuration (glucose vs galactose). These differences correlate with relative rotational isomerization rates measured by ultrasonic relaxation. In short HA chains, the nonreducing and reducing terminal residues show much greater mobility than penultimate residues, and these are in turn more mobile than interior residues. In contrast, the more rigid hydrogen-bonded conformation of an HA tetrasaccharide in dimethyl sulfoxide solution shows less position-dependent variation in T1. These data are in accord with the dynamic nature of conformation-stabilizing hydrogen bonds for HA chains in aqueous solution.

Ammonium ion dynamics in NH4Br at high pressure - type of reorientation

Proton NMR relaxation measurements were performed for ammonium bromide at pressures up to 800 MPa. The correlation functions for ammonium ion reorientation were calculated by Monte Carlo method. Knowledge of these functions allow calculation of the NMR relaxation times T1. Comparison between experimental and calculated values of relaxation times gives some insight into the model of ammonium ion reorientation.

3He spin diffusion measurements in 3He- 4He mixture films

3He spin diffusion measurements for 3He-4He mixture films on Nuclepore are reported as a function of 3He coverage for 0. 030</=T</=0.070 K. We report measurements for a constant 4He coverage of 5.0 bulk density atomic layers, with 3He coverages between 0.10 and 0.98 layers. We find that the 3He spin diffusion coefficient, as measured by Hahn spin echoes, has an unexplained maximum as a function of 3He coverage and that the temperature dependence is somewhat weaker than predicted. Comparison measurements are presented for the NMR relaxation times T1 and T2 and the magnetization over the same 3He coverage range.

Optimized MR imaging for polyacrylamide gel dosimetry

Polyacrylamide gels are a powerful tool to measure radiation dose by quantifying the NMR T2 relaxation times of the irradiated gel. The exploitation of these radiation sensitive gels in clinical radiotherapy requires accurate mapping of T2 values. This paper describes the optimization strategy used to identify accurate and practical methods of measuring the range of T2 values typical of gel dosimeters (140-700 ms). The MR imaging techniques used to measure T2 values and the choice of image acquisition parameters are described. Four sequences are compared and the results are analysed in terms of accuracy, signal-to-noise ratio and acquisition time. A multiple spin echo sequence was found to yield the most accurate results (98.9%). Single spin echo sequences, such as Hahn spin echo and EPI spin echo, were found to measure gel T2 values with an accuracy of 90.1%. This paper reports the importance of careful selection and optimization of the MR imaging sequences for accurate and reliable polyacrylamide gel dosimetry.

Dynamic relaxometry: application to iron uptake by ferritin.

We introduce dynamic relaxometry as a novel technique for studying biochemical reactions, such as those leading to mineral formation (biomineralization). This technique was applied to follow the time course of iron oxidation and hydrolysis by the protein ferritin. Horse spleen apoferritin was loaded with single additions of 4, 10, 20, 40, and 100 ferrous ions per protein, and with multiple additions of 4, 10, 20, and 100 ferrous ions. The NMR T2 relaxation time was then measured sequentially and continuously for up to 24 h. At low loading factors of 4-10 Fe atoms/molecule, the iron is rapidly bound and oxidized by the protein on a time scale of approximately 15 s to several minutes. At intermediate loading factors (10-40), rapid initial oxidation was observed, followed by the formation of antiferromagnetic clusters. This process occurred at a much slower rate and continued for up to several hours, but was inhibited at lower pH values. At higher loading factors (40-1000), iron oxidation may occur directly on the core, and this process may continue for up to 24 h following the initial loading. Dynamic relaxometry appears to be a potentially powerful technique that may well have applications beyond the study of iron uptake by the ferritin protein.

Li+ Influx and Binding, and Li+/Mg2+ Competition in Bovine Chromaffin Cell Suspensions as Studied by 7Li NMR and Fluorescence Spectroscopy

Li+ influx by bovine chromaffin cells, obtained from bovine adrenal medulla, was studied in intact cell suspensions using 7Li NMR spectroscopy with the shift reagent [Tm(HDOTP)]4-. The influx rate constants, ki, were determined in the absence and in the presence of two Na+ membrane transport inhibitors. The values obtained indicate that both voltage sensitive Na+ channels and (Na+/K+)-ATPase play an important role in Li+ uptake by these cells. 7Li NMR T1 and T2 relaxation times for intracellular Li+ in bovine chromaffin cells provided a T1/T2 ratio of 305, showing that Li+ is highly, immobilized due to strong binding to intracellular structures. Using fluorescence spectroscopy and the Mg2+ fluorescent probe, furaptra, the free intracellular Mg2+ concentration in the bovine chromaffin cells incubated with 15 mM LiCl was found to increase by about mM after the intracellular Li+ concentration reached a steady state. Therefore, once inside the cell, Li+ is able to displace Mg2+ from its binding sites.

Ammonium Ions Dynamics in NH4Br-Proton NMR T1 Measurements and Simulations

Abstract Monte Carlo simulation of correlation functions for ammonium bromide have been carried out and the NMR relaxation times T1 resulting from these functions were calculated. Results of simulations are compared to experimental values of T1 measured in the temperature range 92 K to 292 K.

A study of the effect of the polymer network and the solvent on the ion conductivity in gels

Ion-conducting gels were produced by photo-polymerisation of acrylates in the presence of lithium hexafluorophosphate (LiPF6) in various solvents. The ionic conductivity of various mixtures containing propylene carbonate and polyethylene glycol diacrylate was measured as a function of temperature before and after polymerisation. The change observed in the conductivity upon polymerisation was explained using a modified free volume theory. The free volume within the mixtures was calculated from the measured density and the van der Waals density of the materials. Using this theory, plots relating the conductivity to the free volume within the mixtures were produced. The measured data were found to cluster around a master curve, explaining the temperature dependence of the conductivity and its reduction upon polymerisation in terms of free volume within various systems. The mobility of the lithium ions within the mixtures before and after polymerisation was studied using 7Li NMR T1 relaxation time measurements. The diffusion coefficients of all the components in the mixtures were also measured using the pulse field gradient NMR technique. The phase structure of the gels and the mobility of the ions were found to be dependent on the polarity of the polymer and the type of solvent used. Transport numbers obtained for Li+ were found to be much lower upon polymerisation in the case of mixtures containing PEGDA and PC whereas other mixtures showed a much lower decrease.

Monte Carlo Calculation of the Correlation Functions for Molecular Rotation-Application to NMR

Abstract The Monte Carlo method for calculating correlation functions for the rotational motion of atoms is presented. This method applies to crystalline solids and allows the determination of the correlation functions for any model of rotating atoms, molecules or ions constituting the material of interest, if only its crystal structure is known. The presented method permits the calculation of the correlation functions for a whole block of unit cells, not only for a single group of particles. The described method can be employed to calculate correlation functions used in the theoretical description of NMR, dielectric or neutron diffraction experiments. As an example the correlation functions and NMR relaxation time T1 of solid benzene are calculated by the Monte Carlo method for the.

Synthesis of and NMR T1 Relaxation in the Bimetallic Trihydride Complexes Cp2TaH2(μ-H)M(CO)5 (M = Cr, Mo, W)

New bimetallic trihydrides Cp2TaH2(μ-H)M(CO)5 (M = Cr (2), Mo (3), W (4)) have been prepared by the room-temperature high-yield reaction between Cp2TaH3 (1) and M(CO)5. The complexes have been characterized by elemental analyses and variable-temperature 1H NMR and IR spectra. The structures and arrangements of the hydride ligands in complexes 2−4, where the central hydride ligand is bound to M, have been determined from 1H NMR and 1H T1 relaxation time data. It has been shown that structural interpretations of 1H T1 measurements, carried out for 1 and bimetallic systems below 295 K, can be made on the basis of the isotropic motion approach and of a model of uncoupled nuclei. The high-temperature saturation transfer experiments in the 1H NMR spectra have shown complexes 2−4 to be stereochemically nonrigid demonstrating an HX/HA exchange on the T1 NMR time scale. It has been established that the energy barrier of the hydride/hydride exchange is decreased in going from complex 1 to bimetallic systems 2−4.

Dynamic structure of proteins in solid state. 1H and 13C NMR relaxation study.

Temperature dependencies of 1H non-selective NMR T1 and T2 relaxation times measured at two resonance frequencies and natural abundance 13C NMR relaxation times T1 and T1r measured at room temperature have been studied in a set of dry and wet solid proteins - Bacterial RNase, lysozyme and Bovine serum albumin (BSA). The proton and carbon data were interpreted in terms of a model supposing three kinds of internal motions in a protein. These are rotation of the methyl protons around the axis of symmetry of the methyl group, and fast and slow oscillations of all atoms. The correlation times of these motions in solid state are found around 10(-11), 10(-9) and 10(-6)s, respectively. All kinds of motion are characterized by the inhomogeneous distribution of the correlation times. The protein dehydration affects only the slow internal motion. The amplitude of the slow motion obtained from the carbon data is substantially less than that obtained from the proton data. This difference can be explained by taking into account different relative inter- and intra- chemical group contributions to the proton and carbon second moments. The comparison of the solid state and solution proton relaxation data showed that the internal protein dynamics in these states is different: the slow motion seems to be few orders of magnitude faster in solution.

NMR Study of β Relaxation in Modified and Blended Isotactic Polypropylene

Broad line NMR and spin-lattice relaxation times T1 have been used to study glass transition phenomena in isotactic polypropylene modified and blended with an ethylene aminoalkylacrylate copolymer. By means of decomposition of spectra into elementary components corresponding to the chains with different mobility a softening due to the polymer modifier was observed. The modification results in an increase of fractions of amorphous and intermediate chains, respectively, gives rise to the enhancement of the chain mobility in amorphous regions, but the higher amount of the polymer modifier, which was used in the range up to 15 wt%, the lower chain mobility in intermediate regions was found out. From the temperature dependences of the spin-lattice relaxation times T1 measured over the temperature range 290–430 K, a broadening of correlation time distribution on the side of the shorter correlation times has been found out to be and it was ascribed to overlapping of relaxation processes in amorphous regions. The sample containing the highest amount of the polymer modifier, in contrast to others, was shown to blend not only on the molecular level but also on the overmolecular.

Anionic Polyelectrolyte Binding to Mixed Cationic−Zwitterionic Surfactant Micelles: A Molecular Perspective from 2H NMR Spectroscopy

Complex formation between an anionic polyelectrolyte poly(sodium styrenesulfonate) (PSSS) and mixed zwitterionic−cationic surfactant micelles was monitored using deuterium NMR spectroscopy of the specifically deuterium-labeled surfactants hexadecylphosphocholine (HDPC-γ-d6) and cetyltrimethylammonium bromide (CTAB-γ-d9). The deuterium signal from both surfactants decreased in direct proportion to the amount of added PSSS, reflecting the phase separation of the charge-neutralized PSSS−surfactant complex. The insoluble complex was preferentially enriched with CTAB over HDPC. The PSSS/CTAB charge ratio was the primary determinant of the degree of complex formation. Deuterium NMR T1 and T2 relaxation time measurements indicated that soluble PSSS−surfactant complexes exist prior to their phase separation. Within these soluble complexes motions of the cationic surfactant CTAB, but not the zwitterionic surfactant HDPC, were hindered. Quadrupolar echo deuterium NMR spectra of the phase-separated complexes showed that the flocculated material is inhomogeneous in composition and that surfactant mobility within the insoluble complex varies according to the CTAB/HDPC ratio.