Longitudinal Relaxation Time Measurements Research Articles

Enantiodifferentiation of phytoalexin spirobrassinin derivatives using the chiral solvating agent ( R)-(+)-1,1′-bi-2-naphthol in conjunction with molecular modeling

The enantioresolutions of chiral phytoalexins spirobrassinin, 1-methoxyspirobrassinin and cis- and trans-1-methoxyspirobrassinol methyl ethers were obtained by solution-state 1H NMR spectroscopy using the chiral solvating agents ( R)-(+)-1,1′-bi-2-naphthol and, for 1-methoxyspirobrassinin, ( R)-(−)-1-phenyl-2,2,2-trifluoroethanol. In addition to the evaluation of the magnitude of the chemical shift differences, several other aspects with respect to enantiodifferentiation were also examined, namely titration of the enantiomers with the chiral solvating agents, signal degradation in the presence of excess, solid binaphthol, measurement of longitudinal relaxation times ( T 1) and diffusion coefficients ( D), conducting enantiomeric titrations of spirobrassinin under SIDA and atypical SIDA conditions, and conducting chromatographic-type examinations (‘TLC in an NMR tube’). To rationalize the observed spectroscopic and adsorption behavior, the conformations, geometries, and energies of the complexes were calculated by DFT calculations at the B3LYP/TZVP{6-31G(d,p)} level of theory with solvation simulated by IEF-PCM. Chemical shifts were assessed by making a comparison with population-weighted averaged shieldings calculated by DFT at the B3LYP/cc-pVTZ level of theory using the GIAO method. The reliability of the interpretations with respect to the assignment of the configuration was very dependent on obtaining a large calculated Δ G and/or adopting a holistic approach by evaluating several measured parameters.

Quantitative proton MRI and MRS of the rat brain with a 3 T clinical MR scanner

To demonstrate the capability of a clinical 3T human scanner in performing quantitative MR experiments in the rat brain. In vivo, measurements on eight Wistar rats were performed. Longitudinal relaxation time (T1) and transverse relaxation time (T2) measurements were set up at a spatial resolution of 0.3×0.3×1mm(3). Diffusion-weighted imaging was also applied and the evaluation included both mono- and biexponential approaches (b-value up to 6000s/mm(2)). Besides quantitative imaging, the rat brain was also scanned at a microscopic resolution of 130×130×130μm(3). Quantitative proton spectroscopy was also carried out on the rat brain with water as internal reference. T1 and T2 for the rat brain cortex were 1272±85ms and 75±2ms, respectively. Diffusion-weighted imaging yielded accurate diffusion coefficient measurements at both low and high b-value ranges. The concentrations of MR visible metabolites were determined for the major resonances (i.e., N-acetyl-aspartate, choline and creatine) with acceptable accuracy. The results suggest that quantitative imaging and spectroscopy can be carried out on small animals on high-field clinical scanners.

Highlighting Cavities in Proteins by NMR Using Sulfur Hexafluoride as a Spy Molecule

Cavities in proteins can be studied experimentally by using some detectable atoms, such as xenon, or molecules which act as reporter, such as a spy. The interest of sulfur hexafluoride (SF(6)) for probing hydrophobic cavities by solution-state NMR is investigated. The wheat nonspecific lipid transfer protein (LTP) was selected as a model system for this purpose. The binding of SF(6) is straightforwardly detected by the (19)F chemical shift, line width, or longitudinal relaxation time measurements, which can be carried out at low SF(6) concentration without interference from resonances of the protein. Most interestingly, the binding of SF(6) gives rise to selective intermolecular (1)H{(19)F} heteronuclear Overhauser effects (HOEs). Molecular dynamics simulation and NMR spectrum modeling show that the experimental HOESY spectra are consistent with (1)H{(19)F} HOEs arising from SF(6) in the cavity of LTP. SF(6) is found to be an advantageous alternative to hyperpolarized (129)Xe and small organic compounds for probing cavities in proteins by solution-state NMR.

How to perform accurate and reliable measurements of longitudinal and transverse relaxation times of MRI contrast media in aqueous solutions

Knowledge of the longitudinal and transverse relaxation times (T(1) and T(2)) of water protons in an aqueous solution of an MRI contrast agent is essential for its characterization. These parameters can be measured at low field on low resolution spectrometers or at high field on high resolution spectrometers. The reliability and the accuracy of T(1) and T(2) measurements rely on several experimental settings and on the equation used to fit the data. Examples of the importance of careful adjustment of the most important parameters are illustrated through several measurements performed on a low-resolution, low-magnetic field instrument. In addition, some specificities of T(1) and T(2) measurements on high-resolution, high-magnetic field spectrometers are pointed out.

Location of a Metallic Cation Complexed in a Calixarene Cavity As Determined by Calixarene 13C Spin Relaxation. Application to Cesium and Thallium Complexed by p-Sulfonatocalix[4]arene in Water

This study deals with the exact location of the monovalent metal cations Cs(+) and Tl(+) which are complexed by the p-sulfonatocalix[4]arene in water. This determination rests on the measurements of longitudinal relaxation times of carbon-13 not directly bonded to protons. The difference between the relaxation times of the free calixarene and of the complex definitely demonstrates that the monovalent metal cation is well inside the calixarene cavity. These features are in fact enhanced by the presence of paramagnetic species which act in a different way in the complexed form. Experimental results also show without any ambiguity that the calixarene cavity is essentially hydrophobic. Finally, it is observed that thallium is more mobile than cesium within the calixarene cavity.

Quantitative ATP synthesis in human liver measured by localized 31P spectroscopy using the magnetization transfer experiment

The liver plays a central role in intermediate metabolism. Accumulation of liver fat (steatosis) predisposes to various liver diseases. Steatosis and abnormal muscle energy metabolism are found in insulin-resistant and type-2 diabetic states. To examine hepatic energy metabolism, we measured hepatocellular lipid content, using proton MRS, and rates of hepatic ATP synthesis in vivo, using the 31P magnetization transfer experiment. A suitable localization scheme was developed and applied to the measurements of longitudinal relaxation times (T1) in six healthy volunteers and the ATP-synthesis experiment in nine healthy volunteers. Liver 31P spectra were modelled and quantified successfully using a time domain fit and the AMARES (advanced method for accurate, robust and efficient spectral fitting of MRS data with use of prior knowledge) algorithm describing the essential components of the dataset. The measured T1 relaxation times are comparable to values reported previously at lower field strengths. All nine subjects in whom saturation transfer was measured had low hepatocellular lipid content (1.5 +/- 0.2% MR signal; mean +/- SEM). The exchange rate constant (k) obtained was 0.30 +/- 0.02 s(-1), and the rate of ATP synthesis was 29.5 +/- 1.8 mM/min. The measured rate of ATP synthesis is about three times higher than in human skeletal muscle and human visual cortex, but only about half of that measured in perfused rat liver. In conclusion, 31P MRS at 3 T provides sufficient sensitivity to detect magnetization transfer effects and can therefore be used to assess ATP synthesis in human liver.

Solid state and solution properties of lanthanide(III) complexes of a tetraiminodiphenolate macrocyclic ligand. X-ray structure, 1H NMR and luminescence spectral studies

The lanthanide(III) complexes of composition [Ln(LH 2)(H 2O) 3Cl]Cl 2 (Ln = La–Lu and Y, 1– 15) derived from the tetraiminodiphenolate macrocyclic ligand L 2− have been prepared and characterized. In these compounds, the two uncoordinated imine nitrogens of the macrocycle are protonated and hydrogen-bonded with the metal-bound phenolate oxygens and thereby provide a zwitterionic structure to the ligand. The X-ray crystal structure of the compounds of La and Nd have been determined and they are found to be isostructural. The coordination polyhedra for the eight-coordinated metal centre in the complex cation [Ln(N 2O 2)(O 3Cl)] 2+ can be described as distorted square antiprism. Intermolecular hydrogen-bondings involving the three coordinated water molecules and the two uncoordinated chloride ions give rise to the 2-D network in which the chlorides are triply hydrogen-bridged and the water molecules are doubly hydrogen-bridged. Moreover, the aromatic rings in this network are involved in π–π interaction in two different ways. 1H NMR spectra of the complexes in (CD 3) 2SO have been studied. The spectral assignments for the paramagnetic complexes of Ce–Eu have been made from { 1H– 1H} COSY spectra and longitudinal relaxation time ( T 1) measurements. It is inferred that the complex species [Ln(LH 2){(CD 3) 2SO} 4] 3+ that exist in solution are isostructural for the compounds of La–Eu. The contact and pseudo-contact contribution to the isotropic paramagnetic shifts in the complexes of Ce–Eu have been estimated. The luminescence spectra of the complexes of La, Sm, Eu and Tb have been studied in methanol–ethanol (1:4) glassy matrix and in the solid state at 77 K, and the quantum yields have been estimated.

Processing and nanodispersion: A quantitative approach for polylactide nanocomposite

In this work, the nanodispersion of organomodified montmorillonite (MMT) in polylactide (PLA), prepared by varying the processing conditions, is investigated by solid-state nuclear magnetic resonance (NMR). The method is based on T 1 H (proton longitudinal relaxation time) measurement as a quantitative indicator of the dispersion. It permits extraction of two parameters: (i) f is the fraction of the potentially available clay surface, which has been transformed into polymer/clay interfaces and (ii) ε is a relative measure of the homogeneity of the dispersion of these actual polymer/clay interfaces. The NMR approach is consistent with transmission electron microscopy (TEM) results but, as a bonus, NMR allows providing of a quantitative view of the whole sample, while TEM is mainly qualitative and looks only at a tiny volume of the material. This critical advantage is used, for the first time, to quantitatively investigate the influence of the processing conditions on the nanomorphology of PLA/MMT nanocomposite. It is revealed that high mechanical energy for a short time (∼1 min) followed by low shear stress for a reasonable residence time (∼5 min) results in the delamination of the clay platelets (high f value) associated with a very high degree of homogeneity ( ε=97%). Longer times and higher shear lead to the reagglomeration of the platelets.

Encoding of diffusion and T1 in the CPMG echo shape: single-shot D and T1 measurements in grossly inhomogeneous fields

Abstract We present a new approach of NMR measurements in the presence of grossly inhomogeneous fields where information is encoded in the echo shape of CPMG trains. The method is based on sequences that consist of an initial encoding sequence that generates echoes with contributions from at least two different coherence pathways that are then both refocused many times by a long string of closely spaced identical pulses. The generated echoes quickly assume an asymptotic shape that encodes the information of interest. High signal-to-noise ratios can be achieved by averaging the large number of echoes. We demonstrate this approach with different implementations of the measurements of longitudinal relaxation time, T 1 , and diffusion coefficient, D . It is shown that the method can be used for novel single-shot measurements.

Encoding of diffusion and T1 in the CPMG echo shape: Single-shot D and T1 measurements in grossly inhomogeneous fields

We present a new approach of NMR measurements in the presence of grossly inhomogeneous fields where information is encoded in the echo shape of CPMG trains. The method is based on sequences that consist of an initial encoding sequence that generates echoes with contributions from at least two different coherence pathways that are then both refocused many times by a long string of closely spaced identical pulses. The generated echoes quickly assume an asymptotic shape that encodes the information of interest. High signal-to-noise ratios can be achieved by averaging the large number of echoes. We demonstrate this approach with different implementations of the measurements of longitudinal relaxation time, T 1, and diffusion coefficient, D. It is shown that the method can be used for novel single-shot measurements.

The influence of nitrogen-15 proton-driven spin diffusion on the measurement of nitrogen-15 longitudinal relaxation times

The effect of nitrogen-15 proton-driven spin diffusion on quantitative 15N T 1 measurements in solid proteins is investigated, and the impact on the measurement of dynamic parameters is assessed. A simple model of exchange between neighboring nitrogens is used to reproduce the evolution of 15N spin systems whose longitudinal relaxation rates and exchange rates are compatible with experimental measurements. We show that the induced error in the measured T 1 and its effect on the determination of dynamics parameters is likely to be less than the current experimental error. The use of deuterated protein samples is shown to have a small but sometimes visible effect, and may also considerably slow down or even suppress the exchange of magnetization due to spin diffusion.

Low- and high-resolution nuclear magnetic resonance (NMR) characterisation of hyaluronan-based native and sulfated hydrogels

Hyaluronan-based hydrogels were synthesised using different crosslinking agents, such as 1,3-diaminopropane (1,3-DAP) and 1,6-diaminohexane (1,6-DAE). The hydrogels were sulfated to provide materials (Hyal-1,3-DAP, Hyal-1,6-DAE, HyalS-1,3-DAP and HyalS-1,6-DAE) that were characterised by both high- and low-resolution nuclear magnetic resonance (NMR) spectroscopy. The 13C NMR spectra of the materials were analysed to identify, characterise and study the crosslinking degree of the hydrogels. The crosslinking degree was also determined by potentiometric titration and the effectiveness of the two techniques was compared. Measurements of longitudinal relaxation times (spin–lattice) and of NOE enhancement were used to study the mobility of the hydrogels. Low-resolution NMR studies allowed the determination of the water transport properties in the hydrogels. In addition, the swelling degree for the various hydrogels was calculated as a function of the longitudinal and transversal relaxation times of the water molecules. Lastly, the self-diffusion coefficients of the water in interaction with the four polysaccharides were measured by the pulsed field gradient spin echo (PFGSE) sequence.

Fast and simultaneous measurement of longitudinal and transverse NMR relaxation times in a single continuous wave free precession experiment

The purpose of this communication is to describe a method for rapid and simultaneous determination of longitudinal ( T 1) and transversel ( T 2) relaxation times, based on a single continuous wave free precession (CWFP) experiment which employs RF pulses with a π/2 flip angle. We analyze several examples, involving nuclei such as 1H, 31P, and 19F, where good agreement with T 1 and T 2 measurements obtained by traditional methods is apparent. We also compare with the more time-consuming steady-state free precession (SSFP) method of Kronenbitter and Schwenk where several experiments are needed to determine the optimum flip angle. The role of an inhomogeneous magnetic field on the observed decays and its effect upon the accuracy of relaxation times obtained by these methods is examined by comparing numerical simulations with experimental data. Possible sources of error and conditions to minimize its effects are described.

Intramolecular hydrogen bonding in transition metal clusters: NMR evidence for an Os–H⋯H–S interaction in H(μ-H)Os 3(CO) 10(HSR) (R=ethyl, cyclopentyl)

The reaction of H 2Os 3(CO) 10 with RSH (R=CH 2CH 3, cyclopentylthiol) at low temperatures in CD 2Cl 2 affords three H(μ-H)Os 3(CO) 10(RSH) adducts, where, by the 13C NMR carbonyl spectrum, it can be shown that the thiol ligand occupies an axial position. The presence of an intermolecular MH⋯HS interaction in the main isomer has been determined by longitudinal relaxation time measurements. The isolation of the H,H dipolar contribution to the terminal hydride relaxation affords a value of 1.97 Å for the ‘unconventional’ hydrogen–hydrogen interaction in H(μ-H)Os 3(CO) 10(RSH) (R=cyclopentylthiol). An intramolecular proton transfer from the thiol ligand RSH to the terminal hydride, via a dihydrogen bonded species, ultimately losing an H 2 molecule. leading to the formation of the stable product (μ-H)Os 3(CO) 10(μ-SR) is proposed.

Relaxation mechanism in several kinds of polyethylene estimated by dynamic mechanical measurements, positron annihilation, X-ray and 13C solid-state NMR

Relaxation processes of several kinds of polyethylene films and fibers with different molecular orientational degrees and crystallinities were extensively investigated by the dynamic mechanical relaxation, positron annihilation and 13C nuclear magnetic relaxation ( 13C NMR). From complex dynamic tensile modulus, the activation energies of α 1 and α 2 relaxations were determined to be 97–118 and 141–176 kJ/mol, respectively. The activation energy of β relaxation was 114–115 kJ/mol. These values were similar to those of α 1 relaxation reported already. For γ relaxation mechanisms, there existed two mechanisms, γ 1 and γ 2, the activation energies being 9–11 and 23–25 kJ/mol, respectively. The values were independent of the molecular orientation and crystallinity. The two local motions indicate that non-crystalline phase composes of two regions of non-crystalline phase, rubber-like amorphous phase and interfacial-like amorphous phase. From 13C NMR measurements of 13C longitudinal relaxation time for the non-crystalline phase, the activation energy was 20.7 kJ/mol. This value is close to the activation energy (23–25 kJ/mol) of the γ 2 relaxation estimated by the dynamic mechanical measurement. The result by 13C NMR did not provide two kinds of activation energy, indicating combined influence of the two correlation times. Even so, the activation energies obtained by 13C NMR indicated that the γ 2 relaxation mainly is due to the motion of the C–C central bond of a short segment (e.g. three to four CH 2) within interfacial-like amorphous phase. The γ and β relaxation peaks by the dynamic mechanical measurements corresponded to the first and second lifetime transition of ortho-positronium indicating, in turn, a drastic change in free volume by local mode relaxation.

A 13C NMR Study of the Products and Mechanism of the Thermal Oxidative Degradation of Poly(ethylene oxide)

AbstractThe products and mechanism of the thermal oxidative degradation of poly(ethylene oxide) at 150 °C have been analysed using 13C NMR spectroscopy. The analysis was assisted by the use of distortionless enhancement by polarisation transfer spectra, longitudinal relaxation time measurements, long‐range 13C1H coupling and chemical shift simulation software. The major result of degradation was chain scission resulting in two formate ester chain ends. Minor products were in‐chain esters, peroxy groups, oxymethylene links and hydroxy and methoxy chain ends. Initial steps for the mechanism of chain scission.magnified imageInitial steps for the mechanism of chain scission.

19F-MRI of perflubron for measurement of oxygen partial pressure in porcine lungs during partial liquid ventilation.

A method for in vivo measurement of oxygen partial pressure (pO2) in porcine lungs during partial liquid ventilation (PLV) with perflubron (PFOB) was developed. A pulse sequence for high-resolution MRI of the distribution of PFOB in the lung after intratracheal administration was developed as well. Moreover, quantitative measurements of longitudinal relaxation time T(1) of (19)F resonances for assessment of regional pO2 are described. Due to the need to acquire data during a single expiratory breathhold, only low SNRs were achieved in vivo. Therefore, simulations were performed to investigate the influence of background noise on T(1) values calculated from data with low SNR. Based on these simulations, a postprocessing strategy was developed to correct for systematic errors by background noise prior to quantitative analysis. Results of a pilot study in pigs under conditions of PLV are presented.

Measurement of longitudinal relaxation times in crowded 1H NMR spectra using one- and two-dimensional maximum quantum (MAXY) NMR spectroscopy

Methods for measuring longitudinal relaxation times of protons in heavily overlapped 1H NMR spectra are introduced and exemplified using a solution of cholesteryl acetate. The methods are based on 1-dimensional and 2-dimensional maximum quantum NMR spectroscopy (MAXY), which makes possible the selective detection of CH, CH2 and CH3 1H NMR resonances. A modification of the BIRD pulse sequence to achieve selective inversion of protons bonded to either 12C or 13C is given. The approach should find application in studies of molecular dynamics where isotopic enrichment is not possible and the level of available sample dictates the use of 1H NMR spectroscopy.

Intermolecular dipole-dipole relaxation of (129)Xe dissolved in water.

Intermolecular (129)Xe-(1)H nuclear Overhauser effects and (129)Xe longitudinal relaxation time measurements were used to demonstrate that the dipole-dipole coupling is the dominant relaxation mechanism for (129)Xe in water, at room temperature. (129)Xe-(1)H cross-relaxation rates were derived to be sigma(XeH) approximately 3.2 +/- 0.3 x 10(-3) s(-1), independent of xenon pressure (in the range of 1-10 bar) and of the presence of oxygen. Corresponding xenon-proton internuclear distances were calculated to be 2.69 +/- 0.12 A. Using the magnitude of the dipole-dipole coupling and the spin density ratio between dissolved xenon and bulk water, it is estimated that (129)Xe-(1)H spin polarization-induced nuclear Overhauser effects would yield little net proton signal enhancement in water.

Determination of Binding Constants for Cyclodextrin Complexes with Alkanols by the 1H NMR Measurements of Longitudinal Relaxation Time Using Tetramethylammonium Chloride as an Internal Reference

The longitudinal relaxation times (T1)of 1H NMR signals for a variety of alkanols in D2Omarkedly decreased with increasing concentrations (c)of α- and β-cyclodextrins (CD). Tetramethylammoniumchloride (TMA) used as an internal reference wasavailable for evaluating an effect of solutionviscosity on relaxation rates R(=1/T1), since TMAshowed no appreciable interaction with CD. Changes inthe ratio of R for alkanol protons to R for TMAprotons with c were analyzed by the curve-fittingmethod to give Ka. These Ka values agreedwell with those obtained by the analysis of changesin δ, indicating that T1 measurement is available forthe determination of Ka for CD complexes. 2D ROESYspectra provided definite information on the molecularstructures of CD complexes with alkanols.