Proton Spin-lattice Relaxation Rate Research Articles

Methyl reorientation in solid 3-ethylchrysene and 3-isopropylchrysene

We have measured the proton spin-lattice relaxation rate as a function of temperature in polycrystalline 3-ethylchrysene at nuclear magnetic resonance Larmor frequencies of 53.0 and 22.5 MHz and in polycrystalline 3-isopropylchrysene at 53.0, 22.5 and 8.50 MHz. The syntheses of these new compounds are presented. The relatively large chrysene backbone creates an ideal and unique environment for the alkyl groups such that methyl group rotation is the only motion on the nuclear magnetic resonance Larmor frequency timescale over a large temperature range. The relaxation rate data are interpreted in terms of the simplest possible dynamical model; that of random hopping for the methyl group(s), all of which are equivalent in the solid state. The barriers of 11–12 kJ mol −1 are typical for methyl groups in `isolated' ethyl and isopropyl groups.

A low-field paramagnetic nuclear spin relaxation theory

A low-field theory of paramagnetically enhanced proton spin-lattice relaxation is developed resulting in closed analytical expressions. The theory describes paramagnetically enhanced water proton spin-lattice relaxation in complexes of transition metal ions with electron spin quantum number S=1. In the low-field regime the electron spin system is dominated by a static or permanent zero-field splitting interaction which is much larger than the Zeeman interaction. The electron spin is thus quantized in the molecular fixed principal frame of the static ZFS-interaction rather than in the laboratory fixed frame. In the molecular fixed frame the zero-field splitting Hamiltonian is characterized by an axial parameter (D) and a rhombic parameter (E). It is shown how the relative magnitude of D and E strongly influence the magnitude of the paramagnetically enhanced proton spin lattice relaxation rate. In describing electron spin relaxation in the molecular fixed frame, we consider a transient ZFS-interaction and a reorientation modulated Zeeman interaction as the two main relaxation mechanisms. Then, using Redfield theory we derive expressions for the relevant electron spin relaxation rates in terms of spectral densities. In contrast to the Zeeman or high field regime, the electron spin system in the low field limit does not have a permanent magnetic moment. The lack of this magnetic moment implies that electron spin-lattice relaxation processes do not influence the paramagnetically enhanced proton spin-lattice relaxation rates. Instead three spin–spin relaxation rates determine the enhanced proton spin-lattice relaxation profile. We express the electron spin–spin relaxation rates in terms of spectral densities determined by the mean square value of the transient zero-field splitting interaction, its characteristic correlation time, and the reorientational correlation time of rank 1.

Estimation of the spin-lattice relaxation time constants of low molecular weight solutes in dilute complex aqueous solutions: application to urinary metabolites

Proton spin–lattice relaxation rate constants (T1 values) of some selected urinary metabolites were analysed extensively by two different least-squares minimization procedures, based on the simplex and the Marquardt–Levenberg algorithms. Comparative multiplicate analysis under different experimental conditions using laboratory solutions of some of the urinary metabolites was employed in addition to direct measurements on human urine samples. The T1 values estimated for most of the metabolites by the two methods agreed to within 10–15%. © 1998 John Wiley & Sons, Ltd.

NMR Studies of hydrogen diffusion in ZrBe 2H 1.4

The diffusivity of hydrogen in the metal-hydride ZrBe 2H 1 . 4 has been studied by different nuclear magnetic resonance (NMR) techniques. The proton spin-lattice relaxation rate Γ 1 has been measured at 27.7 MHz and 67.7 MHz in the temperature range from 175 K to 500 K. At four distinct temperatures the frequency dependence of Γ 1 has been investigated between 0.31 MHz and 29.2 MHz using the field-cycling (FC) NMR technique. These studies indicate that the hydrogen diffusion is restricted to two dimensions within each particle of the powdered sample. The structure of ZrBe 2H 1 . 4 suggests that the hydrogen diffusion takes place in the xy planes formed by the Zr atoms. The variation of the echo amplitude with the applied field gradient in pulsed-field-gradient (PFG) spin-echo experiments confirmed the two-dimensional character of the hydrogen diffusion. The PFG data taken between 272 K and 478 K yielded directly the diffusivities in the xy plane D xy. The temperature dependence of D xy is well represented by an Arrhenius law with an activation enthalpy H a=(0.25±0.02) eV. This activation enthalpy is in very good agreement with values deduced from both the frequency and the temperature dependence of Γ 1.

Synthesis and complexation of Gd3+, Ca2+, Cu2+ and Zn2+ by 3,6,10-tri(carboxymethyl)-3,6,10-triazadodecanedioic acid

3,6,10-Tri(carboxymethyl)-3,6,10-triazadodecanedioic acid (H5L), was synthesized and its protonation constants were determined by potentiometric titration in 0.10 mol dm–3 Me4NNO3 and by NMR pH titration at 25.0 ± 0.1 °C. Stability and selectivity constants have been measured to evaluate the possibility of using the corresponding gadolinium(III) complex as a magnetic resonance imaging contrast agent. The formations of gadolinium(III), copper(II), zinc(II) and calcium(II) complexes were investigated quantitatively by potentiometry. The stability constant for the gadolinium(III) complex is larger than those of CaII, ZnII and CuII for this octadentate ligand. The selectivity constants and modified selectivity constants of the ligand for Gd3+ over endogenously available metal ions were calculated. The spin–lattice relaxivity R1 for the gadolinium(III) complex was also determined. It was found to decrease with increasing pH below 4 and became invariant with respect to pH over the range 4–10. Oxygen-17 NMR shifts showed that the [DyL]2– complex had one inner-sphere water molecule. The water proton spin–lattice relaxation rate for the [GdL]2– complex was also consistent with one inner-sphere co-ordination position.

Impact of chemical stimuli and temperature on water transport and mobility in germinating rape seeds by pulsed 1H NMR spectroscopy

Pulsed 1H NMR in vivo difference spectroscopy and germination were combined to elucidate the effect of exogenously applied estriol or ethanol on water retention in fully hydrated rape seeds. The presence of estriol or ethanol in the imbibition medium resulted in a reduction of the photoreversible phytochrome. An approach to the assessment of cell membrane permeability related to the intracellular water exchange rate, using the bi-exponential feature in the water proton spin–lattice relaxation rate R1 and spin–spin relaxation rate R2, is presented. The intracellular water exchange rate increases approximately two- and three-fold in seeds pre-treated with estriol or ethanol solution in comparison with water imbibed seeds. The activation energy for the germination rate was 61±6, 42±4 and 37±4 kJ mol-1 and the transition temperature was 281±2, 286±2 and 284±2 K for seeds imbibed with water, estriol and ethanol solution, respectively. The temperature dependence of the intracellular water exchange rate showed comparable values of activation energy and transition temperatures. All the results support the hypothesis that the two effects, thermal activation and increase in the seed cell membrane permeability, combine during water transport into the cell and result in the germination of rape seeds imbibed with estriol or ethanol solution. © 1997 John Wiley & Sons, Ltd.

Impact of chemical stimuli and temperature on water transport and mobility in germinating rape seeds by pulsed1H NMR spectroscopy

Pulsed 1H NMR in vivo difference spectroscopy and germination were combined to elucidate the effect of exogenously applied estriol or ethanol on water retention in fully hydrated rape seeds. The presence of estriol or ethanol in the imbibition medium resulted in a reduction of the photoreversible phytochrome. An approach to the assessment of cell membrane permeability related to the intracellular water exchange rate, using the bi-exponential feature in the water proton spin–lattice relaxation rate R1 and spin–spin relaxation rate R2, is presented. The intracellular water exchange rate increases approximately two- and three-fold in seeds pre-treated with estriol or ethanol solution in comparison with water imbibed seeds. The activation energy for the germination rate was 61±6, 42±4 and 37±4 kJ mol-1 and the transition temperature was 281±2, 286±2 and 284±2 K for seeds imbibed with water, estriol and ethanol solution, respectively. The temperature dependence of the intracellular water exchange rate showed comparable values of activation energy and transition temperatures. All the results support the hypothesis that the two effects, thermal activation and increase in the seed cell membrane permeability, combine during water transport into the cell and result in the germination of rape seeds imbibed with estriol or ethanol solution. © 1997 John Wiley & Sons, Ltd.

Proton Spin-lattice Relaxation in Amorphous States of Organic Molecular Solids

Abstract We have measured the temperature dependence of the proton spin-lattice relaxation rate R at 8.50 and 22.5 MHz in solid 1,3,5-tri-ethyl-benzene and solid 1,2,4-tri-ethyl-benzene. Analysis of the data strongly suggests that we are studying amorphous states in these slowly solidified organic solids (that are liquids at room temperature). The ethyl groups are static on the Larmor frequency time-scale. There are no simple-model interpretations of the data, but a reasonable model for the dominantly-occurring amorphous state data observed with 1,3,5-tri-ethyl-benzene suggests that two of the three methyl groups are reorienting and the third is static on the proton Larmor frequency time scale. The same approach for the two amorphous states observed in 1,2,4-tri-ethyl-benzene suggests that all three methyl groups are reorienting in one state and that three of the six methyl groups in each pair of molecules are turned off in a second state. We discuss that, whereas specific dynamical statements are model dependent, the proton spin relaxation technique does make some general qualitative statements about the mesostructure of the solid.

Investigation by NMR Spectroscopy and Molecular Modeling of the Conformations of Some Modified Disaccharide Antigens for Shigella Dysenteriae Type 1

The O-polysaccharide of Shigella dysenteriae type 1 is made up of multiple repeats of the linear tetrasaccharide 3)-α-L-Rhap-(1→2)-α-D-Galp-(1→3)-α-D-GlcpNAc-(1→3)-α-L-Rhap-(1→, for which the antigenic determinant for a murine monoclonal IgM antibody is the disaccharide α-L-Rhap-(1→2)-α-D-Galp. This disaccharide and various analogs have been studied by 2D NOESY, ROESY, and TOCSY NMR spectroscopy, in conjunction with proton spin-lattice relaxation rate measurements, restrained molecular mechanics, and restrained molecular dynamics with simulated annealing. It has been found that replacement of any single hydroxyl group in the determinant by a hydrogen atom, or replacement of any single hydroxyl group in the Gal residue by a fluorine atom has little if any influence on the conformation of the resulting derivatives. cPresent address: Institut Pasteur, Paris, France

Hydrogen and deuterium diffusion in titanium dihydrides/dideuterides

The macroscopic diffusivities D of hydrogen in titanium dihydrides. Ti1H1 (1.65≤x≤2.02), and of deuterium in titanium dideuterides. Ti2H1 (x=1.74, 1.98), have been measured by means of pulsed-field-gradient (PFG) nuclear magnetic resonance over wide temperature ranges. The effective activation enthalpy for hydrogen diffusion Ha1H, obtained by fitting an Arrhenius expression to the diffusivities, increases with increasing hydrogen concentration from Ha1H = 0.55 eV (x = 1.65) to H41H = 0.92 eV (x = 2.02). The proton spin-lattice relaxation rate Γ1, measured on the same samples, reveals also a rapid increase in Ha1H for x →2. The effective activation enthalpy in the deuterides, Ha2H = 0.60 eV (x = 1.74) and Ha2H = 0.67 eV (x= 1.98), depends only weakly on x. A comparison of PFG and Γ1 data indicates that hydrogen atoms jump predominantly between adjacent tetrahedral (T) sites. The observed increase in the effective activation enthalpy with increasing x stands in contradiction to published Ha1H values based on Γ1 or NMR line width data. The literature values Ha1H ≈0.50 eV, which reflect primarily measurements at lower temperature and lower x-values, are nearly independent of the hydrogen concentration x. The present studies show, however, that the concentration dependence of the diffusion parameters of hydrogen in TiH1 is similar to that in ZrH1, where H4 increases sharply as x approaches the stoichiometric limit. If the sublattice formed by the T-sites is almost filled, additional diffusion processes besides the jumps from occupied to empty T-sites must contribute significantly to the hydrogen diffusivity. Several models for such an additional diffusion mechanism are discussed.

Nuclear magnetic resonance studies of hydrogen diffusion in the layer-structured system ZrClH x

The proton spin-lattice relaxation rate Γ 1 in ZrClH 0.5 shows one maximum and two shoulders, which indicates three distinct modes of hydrogen motion. The frequency dependence of the spin-lattice relaxation rate in the rotating frame, Γ 1,ρ, shows clearly that hydrogen diffusion is confined to two dimensions. Direct measurements of the long-range diffusivity D xy were performed by pulsed field gradient (PFG) spin-echo NMR. These PFG experiments confirmed the two-dimensional character of hydrogen diffusion. The temperature dependence of D xy yields an activation enthalpy, H a=0.49 eV, which is in good agreement with the value H a=0.47 eV deduced from the frequency-dependent Γ 1,ρ data. The different processes of hydrogen motion are discussed on an atomistic scale. The Γ 1 and Γ 1,ρ curves measured in ZrClH 1.0 are consistent with a single mechanism of hydrogen diffusion. Both the relaxation data and the PFG data reveal a strongly reduced hydrogen mobility in ZrClH 1.0 compared to ZrClH 0.5. From the analysis of the echo amplitude in the PFG experiments it is concluded that the hydrogen diffusion is restricted to two dimensions as well. The D xy values show Arrhenius behavior with an activation enthalpy of H a=0.58 eV.

Alkaline Earth Metal and Lanthanide(III) Complexes of Ligands Based upon 1,4,7,10-Tetraazacyclododecane-1,7-bis(acetic acid).

The macrocyclic ligand DO2A (1,4,7,10-tetraazacyclododecane-1,7-bis(acetic acid)) was prepared and used as a building block for four new macrocyclic ligands having mixed side-chain chelating groups. These ligands and their complexes with Mg(II), Ca(II), and Ln(III) were studied extensively by potentiometry, high-resolution NMR, and water proton relaxivity measurements. The protonation constants of all compounds compared well with those of other cyclen-based macrocyclic ligands. All Ca(II) complexes were found to be more stable than the corresponding Mg(II) complexes. Trends for the stabilities of the Ln(III) complexes are discussed and compared with literature data, incorporating the effects of water coordination numbers, Ln(III) contraction, and the nature of the side chains and the steric hindrance between them. (1)H NMR titrations of DO2A revealed that the first and second protonations take place preferentially at the secondary ring nitrogens, while the third and fourth involved protonation of the acetates. (17)O NMR shifts showed that the DyDO2A(+) complex had two inner-sphere water molecules. Water proton spin-lattice relaxation rates for the GdDO2A(+) complex were also consistent with water exchange between bulk water and two inner-sphere Gd(III) coordination positions. Upon formation of the diamagnetic complexes of DO2A (Ca(II), Mg(II), La(III), and Lu(III)), all of the macrocyclic ring protons became nonequivalent due to slow conformational rearrangements, while the signals for the acetate CH(2) protons remained a singlet.

Hydration kinetics of modified calcium sulphoaluminate oxides studied by aluminum — 27 NMR spectroscopy and proton magnetic relaxation: Experimental data

Hydration kinetics of Na 2O-, MgO-, BaO- and TiO 2- doped calcium sulphoaluminate oxides, has been followed using 27Al NMR and proton magnetic relaxation measurements. Aluminum — 27 NMR spectra of hydrated pastes show a narrow peak at 80 ppm corresponding to the hydroxy-aluminate ions in a liquid phase. According to the changes of intensity of this lines during the induction period considerable amounts of aluminate were dissolved, especially in the case of Na 2O-doped sulphoaluminate. With the start of crystallization of hydrated phase (marked by the appearance of a line at 0 ppm in the proton decoupled 27Al spectra) the concentration of aluminate ions in the liquid phase decreases. The proton spin-lattice relaxation rates follow a usual pattern, the crystallization being indicated by a sharp increase in the relaxation rate. A significant increase of the proton relaxation rate was also observed during the induction period, when no new solid was yet formed. This relaxation enhancement might be due to the increased viscosity and gel structuring.

Field-Dependent Proton NMR Relaxation in Aqueous Solutions of Ni(II) Ions. A New Interpretation

A new model is presented for nuclear-spin relaxation in paramagnetic transition metal complexes in solution, allowing the electron-spin relaxation to be outside the Redfield limit. The novel feature is that the transient zero-field splitting (ZFS), modulated by distortions of the solvation shell, is allowed to be of rhombic rather than cylindrical symmetry. The model, which assumes that the static ZFS is absent, is applicable to aqueous solutions of transition metal ions. The magnetic-field dependence of the proton spin–lattice relaxation rate enhancement in aqueous solution has been investigated, and calculations are presented for anS= 1 system such as Ni2+(aq), using different degrees of rhombicity of the ZFS and different motional conditions (Redfield limit and slow-motion regime). The new model is also applied to fit the previously reported data for the field dependence of proton relaxation in aqueous solution of Ni(ClO4)2at low pH [J. Kowalewski, T. Larsson, and P.-O. Westlund,J. Magn. Reson.74, 56 (1987)]. The inclusion of rhombicity, motivated by recent theoretical work, provides a model which performs as well as the earlierad hocmodel.

Proton tunneling and local symmetry of the hydrogen bond in Rb 3H(SO 4) 2

The 87Rb NMR spectrum of polycrystalline Rb 3H(SO 4) 2 indicated that this substance does not possess the center of inversion at the midpoint of the dimeric unit (SO 4)⋯H⋯(SO 4) at and below room temperature in contrast to the result of an X-ray diffraction experiment. The proton spin-lattice relaxation rate showed a hump near 30 K. This phenomenon was attributed to the phonon-assisted tunneling of proton in the asymmetric hydrogen bond in the dimeric unit (SO 4)⋯H⋯(SO 4).

Water molecule binding and lifetimes on the DNA duplex d(CGCGAATTCGCG)2.

Measurements of the water proton spin-lattice relaxation rate for aqueous solutions of the palindromic dodecamer, d(CGCGAATTCGCG)2, are reported as a function of the magnetic field strength. The magnitude of the relaxation rates at low magnetic field strengths and the shape of the relaxation dispersion curve permit assessment of the number of water molecules which may be considered bound to the DNA for a time equal to or longer than the rotational correlation time of the duplex. The data are examined using limiting models that arbitrarily use the measured rotational correlation time of the polynucleotide complex as a reference point for the water molecule lifetime. If it is assumed that water molecules are bound at DNA sites for times as long as or longer than the rotational correlation time of the duplex, then the magnitude of the relaxation rates at low field require that there may be only two or three such water sites. However, if the lifetime constraint is relaxed, and we assume that the number of water molecules bound to the DNA is more nearly the number identified in the X-ray structures, then the average water molecule lifetime is on the order of 1 ns. Measurements of 1H NOESY spectra demonstrate that some water molecules must have lifetimes sufficiently long that negative Overhauser effects are observed. Taken together, these results suggest a distribution of water molecule lifetimes in which most of the DNA-bound water molecule lifetimes are shorter than the rotational correlation time of the duplex, but where some have lifetimes of at least 1 ns under these concentrated conditions.

Noninvasive measurement of protein concentration

At selected magnetic field strengths, protein and water proton spin-lattice relaxation rates are sensitive to the concentration of rotationally immobilized peptide nitrogen because of field dependent heteronuclear cross relaxation coupling between protein proton and nitrogen-14 spins that is carried to the water by proton homonuclear cross-relaxation. Measurement of the water proton spin-lattice relaxation time, or a signal amplitude proportional to it, may provide a noninvasive measure of peptide bond concentration, which provides a direct measure of immobilized protein content in most tissues. The approach using protein gels in two magnetic field strengths is demonstrated. At 66.7 mT; the proton Zeeman energy matches one of the peptide nitrogen transitions dominated by the unaveraged nuclear electric quadrupole interaction; cross-relaxation between the protons and nitrogen-14 is efficient. At 77.5 mT, the proton Zeeman energy is not matched with the nitrogen energy and proton-nitrogen cross-relaxation is not efficient. It is shown that the difference in the water proton spin-lattice relaxation rates on and off the energy level match condition is a linear function of the rotationally immobilized protein concentration.

Translational Diffusion of Liquids Confined in Nanopores

ABSTRACTWe present a theory of nuclear spin relaxation and experiments to probe the microdynamics of polar solvents at the surface of calibrated microporous glass beads. The theory is tested using the magnetic field dependence of the proton spin-lattice relaxation rate recorded with a field-switched magnetic relaxation dispersion spectrometer.

Hydrogen in the A15 compounds Nb 3Sn and Nb 3Al: a nuclear magnetic resonance study

Nuclear magnetic resonance measurements of the 119Sn, 27Al Knight shifts and the 119Sn, 27Al and 1H spin-lattice relaxation rates in the A15-type compounds Nb 3SnH x (D x ) ( x = 0, 0.7 and 1.2) and Nb 3AlH x ( x = 0, 2.3) have been performed over the temperature range 10–460 K. The low temperature 119Sn Knight shift and spin-lattice relaxation data indicate that indicate that in Nb 3SnH x (D x ) the density of electron states at the Fermi level decreases strongly with increasing x. The hydrogen mobility in Nb 3Al is found to be much higher than that in Nb 3Sn. The behaviour of the proton spin-lattice relaxation rate for NB 3AlH 2.3 in a wide range of resonance frequencies (9–90 MHz) can be described by the model using a double-peak distribution of activation energies.

Pulsed-field-gradient NMR study of hydrogen diffusion in the hydrogen-stabilized Laves phase C15ZrTi 2H 3.7

Hydrogen diffusion in C15ZrTi 2H 3.7 has been measured in the temperature range 230 K to 450 K by means of pulsed-field-gradient nuclear magnetic resonance. The temperature dependence of the diffusivity shows an Arrhenius behavior, D = D 0· exp(− H a k BT ) , with an activation enthalpy H a = 0.22 eV and a pre-exponential factor D 0 = 1.8 · 10 −8 m 2 s −1. These results are in good agreement with the values derived from the proton spin-lattice relaxation rate Γ 1 measured on the same sample. Comparison of the results on D and Γ 1 yields additional information on the mechanisms of hydrogen diffusion. This permits distinction between long-range diffusion and localized motion.