Nuclear Magnetic Resonance Relaxation Rates Research Articles

Early Indications for Manganese Oxidation State Changes During Photosynthetic Oxygen Production: A Personal Account

One of the major questions yet to be answered in photosynthesis research today is what is the chemical mechanism for the oxidation of water into molecular oxygen. It is well established that an inorganic cluster of four manganese ions and at least one calcium ion form the catalytic core. As the oxidation potential generated by the Photosystem II reaction center is accumulated over the four sequential steps needed to produce O(2), changes in the oxidation state of the catalytic manganese occur, though the formal oxidation states that are involved are still a matter of considerable debate. Much of what is currently known has come from direct measurements of the catalytic manganese using electron paramagnetic resonance (EPR) and X-ray spectroscopy. However, in the early attempts to attack this problem, the catalytic manganese was monitored indirectly by its paramagnetic effect on the nuclear magnetic resonance (NMR) relaxation rates of solvent water protons. In this contribution, a description of the proton relaxation rate phenomenon and its use to indicate manganese oxidation state changes during O(2) production is presented.

The N-terminal domain (IF2N) of bacterial translation initiation factor IF2 is connected to the conserved C-terminal domains by a flexible linker.

Bacterial translation initiation factor IF2 is a multidomain protein that is an essential component of a system for ensuring that protein synthesis begins at the correct codon within a messenger RNA. Full-length IF2 from Escherichia coli and seven fragments of the protein were expressed, purified, and characterized using nuclear magnetic resonance (NMR) and circular dichroism (CD) methods. Interestingly, resonances of the 6 kD IF2N domain located at the extreme N terminus of IF2 can be clearly identified within the NMR spectra of the full-length 97-kD protein. (15)N NMR relaxation rate data indicate that (1) the IF2N domain is internally well ordered and tumbles in solution in a manner that is independent of the other domains of the IF2 protein, and (2) the IF2N domain is connected to the C-terminal regions of IF2 by a flexible linker. Chemical shifts of resonances within the isolated IF2N domain do not significantly differ from those of the corresponding residues within the context of the full-length 97-kD protein, indicating that IF2N is a structurally independent unit that does not strongly interact with other regions of IF2. CD and NMR data together provide evidence that Domains I-III of IF2 have unstructured and flexible regions as well as substantial helical content; CD data indicate that the helical content of these regions decreases significantly at temperatures above 35 degrees C. The features of structurally well-ordered N- and C-terminal domains connected by a flexible linker with significant helical content are reminiscent of another translation initiation factor, IF3.

99,101RuNMR study ofEuSr2RuCu2O8:Coexistence of ferromagnetism and superconductivity

The copper-ruthenium oxide ${\mathrm{EuSr}}_{2}{\mathrm{RuCu}}_{2}{\mathrm{O}}_{8}$ (EuRu1212) has been studied by means of nuclear-magnetic resonance (NMR) and nuclear spin-lattice relaxation rate ${(T}_{1}^{\ensuremath{-}1})$ experiments together with measurements of x-ray diffraction, electrical resistivity, and magnetization. This compound shows the ferromagneticlike transition at ${T}_{\mathrm{M}}\ensuremath{\sim}130\mathrm{K},$ and the superconducting transition at ${T}_{\mathrm{c}}^{\mathrm{on}}\ensuremath{\sim}30\mathrm{K}.$ The EuRu1212 does not show an Invar effect below ${T}_{\mathrm{M}}$ in the temperature dependence of the volume. The ${}^{99,101}\mathrm{Ru}$ NMR signals under zero applied field at 4.2 K have been observed in the frequency ranges from 30 to 80 MHz and from 110 to 150 MHz, both of which could be identified as ferromagnetic resonances even below ${T}_{\mathrm{c}}^{\mathrm{on}}.$ These results clearly indicate that the Ru moments have a ferromagnetic component even below ${T}_{\mathrm{c}}^{\mathrm{on}}.$ These frequency-swept spectra around 60 MHz and around 130 MHz are ascribed to ${}^{99,101}\mathrm{Ru}$ nuclei of ${\mathrm{Ru}}^{4+}(S=1)$ and to those of ${\mathrm{Ru}}^{5+}$ $(S=\frac{3}{2}),$ respectively. These results reveal that the Ru ions are in a mixed valent state and that the Ru 4d electrons are not itinerant but localized. The ${}^{101}\mathrm{Ru}$ spin-lattice relaxation rate ${T}_{1}^{\ensuremath{-}1}$ decreases steeply just below ${T}_{\mathrm{c}}^{\mathrm{on}},$ because of the formation of superconducting gap. The divergent behavior of ${(T}_{1}{T)}^{\ensuremath{-}1}$ below $\ensuremath{\sim}10\mathrm{K}$ indicates that the ``spontaneous vortex phase'' occurs in the superconducting state, in which the transferred field from the Ru sublattice to the ${\mathrm{CuO}}_{2}$ layer is present between the lower and the upper critical field.

Localized Charge Transfer in CsAu·NH3: 1H and 133Cs Nuclear Magnetic Resonance

We report results from 1H and 133Cs nuclear magnetic resonance (NMR) studies on CsAu·NH3. This compound has recently been synthesized and belongs to the family of ammoniates. The intense blue color of the material, which is not observed in the parent compounds, suggests that a charge-transfer electronic state is formed. The 1H NMR line shape and relaxation rates are found to be very similar to the corresponding properties in pure ammonia. However, the 133Cs-NMR properties of CsAu·NH3 strongly deviate from the corresponding ones in CsAu, and we show that the former can be interpreted consistently within a localized charge distribution due to either an anharmonic charge density wave state induced by a charge transfer process or an Anderson localization mechanism.

Blood NMR relaxation in the rotating frame: mechanistic implications

The rotating frame nuclear magnetic resonance relaxation rate R 1 ρ in the blood and cell lysate was studied at 4.7 T to provide reference values for in vivo modeling and to address the mechanisms contributing to net relaxation. A strong dependence on oxygenation, hematocrit, and spin lock field strength B 1 (0.2–1.6 G) was observed in whole blood, whereas in lysate the effects were severely attenuated. The results were further compared to transverse relaxation rate R 2. A good agreement in low-field asymptotes of these two relaxation rates was found. R 1 ρ field dispersion was fitted to Lorenzian line shape and resulted in correlation times around 40 μs. The dispersion behavior was related to motional properties of intracellular hemoglobin and effects of susceptibility shift interface across the cell membrane induced by compartmentalization of Hb into cells in blood.

Magnetodendrimers allow endosomal magnetic labeling and in vivo tracking of stem cells.

Magnetic resonance (MR) tracking of magnetically labeled stem and progenitor cells is an emerging technology, leading to an urgent need for magnetic probes that can make cells highly magnetic during their normal expansion in culture. We have developed magnetodendrimers as a versatile class of magnetic tags that can efficiently label mammalian cells, including human neural stem cells (NSCs) and mesenchymal stem cells (MSCs), through a nonspecific membrane adsorption process with subsequent intracellular (non-nuclear) localization in endosomes. The superparamagnetic iron oxide nanocomposites have been optimized to exhibit superior magnetic properties and to induce sufficient MR cell contrast at incubated doses as low as 1 microg iron/ml culture medium. When containing between 9 and 14 pg iron/cell, labeled cells exhibit an ex vivo nuclear magnetic resonance (NMR) relaxation rate (1/T2) as high as 24-39 s-1/mM iron. Labeled cells are unaffected in their viability and proliferating capacity, and labeled human NSCs differentiate normally into neurons. Furthermore, we show here that NSC-derived (and LacZ-transfected), magnetically labeled oligodendroglial progenitors can be readily detected in vivo at least as long as six weeks after transplantation, with an excellent correlation between the obtained MR contrast and staining for beta-galactosidase expression. The availability of magnetodendrimers opens up the possibility of MR tracking of a wide variety of (stem) cell transplants.

Hydration properties and the role of water in taste modalities of sucrose, caffeine, and sucrose-caffeine mixtures.

Solution properties of sapid molecules are informative on their type of hydration (hydrophobic or hydrophilic) and on the extent of the hydration layer. Physicochemical properties (intrinsic viscosity and apparent specific volume) and nuclear magnetic resonance (NMR) relaxation rates R(1) and R(2) for pure sucrose, bitter molecule caffeine, and their mixture were found to be relevant in the interpretation of the effects of these solutes on water mobility. Likewise, surface tension, contact angles with a hydrophobic surface, and the adhesion forces to this type of surface of the aqueous solutions of sapid molecules were found to discriminate between their effects on water cohesion and also between their taste qualities. The interpretation of the two sets of independent experimental results, namely physicochemical and spectroscopic data, helps in the elucidation of the role of water in sweet and bitter taste chemoreception.

Binding site of Fe3+ at purine of ATP as studied by NMR

The binding site of Fe3+ in the purine base of adenosine 5′-triphosphate (ATP) was studied by nuclear magnetic resonance (NMR). The NMR relaxation rates (R1) of 1H and 31P in ATP solutions free of and containing ferric ions were measured in the pH range of 3–10. It was found that Fe3+ selectively enhanced the relaxation rate of protons. In the presence of Fe3+, the R1 of H2 was much bigger than that of H8 at a lower pH (3–4.5), while at a higher pH (5.5–7.5) the R1 of H8 was more enhanced than H2. At a pH of around 5, both H2 and H8, as well as all three phosphorous, showed a sudden jump in R1. When pH>8, Fe3+ failed to show appreciable enhancement of R1 to all protons and phosphorous. The quantitative data of relaxation rate enhancements suggest that the binding site of Fe3+ in ATP is strongly dependent on pH. At lower pH values, Fe3+ binds N1 but at higher pH it binds to N7. When pH is around 5, the whole purine base donates the aromatic π-electrons to the ferric ion, forming a ferrocene-like complex, while when pH>8, ATP could not form complexes with Fe3+.

Charge ordering in a quasi-two-dimensional organic conductor

The first nuclear magnetic resonance (NMR) evidence of charge ordering in a quasi-two-dimensional organic conductor is presented. We have performed ${}^{13}\mathrm{C}$ NMR shift and relaxation rate measurements for the organic conductor, $\ensuremath{\theta}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{RbZn}(\mathrm{SCN}{)}_{4},$ with a quasi-two-dimensional quarter-filled band. Below the metal-insulator transition at 195 K, the NMR spectrum splits into two kinds of lines. The spin shifts and relaxation rates are quite different between the two lines, showing charge disproportionation of 1:6. This result demonstrates a route to the correlation-induced insulator transition in organic conductors.

A Proton Nuclear Magnetic Resonance Relaxation Study of C12E6/D2O

Aqueous solutions of the nonionic surfactant, n-dodecylhexaoxyethylene glycol [C12H25(OCH2CH2)6OH], termed C12E6, form various phases at room temperature: there is a micellar phase at high water content, hexagonal, cubic, and lamellar mesophases at intermediate water contents, and an isotropic phase at very low water content. Proton nuclear magnetic resonance (NMR) relaxation rates have been measured for the C12E6/D2O system as a function of composition, spectral frequency, and temperature. The results are interpreted in terms of the possible molecular motions for the C12E6 molecule in the different phases. A review of relaxation theory is presented with special attention to molecular exchange between sites that are associated with different relaxation rates.

Correlated spin dynamics in 2-D quantum Heisenberg antiferromagnets from NMR relaxation in copper formiate tetradeuterate

Proton nuclear magnetic resonance (NMR) T1 in a single crystal of copper formiate tetradeuterate is used to study the correlated Cu2+ spin dynamics and to derive the temperature behavior of the in-plane magnetic correlation length. The results are compared with the predictions of recent theoretical models for the spin dynamics in planar quantum Heisenberg antiferromagnets in a wide temperature range (from the Neel temperature up to a reduced temperatureT/J ∼ 1.4, withJ in-plane exchange integral). In particular, it is shown that, in contrast to the predictions of the nonlinear σ model, no crossover to a quantum critical regime occurs and that the experimental findings are well reproduced by deriving the NMR relaxation rate in the framework of the standard mode-mode coupling theory.

NMR probe of pseudogap characteristics inFe2+xV1−xAl

We report the results of a ${}^{51}\mathrm{V}$ nuclear magnetic resonance (NMR) study of ${\mathrm{Fe}}_{2+x}{\mathrm{V}}_{1\ensuremath{-}x}\mathrm{Al},$ near the ordered composition with $x=0,$ at temperatures between 4 and 500 K. All compositions are found to be metallic or semimetallic, though electrical resistivity has indicated a metal-insulator transition at $x=0.$ The low-temperature NMR relaxation rates for stoichiometric $(x=0)$ and nonstoichiometric $(x=0.10$ and $\ensuremath{-}0.05)$ compounds follow a Korringa law, associated with a finite density of carriers at the Fermi level. High-temperature relaxation rates for $x<~0$ go over to a semiconductorlike activated form, providing information about the gap structure near the Fermi energy. The results are consistent with pseudogap features identified by recent band-structure calculations, although with smaller energy splittings. We analyze pseudogap changes vs composition, and compare the band-filling behavior to a recent coherent potential approximation calculation.

Theory of self-diffusion in alkali metals: II. The influence of divacancies in Li

The formation energies and the binding energies of divacancies in bcc Li are calculated using the ab initio electron theory in a supercell approach. The results show that the activation energy for self-diffusion via divacancies should be much larger than that for self-diffusion via monovacancies. This is in contrast to the case for an analysis of the experimental self-diffusion data from the nuclear magnetic resonance relaxation rates in terms of a combined monovacancy-divacancy mechanism which would yield similar activation energies for monovacancies and divacancies. It is therefore concluded that a contribution from divacancies cannot explain the observed curvature of the Arrhenius plot for the self-diffusion data for Li at high temperatures.

NMR Spectroscopy of Phase Transitions in Minerals

Nuclear magnetic resonance (NMR) spectroscopy has been used extensively since the 1960’s to study phase transformations. When Lippmaa et al. (1980) presented the first significant high-resolution solid-state NMR spectroscopic study of minerals, the study of structural phase transitions by NMR was a mature field, appearing primarily in the physics literature (e.g., Blinc 1981; Rigamonti 1984). The sensitivity of NMR spectroscopy to short-range structure (first- and second-coordination spheres) and low-frequency dynamics (i.e., frequencies much lower than the thermal vibrations of atoms) make it useful for determining changes in the structure and dynamics of solids that occur near phase transitions. This combination of characteristic time- and distance-scales is not easily accessible by other techniques. Widely studied phase transformations include order-disorder and displacive transitions in compounds with perovskite, antifluorite, β-K2SO4 (A2BX4), KH2PO4 (“KDP”), and other structure types, including some mineral phases (e.g., colemanite; Theveneau and Papon 1976). These studies used primarily single-crystal techniques and were mostly limited to phases that exhibit high symmetry and possess one crystallographic site for the nucleus studied. In some cases, the NMR data can help distinguish order-disorder from purely displacive transition mechanisms. Also, careful measurement of NMR relaxation rates provides information on the spectral density at low frequencies, which reflects softening lattice vibrations or freezing-in of rotational disorder modes. Many phase transitions interesting to mineralogists became accessible to NMR spectroscopy in the late 1980’s with the commercial availability of magic-angle-spinning (MAS) NMR probe assemblies capable of operation at extended temperatures (up to about 600°C) and the development of high-temperature NMR capability at Stanford (Stebbins 1995b). These technical advances enabled many of the structural transitions that occur in minerals to be studied in situ by NMR, including those in phases for which large single crystals are not available. MAS-NMR …

Uncertainty analysis in polymer gel dosimetry

Verification of advanced radiotherapy treatment modalities requires measurement of three-dimensional absorbed dose distributions with high spatial resolution and precision. Polymer gel dosimeters combined with magnetic resonance imaging may be able to fulfil this requirement. However, verification requires that the uncertainty in the dosimeter is well known. One method of estimating the overall uncertainty in polymer gel dosimeters involves the propagation of the uncertainty in the R2 (nuclear magnetic resonance relaxation rate) map and the uncertainties in the calibration data. This work shows that using this method with current data suggests that the lowest uncertainty currently obtainable is about 3% at 8 Gy and 7% at 2 Gy. Furthermore, the most significant reductions in overall uncertainty will be achieved by reducing the noise in the R2 map.

Deuteron NMR relaxometry applied to confined liquid crystals

We discuss the capability of deuteron nuclear magnetic resonance (NMR) spectroscopy and relaxometry to reveal molecular ordering and dynamics in confined liquid crystals. The attention is focused on the high-temperature phase above the nematic-isotropic transition, which is — in the absence of the long-range orientational order — very suitable for the study of surface interactions. Deuteron NMR spectra and relaxation rates are presented for two representatives of confined liquidcrystal systems: 8CB in cylindrical cavities of Anopore membranes and 5CB with an embedded polymer network. A substantial increase in the transverse spin relaxation rate, stimulated by the surface-induced order in enclosures, has been observed. In cylindrical cavities, it exhibits a strong temperature dependence on approaching the phase transition, whereas in the polymer network dispersion it is temperature-independent. The increase of T2−1 provides information on the effect of spatial constraints on molecular mobility and on the surface orientational order parameter. Using deuteron relaxometry, one can measure the degree of orientational order in the isotropic phase not only in cylindrical but also in spherical cavities and enclosures of irregular shape, where the standard approach based on quadrupolar splitting of the NMR spectrum fails.

Influence of cholesterol on dynamics of dimyristoylphosphatidylcholine bilayers as studied by deuterium NMR relaxation

Investigation of the deuterium (2H) nuclear magnetic resonance (NMR) relaxation rates of lipid bilayers containing cholesterol can yield new insights regarding its role in membrane function and dynamics. Spin-lattice (R1Z) and quadrupolar order (R1Q) H2 NMR relaxation rates were measured at 46.1 and 76.8 MHz for macroscopically oriented bilayers of 1,2-diperdeuteriomyristoyl-sn-glycero-3-phosphocholine (DMPC-d54) containing cholesterol (1/1 molar ratio) in the liquid-ordered phase at 40 °C. The data for various segmental positions along the DMPC-d54 acyl chain were simultaneously fitted to a composite membrane deformation model, including fast segmental motions which preaverage the coupling tensor along the lipid acyl chain, slow molecular reorientations, and small-amplitude collective fluctuations. In contrast to pure DMPC-d54 in the liquid-crystalline (Lα) phase, for the DMPC-d54:cholesterol (1/1) system a linear square-law functional dependence of the relaxation rates on the order parameter (quadrupolar splitting) does not appear evident. Moreover, for acyl segments closer to the top of the chain, the angular anisotropy of the H2 R1Z and R1Q relaxation rates is more pronounced than toward the chain terminus. The residual (preaveraged) coupling tensor has its greatest effective asymmetry parameter near the polar groups, decreasing for the groups closest to the end of the chain. The results suggest that axial rotations of the phospholipid molecules occur at a somewhat higher rate than in pure bilayers, as a consequence of the higher ordering and reduction of chain entanglement. On the other hand, the rigid cholesterol molecule appears to undergo somewhat slower axial rotation, possibly due to its noncylindrical shape. Collective motions are found to be less predominant in the case of DMPC-d54:cholesterol than for pure DMPC-d54, which may indicate an increased dynamical rigidity of lipid bilayers containing cholesterol versus pure lipid systems.

Dynamics of lipid bilayers from comparative analysis of H2 and C13 nuclear magnetic resonance relaxation data as a function of frequency and temperature

Analysis of the nuclear spin relaxation rates of lipid membranes provides a powerful means of studying the dynamics of these important biological representatives of soft matter. Here, temperature- and frequency-dependent H2 and C13 nuclear magnetic resonance (NMR) relaxation rates for vesicles and multilamellar dispersions of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) in the liquid–crystalline state have been fitted simultaneously to various dynamic models for different positions of the acyl chains. The data include H2 R1Z rates (Zeeman order of electric quadrupolar interaction) acquired at 12 external magnetic field strengths from 0.382 to 14.6 T, corresponding to a frequency range from ωD/2π=2.50–95.3 MHz; and H2 R1Q rates (quadrupolar order of electric quadrupolar interaction) at 15.3, 46.1, and 76.8 MHz. Moreover, C13 R1Z data (Zeeman order of magnetic dipolar interaction) for DMPC are included at six magnetic field strengths, ranging from 1.40 to 17.6 T, thereby enabling extension of the frequency range to effectively (ωC+ωH)/2π=938.7 MHz. Use of the generalized approach allows formulation of noncollective segmental and molecular diffusion models, as well as collective director fluctuation models, which were tested by fitting the H2 R1Z data at different frequencies and temperatures (30 °C and 50 °C). The corresponding C13 relaxation rates were predicted theoretically and compared to experiment, thus allowing one to unify the C13 and H2 NMR data for bilayer lipids in the fluid state. A further new aspect is that the spectral densities of motion have been explicitly calculated from the H2 R1Z and R1Q data at 40 °C. We conclude that the relaxation in fluid membrane bilayers is governed predominantly by relatively slow motions, which modulate the residual coupling remaining from faster local motions (order fluctuations). Only the molecular diffusion model, including an additional slow motional process, and the membrane deformation model describing three-dimensional collective fluctuations fit the H2 NMR data and predict the C13 NMR data in the MHz range. Orientational correlation functions have been calculated, which emphasizes the importance of NMR relaxation as a unique tool for investigating the dynamics of lipid bilayers and biological membranes.

Paramagnetically induced nuclear magnetic resonance relaxation in solutions containing S⩾1 ions: A molecular-frame theoretical and physical model

The enhancement of nuclear magnetic resonance (NMR) relaxation rates produced by paramagnetic solutes is physically rather different for electron spin S=1/2 paramagnetic species than for S⩾1 species due to the presence of zero-field splitting interactions in the electron spin Hamiltonians of the latter. When the zfs energy is larger than the electronic Zeeman energy, the electron spin precessional motion is spatially quantized with respect to the molecule-fixed principal axis system (PAS) of the zfs tensor rather than along the external laboratory magnetic field. An analytical theory of the orthorhombic zfs limit has been derived in which the motion of the electron spin variables is described in the zfs-PAS and that of the nuclear spin variables in the laboratory coordinate frame. The resulting theoretical expressions are simple in form and suggest a physically transparent interpretation of the experiment. The NMR relaxation enhancement R1p results from additive contributions, R1x, R1y, and R1z, arising from the molecular-frame Cartesian components of the time-dependent electron spin magnetic moment operator μr(t). Each Cartesian component R1r depends on the dipolar power density at the nuclear Larmor frequency that is produced by the corresponding Cartesian component of μr(t). The theory displays the dependence of the relaxation enhancement on the variables of molecular structure in a very simple and physically transparent form: R1r∝r−6[1+P2(cos θr)], where r is the interspin distance and cos θr is the direction cosine of the interspin vector with the rth principal axis of the zfs tensor. New experimental data are presented for the model S=1 complex [trans-Ni(II)(acac)2(H2O)2] (acac=acetylacetonato) in dioxane solvent. The magnetic field dependence of the proton T1 of the axial water ligands has been measured over the range 0.15–1.5 T, the lower end of which corresponds to the zfs limit. The experimental data have been analyzed using the new analytical theory for the zfs-limit regime in conjunction with spin dynamics simulations in the intermediate regime. Dipolar density power plots are presented as graphical devices which clearly exhibit the physical information in the experiment, and which permit a rapid differentiation of the sensitive and insensitive parameters of theory. The data analysis depends strongly on the zfs parameter |E| and on the electron spin relaxation time τS,z along the zfs-PAS z-axis, but only very weakly on the other parameters of theory. A fit of the data to theory provided the values |E|=1.8±0.1 cm−1 and τS,z=8.0±0.3 ps.

The phases of polymers as determined by NMR

Nuclear magnetic resonance (NMR) spectroscopy has been used to study the morphology and dynamics in semicrystalline polymers. Dynamics may be observed through NMR relaxation rates that are sensitive to motions in the 1-10 8 Hz range, or through modulation of anisotropic magnetic interactions, such as the chemical shift and dipole-dipole interactions. Morphological structure may be inferred through NMR measurements of polymer dynamics or investigated directly through studies of the magnetic interactions. Here, we discuss the study of morphological structure in semicrystalline polymers using NMR, and review results on poly(ethylene terephthalate) that address the question of the number of phases in this semicrystalline polymer.