Relaxation Rate Data Research Articles

1H and 17O NMR relaxometric study in aqueous solution of Gd(III) complexes of EGTA-like derivatives bearing methylenephosphonic groups

The Gd(III) complexes of three new octadentate chelators, prepared by substitution of four, two, and one carboxylate groups of EGTA with phosphonate groups, have been investigated by 1H and 17O NMR relaxometric techniques in aqueous solutions. The analysis of the solvent proton relaxivity data as a function of pH, temperature, and magnetic field strength (nuclear magnetic relaxation dispersion (NMRD) profiles) in combination with the 17O transverse relaxation rate data at variable temperature allowed assessing the hydration state of the complexes, the occurrence of pH-dependent oligomerization processes for the tetraphosphonate derivative, the presence of a well-defined second sphere of hydration that markedly contributes to the relaxivity, and the values of the structural and dynamic relaxation parameters. In addition, in the case of the monophosphonate derivative the presence of a coordinated water molecule has allowed evaluation of the kinetic parameters of the exchange process, highly relevant for the possible use of this Gd(III) complex as an MRI probe. The rate of exchange of the water molecule, (298)k(ex) = 4.2 x 10(8)s(-1), is one of the highest measured so far for a nonacoordinate Gd(III) chelate and optimal for developing contrast-enhancing probes of high efficacy at high magnetic fields.

Monomer Concentrations in Simple Micellar Systems from NMR Relaxation Times and C‐13 Chemical Shifts

The monomer concentrations for some simple micelle forming surfactants, octylammonium bromide (OTAB), sodium decyl sulphate, SDecS, and dodecyltrimethylammonium bromide (DTAB) were determined from the NMR‐paramagnetic relaxation enhancement experiment (the NMR‐PRE) on the H‐1 spin lattice relaxation times for the surfactant protons as a function of the surfactant concentration above and below the critical micelle concentration (cmc region). The NMR relaxation times, determined from the inversion recovery method, decrease steadily as the concentration of the surfactant is increased above the cmc value. A second, simple method is proposed whereby the p‐value of the surfactant in the micelle is determined from the two‐site model via extrapolation of the observed relaxation rate data to high concentrations. The p‐values and the calculated monomer concentrations are compared to each other and to those obtained using the mass‐action and phase separation models. In most cases, the monomer concentrations appear to remain constant at concentrations up to 2–3 times the cmc value, and then begin to decrease thereafter. These results are compared to the trends in monomer concentrations obtained from other NMR experiments in other surfactant systems.

Backbone Dynamics in an Intramolecular Prolylpeptide–SH3 Complex from the Diphtheria Toxin Repressor, DtxR

The diphtheria toxin repressor contains an SH3-like domain that forms an intramolecular complex with a proline-rich (Pr) peptide segment and stabilizes the inactive state of the repressor. Upon activation of diphtheria toxin repressor (DtxR) by transition metals, this intramolecular complex must dissociate as the SH3 domain and Pr segment form different interactions in the active repressor. Here we investigate the dynamics of this intramolecular complex using backbone amide nuclear spin relaxation rates determined using NMR spectroscopy and molecular dynamics trajectories. The SH3 domain in the unbound and bound states showed typical dynamics in that the secondary structures were fairly ordered with high generalized order parameters and low effective correlation times, while residues in the loops connecting β-strands exhibited reduced generalized order parameters and required additional motional terms to adequately model the relaxation rates. Residues forming the Pr segment exhibited low-order parameters with internal rotational correlation times on the order of 0.6 ns–1 ns. Further analysis showed that the SH3 domain was rich in millisecond time scale motions while the Pr segment exhibited motions on the 100 μs time scale. Molecular dynamics simulations indicated structural rearrangements that may contribute to the observed relaxation rates and, together with the observed relaxation rate data, suggested that the Pr segment exhibits a binding↔unbinding equilibrium. The results here provide new insights into the nature of the intramolecular complex and provide a better understanding of the biological role of the SH3 domain in regulating DtxR activity.

Ga NQR relaxation rates in superconductor [formula omitted

Nuclear quadupole resonance (NQR) measurements have been performed on the Pu-based superconductor PuRhGa 5 with a transition temperature of ∼ 8.8 K . NQR lines ascribed to Ga 69 ( 2 ) , Ga 71 ( 2 ) , and Ga 69 ( 1 ) have been found in zero field. Spin–lattice relaxation rate ( 1 / T 1 ) data for these lines have been measured in the superconducting state. The temperature variation of 1 / T 1 can be interpreted in terms of a line-nodal superconducting gap. Data for both the Ga 69 ( 2 ) and Ga 69 ( 1 ) NQR lines show a constant- T 1 T behavior between T c and T * ∼ 30 K , which suggests that a coherent Fermi-liquid state sets in below T * . The modified Korringa relation using 1 / T 1 and Knight shift data in an applied field indicates that the normal state of PuRhGa 5 can be regarded as an antiferromagnetically correlated metal.

31P NMRstudy of Na2CuP2O7: an S = 1/2 two dimensional Heisenberg antiferromagnetic system

The magnetic properties of Na2CuP2O7 were investigated by means of 31P nuclear magnetic resonance (NMR), magnetic susceptibility, and heat capacity measurements. We reportthe 31P NMR shift, the spin–lattice , and spin–spin (1/T2) relaxation rate data as a function of temperatureT. The temperature dependence of the NMR shiftK(T) is welldescribed by the S = 1/2 square lattice Heisenberg antiferromagnetic model with an intraplanar exchange of K and a hyperfine coupling A = 3533 ± 185 Oe /μB. The 31P NMR spectrum was found to broaden abruptly belowT∼10 K,signifying some kind of transition. However, no anomaly was noticed in the bulk susceptibility data downto 1.8 K. The heat capacity appears to have a weak maximum around10 K. With decrease in temperature, the spin–lattice relaxation rate1/T1 decreases monotonically and appears to agree well with the high temperature series expansion expressionfor a S = 1/2 2D square lattice.

Accessibility of tobacco lipid transfer protein cavity revealed by15N NMR relaxation studies and molecular dynamics simulations

Plant LTP1 are small helical proteins stabilized by four disulfide bridges and are characterized by the presence of an internal cavity, in which various hydrophobic ligands can be inserted. Recently, we have determined the solution structure of the recombinant tobacco LTP1_1. Unexpectedly, despite a global fold very similar to the structures already known for cereal seed LTP1, its binding properties are different: Tobacco LTP1_1 is able to bind only one monoacylated lipid, whereas cereal LTP1 can bind either one or two. The 3D structure of tobacco LTP1_1 revealed the presence of a hydrophobic cluster, not observed on cereal LTP1 structures, which may hinder one of the two entrances of the cavity defined for wheat LTP1. To better understand the mechanism of lipid entrance for tobacco LTP1_1 and to define the regions of the protein monitoring the accessibility of the cavity, we have complemented our structural data by the study of the internal dynamics of tobacco LTP1_1, using (15)N magnetic relaxation rate data and MD simulations at room and high temperatures. This work allowed us to define two regions of the protein experiencing the largest motions. These two regions delineate a portal that opens up during the simulation constituting a unique entrance of the hydrophobic cavity, in contrast with wheat LTP1 where two routes were detected. The hydrophobic interactions resulting from a few point mutations are strong enough to completely block the second portal so that the accessibility of the cavity is restricted to one entrance, explaining why this particular LTP1 binds only one lipid molecule.

Study of one-dimensional nature ofS=1∕2(Sr,Ba)2Cu(PO4)2andBaCuP2O7viaP31NMR

The magnetic behavior of the low-dimensional phosphates (Sr,Ba)_2 Cu(PO_4)_2 and BaCuP_2O_7 was investigated by means of magnetic susceptibility and ^{31}P nuclear magnetic resonance (NMR) measurements. We present here the NMR shift K(T), the spin-lattice 1/T_1 and spin-spin 1/T_2 relaxation-rate data over a wide temperature range 0.02 K < T < 300 K. The T-dependence of the NMR K(T) is well described by the S=1/2 Heisenberg antiferromagnetic chain model with an intrachain exchange of J/k_B = 165 K, 151 K, and 108 K in Sr_2Cu(PO_4)_2, Ba_2Cu(PO_4)_2, and BaCuP_2O_7, respectively. Our measurements suggest the presence of magnetic ordering at 0.8 K in BaCuP_2O_7 (J/k_B = 108 K). For all the samples, we find that 1/T_1 is nearly T-independent at low-temperatures (1 K < T < 10 K), which is theoretically expected for 1D chains when relaxation is dominated by fluctuations of the staggered susceptibility. At high temperatures, 1/T_1 varies nearly linearly with temperature.

Carrier concentration independent antiferromagnetic spin fluctuations in the electron-doped high-temperature superconducting cupratePr2−xCexCuO4

We have performed ${}^{63}\mathrm{Cu}$ NMR measurements on the electron-doped high-temperature superconducting cuprate (HTSC) ${\mathrm{Pr}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}$ $(x=0.10,$ 0.15, and 0.20), at 9 T, which is sufficient to suppress ${T}_{c}$ to zero. The Cu spin-lattice relaxation rate and the Cu spin-lattice relaxation rate anisotropy can be consistently interpreted in terms of coupling to antiferromagnetic spin fluctuations for temperatures as low as 6 K. We find that the spin-fluctuation spectrum probed by the Cu spin-lattice relaxation rate does not change with increasing electronic concentration, contrary to a recent theoretical predication. There is no evidence in the Cu spin-lattice relaxation rate data for a temperature-independent spin gap that is as large as theoretically predicted or as large as the normal-state pseudogap energy reported from infrared reflectance measurements on the electron-doped HTSC ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}.$ The Cu nuclear quadrupole resonance frequency is significantly smaller than that observed in the hole doped HTSC, which implies a nearly complete cancellation of the Cu $3d,$ O $2p,$ and nuclei contributions to the electric-field gradient at the Cu nucleus. The orbital shift anisotropy is similar to that observed in the hole doped HTSC, implying a similar relative splitting of the Cu $3d$ orbitals.

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.

NMR structure of an archaeal homologue of ribonuclease P protein Rpp29.

A protein component of the Archaeoglobus fulgidus RNase P was expressed in Escherichia coli, purified, and structurally characterized using multidimensional NMR methods. The dominant structural feature of this 11 kDa protein is a sheet of six antiparallel beta-strands, wrapped around a core of conserved hydrophobic amino acids. Amide proton exchange and (15)N relaxation rate data provide evidence that the first 16 residues of the protein, located before the start of the first beta-strand, and the last 24 residues, located past the end of the last beta-strand, are relatively flexible; this contrasts with the relatively rigid and well-defined structure of the beta-sheet. Amino acid sequence comparisons among a diverse set of species indicate that the A. fulgidus protein is homologous to the human RNase P protein Rpp29, yeast RNase P protein Pop4, and a known archaeal RNase P protein from Methanobacter thermoautotrophicus; conserved hydrophobic residues indicate that the homologous protein in each of these species contains a similar beta-sheet structure. Conserved surface residues located in the loop connecting strands beta2 and beta3, the loop connecting strands beta4 and beta5, and in the flexible N- and C-terminal tails are most likely to have specific interactions with the RNA and other proteins of RNase P. The structural model of an RNase P protein component provided by the present work provides an essential step toward eventually understanding the overall architecture of this complex enzyme and the mechanism by which it performs its functions.

Scaling function of the critical binary mixture methanol–cyclohexane

Acoustical attenuation spectra of the methanol–cyclohexane mixture of critical composition have been measured at different temperatures near the critical temperature. The noncritical background contribution of the spectra has been determined and subtracted from the total attenuation to yield the critical contribution. The latter enabled an accurate experimental determination of the scaling function. Regression analysis of the experimental scaling function data in terms of theoretical models yielded the relaxation rate of order parameter fluctuations as a function of temperature. Close agreement with relaxation rate data from other experimental methods has been found. The amplitude of the acoustical attenuation at the critical temperature has been discussed in terms of the adiabatic coupling constant. Its amount is likewise as small (|g|=0.14) as that of the alcohol–alkane system ethanol–dodecane (|g|=0.1).

Solid state proton spin–lattice relaxation in four structurally related organic molecules

We report and interpret the temperature dependence of the proton spin–lattice relaxation rate at 8.50 and 22.5 MHz in four polycrystalline solids composed of structurally related molecules: 2-ethylanthracene, 2- t -butylanthracene, 2-ethylanthraquinone, and 2- t -butylanthraquinone. We have been unable to grow single crystals and therefore do not know the crystal structures. Hence, we use the NMR relaxometry data to make predictions about the solid state structures. As expected, we are able to conclude that the ethyl groups do not reorient in the solid state but that the t -butyl groups do. The anthraquinones have a “simpler” structure than the anthracenes. The best dynamical models suggest that there is a unique crystallographic site for the t -butyl groups in 2- t -butylanthraquinone and two sites, each with half the molecules, for the ethyl groups in 2-ethylanthraquinone. There are also two sites in 2-ethylanthracene, but with unequal weights, suggesting four sites in the unit cell with lower symmetry than the two anthraquinones. Finally, the observed relaxation rate data in 2- t -butylanthracene is very complex and its interpretation demonstrates the uniqueness problem that arises in interpreting relaxometry data without the knowledge of the crystal structure.

Hydrogen in Nb(V 1− yCr y) 2 Laves-phase compounds: neutron diffraction and nuclear magnetic resonance studies

In order to study the positions and mobility of hydrogen in ternary Laves-phase compounds Nb(V 1− y Cr y ) 2, we have performed neutron-diffraction measurements in NbVCr and NbVCrD 0.38 at T=295 K and nuclear magnetic resonance measurements of the 1H, 2D and 51V spin-lattice relaxation rates T 1 −1 in NbVCrH 0.3, NbVCrD 0.38 and NbV 1.4Cr 0.6H 0.6 over the temperature range 11–424 K. It is found that the host-metal lattice of both NbVCr and NbVCrD 0.38 has C15-type structure (space group Fd 3m ), and D atoms in NbVCrD 0.38 occupy tetrahedral g sites coordinated by two Nb atoms and two V(Cr) atoms. For all the samples studied, the temperature dependence of the motional contribution to T 1 −1 shows two peaks. This feature indicates a coexistence of two hydrogen jump processes with well-separated characteristic frequencies. The parameters of H(D) motion derived from the relaxation rate data for Nb(V 1− y Cr y ) 2–H(D) are compared with those for the related Laves-phase system TaV 2–H(D).

Structure and dynamics of translation initiation factor aIF-1A from the archaeon Methanococcus jannaschii determined by NMR spectroscopy.

Translation initiation factor 1A (aIF-1A) from the archaeon Methanococcus jannaschii was expressed in Escherichia coli, purified, and characterized in terms of its structure and dynamics using multidimensional NMR methods. The protein was found to be a member of the OB-fold family of RNA-associated proteins, containing a barrel of five beta-strands, a feature that is shared with the homologous eukaryotic translation initiation factor 1A (eIF-1A), as well as the prokaryotic translation initiation factor IF1. External to the beta barrel, aIF-1A contains an alpha-helix at its C-terminal and a flexible loop at its N-terminal, features that are qualitatively similar to those found in eIF-1A, but not present in prokaryotic IF1. The structural model of aIF-1A, when used in combination with primary sequence information for aIF-1A in divergent species, permitted the most-conserved residues on the protein surface to be identified, including the most likely candidates for direct interaction with the 16S ribosomal RNA and other components of the translational apparatus. Several of the conserved surface residues appear to be unique to the archaea. Nitrogen-15 relaxation and amide exchange rate data were used to characterize the internal motions within aIF-1A, providing evidence that the protein surfaces that are most likely to participate in intermolecular interactions are relatively flexible. A model is proposed, suggesting some specific interactions that may occur between aIF-1A and the small subunit of the archaeal ribosome.

35Cl NMR relaxation study of cement hydrate suspensions

35Cl nuclear magnetic resonance (NMR) T1 and T2 relaxation study of suspensions of the cement hydrate phases portlandite, C4AC̄H11 (an AFm phase), and jennite provides significant insight into the mechanisms of chloride sorption in Portland cement systems. For these three phases, all observed chloride is in rapid exchange (υex>1.6 kHz) between surface and bulk solution sites and is predominantly in a hydrated, solution-like chemical environment. A two-site exchange model for the T1 relaxation rate data yields sorption densities in excellent quantitative agreement with sorption isotherm measurements for portlandite and C4AC̄H11. For jennite, the NMR results underestimate the sorption isotherm results by about 40%, but the sorption densities are low, and the results are probably in agreement within experimental error. The observed sorption densities are much greater than can be accommodated by bonding directly to atoms on the solid surfaces and the amount predicted to be in the Stern and diffuse layers by a Gouy-Chapman calculation assuming sorption of only isolated chloride ions. The significance of understanding the structural and dynamical properties of solution-state chloride and the roles of ion pairs and clusters in the solution near the solid surface are discussed.

Temperature-Jump Relaxation Kinetics of Substrate-Induced Spin-State Transition in Cytochrome P450 (Comparison of the Wild-Type and C334A Mutant P450CAM and P4502B4)

The kinetics of binding of the substrate camphor to the cytochrome P450CAM and the C334A mutant as well as the kinetics of binding of benzphetamine to the wild-type P4502B4 have been studied by the temperature-jump relaxation technique in order to distinguish between the two models for substrate-induced spin-state transition. These models are the bimolecular model in which spin-state transition occurs in parallel with substrate binding, and the two-step spin-equilibrium model in which substrate binding is a separate step preceding the spin-state transition. With all three P450s, the relaxation rate versus concentration data were linear as predicted by the bimolecular model and inconsistent with the spin-equilibrium model, which predicts a curve reaching saturation. With all three P450s, the relaxation rate versus concentration data exhibited maxima. These results are considered to resolve the controversy in favor of the bimolecular model for substrate-induced spin-state transition. In addition, the results suggest that the bimolecular model may be applicable to other P450s as well.

The doping dependence of T* – what is the real high- Tc phase diagram?

Underdoped high- T c superconductors are frequently characterised by a temperature, T *, below which the normal-state pseudogap opens. Two different “phase diagrams” based on the doping ( p) dependence of T * are currently considered: one where T * falls to zero at a critical doping state and the other where T * merges with T c in the overdoped region. By examining the temperature dependence of the NMR Knight shift and relaxation rate, entropy, resistivity, infrared conductivity, Raman scattering, ARPES and tunnelling data, it is concluded that the second scenario is not at all supported. Neither can one distinguish a small and a large pseudogap as is often done. T * is an energy scale, which falls abruptly to zero at p=0.19. Colour figures are available at cond-mat/0005063.

Nuclearmagnetic resonance study of hydrogen diffusion in A15-typeNb3AlHx

Nuclear magnetic resonance measurements of the protonspin-lattice relaxation rate T1-1 for A15-type Nb3AlHx(x = 0.13, 1.77 and 2.75) have been performed over thetemperature range 20-420 K and the resonance frequency range13-90 MHz. The experimental results are analysed to evaluate theelectronic (Korringa) contributions to T1-1 and the parametersof hydrogen diffusion. It is found that the electronic contributionto the relaxation rate decreases with increasing H content. Theeffective activation energies for H diffusion derived from thehigh-temperature relaxation rate data are 0.21 eV for x = 0.13and 0.30 eV for x = 1.77 and 2.75. The behaviour of T1-1at low temperatures (T<270 K for x = 1.77 and 2.75) suggestsa coexistence of at least two frequency scales of H hopping. It isshown that the structure of the sublattice of interstitial dsites (partially occupied by hydrogen) is consistent with thecoexistence of two characteristic jump rates for H atoms.

Unusual proton Zeeman spin relaxation in an organic solid: several crystal polymorphs or different glass structures?

Solid state proton Zeeman relaxation rate R 1z measurements in two isomers of an organic solid (1- and 2-ethylnaphthalene) are reported. The samples are liquids at room temperature and the temperature T and Larmor frequency ω dependence of R 1z depends strongly on how the sample is solidified. Methyl group (CH 3) rotation is responsible for the proton spin relaxation and the methyl groups serve as probes of the local environment. The R 1z measurements clearly distinguish between different solid states due to the differences in local structure at the several-molecule level. The experiments cannot be used to determine the states of these Van der Waals solids although interpreting the relaxation rate data suggests the states are unusual. We propose that these systems might exist in two (2-ethylnaphthalene) or more (1-ethylnaphthalene) polycrystalline polymorphs or that we are observing distinguishable glassy states, or, both. A phase transition is observed in 1-ethylnaphthalene. Variable temperature X-ray studies of organic systems that solidify well below room temperature are difficult, or at least not routine, and proton spin relaxation measurements serve as a convenient starting point for investigating such systems.

Nuclear and Electronic Relaxation of Eu2+(aq): An Extremely Labile Aqua Ion1

Variable-temperature (at 4.7 and 9.4 T) and variable-pressure (9.4 T) 17O nuclear relaxation rates were measured for the Eu(II) aqua ion. Variable-temperature 1H nuclear magnetic relaxation dispersion (NMRD) profiles were recorded. In addition, EPR spectra of the Eu(II) aqua ion are reported as a function of temperature at 0.34, 2.7, 5.4, and 8.1 T. The simultaneous fit of the nuclear and electronic relaxation rate data results in kex298 = 4.4 × 109 s-1 for the water exchange rate with activation parameters ΔH⧧ = 15.7 kJ mol-1 and ΔS⧧ = −7.0 J K-1 mol-1, ΔV⧧ = −11.3 cm3 mol-1 for the volume of activation for exchange, and τR298 = 16.3 ps for the rotational correlation time. Water exchange at Eu2+(aq) occurs via an associative mechanism and has the highest rate ever measured at an aqua ion by magnetic resonance. The high-field EPR spectra, reported here for the first time for divalent Eu, show hyperfine coupling to 151Eu and 153Eu and the coupling constants are determined (37.3 and 16.4 G for 151Eu and 153...