Direct Determination of Motional Correlation Times by 1D MAS and 2D Exchange NMR Techniques

One- and two-dimensional static and magic-angle spinning (MAS) exchange NMR experiments for quantifying slow (τ_c> 1 ms) molecular reorientation dynamics are analyzed, emphasizing the extent to which motional correlation times can be extracteddirectlyfrom the experimental data. The static two-dimensional (2D) exchange NMR experiment provides geometric information, as well as exchange time scales via straightforward and model-free application of Legendre-type orientational autocorrelation functions, particularly for axially symmetric interaction tensors, as often encountered in solid-state²H and¹³C NMR. Under conditions of MAS, increased sensitivity yields higher signal-to-noise spectra, with concomitant improvement in the precision and speed of correlation time measurements, although at the expense of reduced angular (geometric) resolution. For random jump motions, one-dimensional (1D)exchange-inducedsidebands (EIS)¹³C NMR and the recently developed ODESSA and time-reverse ODESSA experiments complement the static and MAS two-dimensional exchange NMR experiments by providing faster means of obtaining motional correlation times. For each of these experiments, the correlation time of a dynamic process may be obtained from a simple exponential fit to the integrated peak intensities measured as a function of mixing time. This is demonstrated on polycrystalline dimethylsulfone, where the reorientation rates from EIS, ODESSA, time-reverse ODESSA, and 2D exchange are shown to be equivalent and consistent with literature values. In the analysis, the advantages and limitations of the different methods are compared and discussed.     

Improved Resolution from Double Constant-Time Evolution of 3D and 4D Triple-Resonance Experiments

Triple-resonance NMR experiments are nearly essential for performing backbone assignments of proteins larger than 15 kDa. Our work extends the double constant-time (2CT) evolution scheme to triple-resonance 3D and 4D experiments. The modifications needed to accomplish 2CT evolution in triple resonance experiments are straight forward, are completely general, and consequently, will yield increased resolution for all out-and-back experiments. We expect that the increased resolution of experiments presented here will be useful in the study of larger proteins (>30 kDa) and in the study of highly helical proteins where¹HN,¹⁵N, and¹³C dimensions are poorly dispersed.     

Reversed isotope effect at formation of hydride/deutéride of Ni-30at.%Cu in high pressure H2/D2 atmosphere

Abstract

Time-dependence ‘in situ’ measurements of the electrical resistance of Ni-30at.%Cu under H₂/D₂ atmosphere up to about 1.2 GPa show a faster formation of the deuteride than of the hydride. This corresponds to a higher diffusivity of the heavier isotope. The effect, being in contrast to the classic rate theory, agrees in tendency with experiments in case of Pd-H/D and with treatments of the quantumstatistic theory.     

Stereochemical Elucidation of Norbornene Derivatives Synthesized as Leukotriene D4 Receptor Antagonists

The complete ¹H and ¹³C NMR assignment and stereochemical elucidation of four norbornene derivatives (1–4) are presented. These derivatives were obtained by an asymmetric Diels–Alder reaction of a chiral dienophile, from D‐glucose, as leukotriene D4 receptor antagonists. NOE measurements of the four products between H‐7b and H‐5 readily allowed discrimination between exo‐ (2, 4) and endo‐ (1, 3) diastereomers with respect to methyl carboxylate and tetrahydrofuro[1,3]dioxolyl groups. Further discrimination and subsequent absolute configuration determination of the two endo‐ and two exo‐ products were performed by NOE peak measurements and molecular modeling calculations of relative interproton distances of the most stable conformations of each isomer.     

Real-time volumetric MR thermometry using 3D echo-shifted sequence under an open source reconstruction platform

PurposeThe aim of this work is to implement real-time 3D MR thermometry for high intensity focused ultrasound (HIFU) monitoring.MethodsVolumetric MR thermometry was implemented based on a 3D echo-shifted sequence with short TR to improve temperature sensitivity. The 3D acquisition was accelerated in two phase encoding directions with controlled aliasing in volumetric parallel imaging (CAIPIRINHA). Image reconstruction was run in an open source reconstruction platform (Gadgetron).ResultsPhantom experiments showed the proposed volumetric thermometry was comparable to the fiber optical thermometer. In-vivo animal experiments in rabbit thigh showed that the temperature error before and after 4× acceleration was less than 0.65 °C. Finally, real-time 3D thermometry with temporal resolution ~3 s and spatial resolution 2 × 2 × 5 mm³ (spatial coverage 192 × 192 × 80 mm³) was achieved with Gadgetron reconstruction.ConclusionReal-time temperature monitoring was achieved in-vivo by using parallel imaging accelerated 3D echo-shifted sequence with Gadgetron reconstruction.     

A C solid-state NMR analysis of vitamin D compounds

¹³C cross-polarization/magic-angle spinning (CP/MAS) solid-state NMR spectroscopy has been employed to analyze four vitamin D compounds, namely vitamin D3 (D3), vitamin D2 (D2), and the precursors ergosterol (Erg) and 7-dehydrocholesterol (7DHC). The ¹³C NMR spectrum of D3 displays a doublet pattern for each of the carbon atoms, while that of Erg contains both singlet and doublet patterns. In the cases of 7DHC and D2, the ¹³C spectra display various multiplet patterns, viz. singlets, doublets, triplets, and quartets. To overcome the signal overlap between the ¹³C resonances of protonated and unprotonated carbons, we have subjected these vitamin D compounds to 1D ¹H-filtered ¹³C CP/MAS and {¹H}/¹³C heteronuclear correlation (Hetcor) NMR experiments. As a result, assisted by solution NMR data, all of the ¹³C resonances have been successfully assigned to the respective carbon atoms of these vitamin D compounds. The ¹³C multiplets are interpreted due to the presence of s-cis and s-trans configurations in the α- and β-molecular conformers, consistent with computer molecular modeling determined by molecular dynamics and energy minimization calculations. To further characterize the ring conformations in D3, we have successfully extracted chemical shift tensor elements for the ¹³C doublets. It is demonstrated that ¹³C solid-state NMR spectroscopy provides a robust and high sensitive means of characterizing molecular conformations in vitamin D compounds.     

Evaluating renal arterial wall by non-enhanced 2D and 3D free-breathing black-blood techniques: Initial experience

ObjectivesTo evaluate the feasibility and reproducibility of 2D and 3D black-blood sequences in measuring morphology of renal arterial wall.MethodsThe 2D and 3D imaging sequences used variable-refocusing-flip-angle and constant-low-refocusing-flip-angle turbo spin echo (TSE) readout respectively, with delicately selected black-blood scheme and respiratory motion trigger for free-breathing imaging. Fourteen healthy subjects and three patients with Takayasu arteritis underwent renal artery wall imaging with 3D double inversion recovery (DIR) TSE and 2D Variable Flip Angle-TSE (VFA-TSE) black-blood sequences at 3.0 T. Four healthy subjects were randomly selected for scan-rescan reproducibility experiments. Signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and morphology of arterial wall were measured and compared using paired-t-test or Wilcoxon signed-rank test between 2D and 3D sequences. The inter-observer, intra-observer and scan-rescan agreements of above measurements were determined using intraclass correlation coefficient (ICC).ResultsThe 2D and 3D imaging sequences showed similar morphological measurements (lumen area, wall area, mean wall thickness and maximum wall thickness) of renal arterial wall (all P > 0.05) and excellent agreement (ICC: 0.853–0.954). Compared to 2D imaging, 3D imaging exhibited significantly lower SNR_lumen (P < 0.01) and SNR_wall (P = 0.037), similar contrast-to-noise ratio (CNR) (P = 0.285), and higher CNR efficiency (CNR_eff) (P < 0.01). Both 2D and 3D imaging showed good to excellent inter-observer (ICC: 0.723–0.997), intra-observer (ICC: 0.749–0.996) and scan-rescan (ICC: 0.710–0.992) reproducibility in measuring renal arterial wall morphology, SNR and CNR, respectively.ConclusionsBoth high-resolution free-breathing 2D VFA-TSE and 3D DIR TSE black-blood sequences are feasible and reproducible in high-resolution renal arterial wall imaging. The 2D imaging has high SNR, whereas 3D imaging has high imaging efficiency.     

The Multidimensional Filter Diagonalization Method: II. Application to 2D Projections of 2D, 3D, and 4D NMR Experiments

The theory of the multidimensional filter diagonalization method (FDM) described in the previous paper (V. A. Mandelshtam, 2000, J. Magn. Reson. 144, 343–356 (2000)) is applied to NMR time signals with up to four independent time variables. Direct projections of the multidimensional time signals produce new kinds of 2D spectra. The resolution obtained by FDM can be far superior to that obtained by conventional phase-sensitive FT processing, and correlation peaks in heteronuclear and homonuclear experiments can be condensed to sharp singlets, removing all spin–spin couplings. Examples of singlet-HSQC and singlet-TOCSY spectra show big gains in resolution. It is not necessary to have a finely digitized spectrum, in which the individual multiplet components are resolved, for the methods to work. Examples of FDM spectra, ranging from simple organic molecules and steroids to metalloproteins, are shown.     

Frontiers in 2D NMR of paramagnetic metalloproteins

The possibilities and the limitations of 2D NMR for the structural characterization of paramagnetic metalloproteins are reviewed. We survey the general strategies for 2D¹H NMR investigations of hyperfine shifted signals. Careful adaptation of classical 2D NMR experiments to fast relaxing systems results in the detection of previously not observed scalar and dipolar connectivities, thus leading to the specific assignment of selected resonances. The approach is of general applicability for paramagnetic metalloproteins. We report here on the application of the application of the method to an iron sulfur protein and a heme protein. In both cases specific assignment of several hyperfine shifted signals, corresponding to active site protons, were obtained; this allowed significant insight into the structure-function relationships of these metalloproteins.     

In vitro experiments on ICOSA6 4D flow MRI measurement for the quantification of velocity and turbulence parameters

PurposeTo perform comprehensive in vitro experiments using six-directional icosahedral flow encoding (ICOSA6) 4D flow magnetic resonance imaging (MRI) under various scan conditions to analyze the robustness of velocity and turbulence quantification.Materials and methodsIn vitro flow phantoms with steady flow rates of 10 and 20 L/min were scanned using both conventional 4D flow MRI and ICOSA6. Experiments focused on comparisons between ICOSA6 and conventional four point (4P) methods, and the effects of contrast agents, velocity encoding range (Venc), and scan direction on velocity and turbulence quantification.ResultsThe results demonstrated that 1) ICOSA6 improves the velocity-to-noise ratio (VNR) of velocity estimation by 33% (on average) and results in similar turbulent kinetic energy (TKE) estimation as the 4P method. 2) Measurements with a contrast agent resulted in more than a 2.5 fold increase in average VNR. However, the improvement of total TKE quantification was not obvious. 3) TKE estimation was less affected by Venc and the scan direction, whereas turbulence production (TP) estimation was largely affected by these measurement conditions. The effects of Venc and scan direction accounted for less than 11.63% of TKE estimation, but up to 33.89% of TP estimation.ConclusionThe ICOSA6 scheme is compatible with conventional 4D flow MRI for velocity and TKE measurement. Contrast agents are effective at increasing VNR, but not signal-to-noise ratio for TKE quantification. The effects of Venc and scan direction influence total TP more than total TKE.     

A combined crossed molecular beam and quasiclassical trajectory study of the Titan-relevant N(2D) + D2O reaction

Following a recent investigation on the N(²D) + H₂O reaction [Homayoon et al., J. Phys. Chem. Lett. 5, 3508 (2014)], we report on an experimental and theoretical study of the isotopologue N(²D) + D₂O reaction. Crossed molecular beam (CMB) experiments were conducted at a collision energy of 10.3 kcal mol^–1. Quasiclassical trajectory calculations were performed on a recent potential energy surface to derive the centre-of-mass functions necessary to simulate the CMB laboratory distributions. Excellent agreement was found. The importance of the channel leading to HON/DON was confirmed. The inclusion of this channel, in addition to that leading to the isomer HNO/DNO, can affect the models considering the coupling between nitrogen and oxygen chemistry in the upper atmosphere of Titan.     

Parametric spectrum analysis of 2D NMR signals. Application to in Vivo J spectroscopy

Parametric modeling techniques for spectrum analysis, based on the linear prediction principle, have previously been proposed to process NMR data. In this paper, they are tested on different practical NMR signals, and especially on in vivo 2D NMR spectroscopy data. The linear prediction version of the maximum entropy method, using AR modeling, and the Prony method are outlined with some considerations about the choice of the AR algorithm. Then simulation and experimental results obtained with the Prony method are presented and compared with those obtained with classical 2D Fourier transform processing. The data processed here result from homonuclear 2D J-resolved spectroscopy experiments performed to measure the spin-spin coupling constants between the three phosphorus nuclei of ATP in the rat brain. The parametric techniques (especially the Prony method) applied in both dimensions yield increased resolution and sensitivity and their ability to process limited data allows the total acquisition time to be reduced without loss of resolution. Although the noise may damage the performances, the results obtained here, on in vivo 2D data, are quite encouraging.     

Molecular mobility in compressed fluid n-butane

Abstract

An autoclave machined from a high strength titanium alloy for high pressure high resolution NMR studies is described. The autoclave has been used at pressures up to 600 MPa and temperatures between 150 K and 450 K. The molecular mobility of fluid n-butane has been studied by proton- and deuteron spin lattice relaxation time measurements of selectively deuterated butanes. Additional information about the dynamic is obtained from isotopic dilution experiments. The p, T dependence of the overall mobility and of the rotation of the methyl groups is derived from the data.     

四维NMR谱

尽管３ＤＮＭＲ谱的分辨率有所提高，但解释较大的三维异核ＮＭＲ谱时，仍存在含糊性。因此，人们希望通过增加维数进一步提高分辨率，在最近几年中出现了４ＤＮＭＲ谱，４ＤＮＭＲ谱常常是说明２ＤＮＭＲ谱的方法，４ＤＮＭＲ实验可简单地看作是由三个２ＤＮＭＲ实验所组成，新的４ＤＮＭＲ技术仍在不断地发展，已有许多将４ＤＮＭＲ谱应用于蛋白质及核酸研究的报道。     

Structural Assignment of Isomeric S‐ and S,N‐1‐Alkoxycarbonylalkylated 6‐Amino‐2‐thiouracil Derivatives by Means of 1H and 13C NMR Spectroscopy

Abstract

The structures of new isomeric 2‐alkoxycarbonylalkylthio‐ and 2‐alkoxy‐ carbonylalkylthio‐1‐alkoxycarbonylalkyl‐6‐aminouracils (1–21) have been established on the basis of the ¹H NMR and ¹³C NMR spectroscopic data. The ¹H NMR and ¹³C NMR spectra of 1–21 have been fully assigned by a combination of two‐dimensional experiments [heteronuclear multiple quantum coherence (HMQC) and heteronuclear multiple bond correlation (HMBC)]. The ¹³C NMR spectra have been shown to be able to differentiate between isomers.     

An NMR approach to the superconducting regime of the spin ladder compound Sr2Ca12Cu24O41

⁶³Cu-NMR experiments of Knight shift and relaxation time T₁ have been performed on the two-leg spin ladders of a Sr₂Ca₁₂Cu₂₄O₄₁ single crystal at several pressures up to the critical pressure for the stabilization of a superconducting ground state. The data confirm the onset of low-lying spin excitations at observed previously [Science 279, 345 (1998)] and reveal a marked decrease of the spin gap under pressures above 20 kbar although a significant fraction of the spin excitations remains gapped at kbar. A comparison between NMR and transport data under pressure suggests that the depression of the spin gap can be ascribed to an increase in the interladder exchange coupling, possibly mediated by the ladder-chain interaction along the b-direction. Received 21 October 1999     

Toward direct determination of conformations of protein building units from multidimensional NMR experiments III

Chemical shielding anisotropy tensors have been determined, within the GIAO-RHF formalism using a smaller [6-31+G(d)] and two medium-size basis sets [6-311++G(d,p) and TZ2P], for all elements of the conformational library (altogether 27 structures) of the hydrophobic model peptide For-L-Phe-NH₂. The individual chemical shifts and their conformational averages have been compared to their experimental counterparts taken from the BioMagnetic Resonance Bank (BMRB). At the highest level of theory applied, for all nuclei but the amide proton, deviations between statistically averaged theoretical and experimental chemical shifts are as low as a few percent. One-dimensional (1D) chemical shift - structure plots do not allow unambiguous identification of backbone conformations. On the other hand, on chemical shift - chemical shift plots of selected nuclei, e.g., ¹H ^N with ¹⁵N or ¹⁵N with ¹³C ^α , regions corresponding to major conformational motifs have been found, providing basis for the identification of peptide conformers solely from NMR shift data. The 2D ¹H ^α -¹³C ^α as well as the 3D ¹H ^α -¹³C ^α -¹³C ^β chemical shift - chemical shift plots appear to be of special importance for direct determination of conformations of protein building units from multidimensional NMR experiments. 48 pairs of ¹H ^α -¹³C ^α data for phenylalanine residues have been extracted from 18 selected proteins and compared to relevant ab initio results, supporting the calculated results. Thus, the appealing idea of establishing backbone folding information of peptides and proteins from chemical shift information alone, obtained from selected multiple-pulse NMR experiments (e.g., 2D-HSQC, 2D-HMQC, and 3D-HNCA), has received further support.     

NMR Spectroscopic Study of Camphanic Acid,Rh2[Camphanate]4, and its Adducts with 1,4-Benzodiazepines

The H and ¹³C NMR spectra of chiral camphanic acid, lactonc of (1S,3R)-1-hydroxy-2,2,3-trimethyl-cyclopentan-1,3-dicarboxylic acid (1), and dirhodium tetracamphanate complex (2) were completely assigned on the basis of one- and two-dimensional NMR experiments. The NMR spectra of the adducts of dirhodium tetracamphanate 2 with 5-pyrido-1, 4-benzodiazepin-2-ones 3 and 4, complexes with catalytic activity, support the assignation and also revealed two different types of axial complexation of these nitrogen ligands.     

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

Abstract

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