High resolution heteronuclear correlation NMR spectroscopy between quadrupolar nuclei and protons in the solid state

A high resolution two-dimensional solid state NMR experiment is presented that correlates half-integer quadrupolar spins with protons. In this experiment the quadrupolar nuclei evolve during t1 under a split-t1, FAM-enhanced MQMAS pulse scheme. After each t1 period ending at the MQMAS echo position, single quantum magnetization is transferred, via a cross polarization process in the mixing time, from the quadrupolar nuclei to the protons. High-resolution proton signals are then detected in the t2 time domain during wPMLG5* homonuclear decoupling. The experiment has been demonstrated on a powder sample of sodium citrate and 23Na-1H 2D correlation spectra have been obtained. From the HETCOR spectra and the regular MQMAS spectrum, the three crystallographically inequivalent Na+ sites in the asymmetric unit were assigned. This MQMAS-wPMLG HETCOR pulse sequence can be used for spectral editing of half-integer quadrupolar nuclei coupled to protons.     

Perspectives of solution NMR spectroscopy for structural and functional studies of integral membrane proteins

This article discusses future perspectives of solution NMR spectroscopy to study structures and functions of integral membrane proteins at atomic resolution, based on a review of recent progress in this area. Several selected examples of structure determinations, as well as functional studies of integral membrane proteins are highlighted. We expect NMR spectroscopy to make future key scientific contributions to understanding membrane protein function, in particular for large membrane protein systems with known three-dimensional structure. Such situations can benefit from the fact that functional NMR studies have substantially less limitations by molecular size than a full de novo structure determination. Therefore, the general potential for NMR spectroscopy to solve biologic key questions associated with integral membrane proteins is very promising.     

Cross polarization, radio frequency field homogeneity, and circuit balancing in high field solid state NMR probes

Homogeneous radio frequency (RF) fields are important for sensitivity and efficiency of magnetization transfer in solid state NMR experiments. If the fields are inhomogeneous the cross polarization (CP) experiment transfers magnetization in only a thin slice of sample rather than throughout the entire volume. Asymmetric patterns have been observed in plots of the CP signal versus RF field mismatch for an 800 MHz solid-state NMR probe where each channel is resonated in a single-ended mode. A simple model of CP shows these patterns can be reproduced if the RF fields for the two nuclei are centered at different places in the coil. Experimental measurements using B1 field imaging, nutation arrays on extremely short NMR samples, and de-tuning experiments involving disks of copper incrementally moved through the coil support this model of spatially offset RF fields. We have found that resonating the high frequency channel in a double-ended or "balanced" mode can alleviate this field offset problem, and have implemented this in a three-channel solid state NMR probe of our own design.     

Double-quantum homonuclear NMR correlation spectroscopy of quadrupolar nuclei subjected to magic-angle spinning and high magnetic field

We present a new application of the symmetry-based dipolar recoupling scheme, for exciting directly double-quantum (2Q) coherences between the central transition of homonuclear half-integer quadrupolar nuclei. With respect to previously published 2Q-recoupling methods (M. Eden, D. Zhou, J. Yu, Chem. Phys. Lett. 431 (2006) 397), the sequence is used without π/2 bracketing pulses and with an original super-cycling. This leads to an improved efficiency (a factor of two for spin-5/2) and to a much higher robustness to radio-frequency field inhomogeneity and resonance offset. The 2Q-coherence excitation performances are demonstrated experimentally by ²⁷Al NMR experiments on the aluminophosphates berlinite, VPI5, AlPO₄-14, and AlPO₄-CJ3. The two-dimensional 2Q–1Q correlation experiments incorporating these recoupling sequences allow the observation of 2Q cross-peaks between central transitions, even at high magnetic field where the difference in offset between octahedral and tetrahedral ²⁷Al sites exceeds 10 kHz.     

Na solid state MAS NMR of sodium halides occluded in zeolites

Occlusion of sodium chloride and sodium bromide in zeolitic pores was performed by heating mixtures of the salts with zeolites NaY and NaA under high vacuum conditions. The obtained samples were subjected to various further pretreatments like washing with water and zinc-exchange, and were investigated spectroscopically with the ²³Na MAS NMR technique at various Zeeman field strengths. In the case of NaY, the halides are occluded in both types of cages of the faujasite structure. About 90% of the sodalite cages are shown to have incorporated salt which is concluded to be part of [Na₄Hal]³⁺ clusters as in the case of sodalite type materials.     

Sensitivity and resolution enhancement in solid-state NMR spectroscopy of bicelles

Magnetically aligned bicelles are becoming attractive model membranes to investigate the structure, dynamics, geometry, and interaction of membrane-associated peptides and proteins using solution- and solid-state NMR experiments. Recent studies have shown that bicelles are more suitable than mechanically aligned bilayers for multidimensional solid-state NMR experiments. In this work, we describe experimental aspects of the natural abundance (13)C and (14)N NMR spectroscopy of DMPC/DHPC bicelles. In particular, approaches to enhance the sensitivity and resolution and to quantify radio-frequency heating effects are presented. Sensitivity of (13)C detection using single pulse excitation, conventional cross-polarization (CP), ramp-CP, and NOE techniques are compared. Our results suggest that the proton decoupling efficiency of the FLOPSY pulse sequence is better than that of continuous wave decoupling, TPPM, SPINAL, and WALTZ sequences. A simple method of monitoring the water proton chemical shift is demonstrated for the measurement of sample temperature and calibration of the radio-frequency-induced heating in the sample. The possibility of using (14)N experiments on bicelles is also discussed.     

Precision and sensitivity optimization of quantitative measurements in solid state NMR

This work presents a methodology for optimizing the precision, accuracy and sensitivity of quantitative solid state NMR measurements based on the external reference method. It is shown that the sample must be exclusively located within and completely span the coil region where the NMR response is directly proportional to the sample amount. We describe two methods to determine this "quantitative" coil volume, based on whether the probe is equipped or not with a gradient coil. In addition, to improve the sensitivity and the accuracy, an optimum rotor packing design is described, which allows the sample volume of the rotor to be matched to the quantitative coil volume. Experiments conducted on adamantane and NaCl, which are representative of a soft and hard material, respectively, show that one order of magnitude increase in experimental precision can be achieved with this methodology. Interestingly, the precision can be further improved by using the ERETIC method in order to compensate for most instrumental instabilities.     

NMR and RAMAN studies of fluids at high pressure

Abstract

Selected results of the studies dealing with the motional dynamics of liquid 2-ethylhexyl cyclohexanecar-boxylate (EHC) and 2-ethylhexyl benzoate (EHB) illustrate the progress in instrumentation development for high-resolution, high-pressure NMR experiments on liquids.

A brief overview of recent laser Raman scattering studies of the collision induced scattering in simple polyatomic molecular fluids is also presented.     

穆斯堡尔谱学在固态物质研究上的发展与成就

1958年,29岁的德国物理学家穆斯堡尔发现γ射线的无反冲发射与共振吸收现象,即穆斯堡尔效应,随之产生了穆斯堡尔谱学,此人亦因该发现于1961年获得诺贝尔物理学奖.穆斯堡尔效应之发现造就了对于核力矩与电磁场间超精细相互作用一种全新的测试技术,其精度可至10-13数量级,从而可分析出固态物质的诸多微观性质.其后,穆斯堡尔谱学已广泛应用于物理、化学、冶金、地质、生物、考古等各领域.     

SPINEVOLUTION: a powerful tool for the simulation of solid and liquid state NMR experiments

Exact numerical simulations of NMR experiments are often required for the development of new techniques and for the extraction of structural and dynamic information from the spectra. Simulations of solid-state magic angle spinning (MAS) experiments can be particularly demanding both computationally and in terms of the programming required to carry them out, even if special simulation software is used. We recently developed a number of approaches that dramatically improve the efficiency and allow a high degree of automation of these computations. In the present paper, we describe SPINEVOLUTION, a highly optimized computer program that implements the new methodology. The algorithms used in the program will be described separately. Although particularly efficient for the simulation of experiments with complex pulse sequences and multi-spin systems in solids, SPINEVOLUTION is a versatile and easy to use tool for the simulation and optimization of virtually any NMR experiment. The performance of SPINEVOLUTION was compared with that of another recently developed NMR simulation package, SIMPSON. Benchmarked on a series of examples, SPINEVOLUTION was consistently found to be orders of magnitude faster. At the time of publication, the program is available gratis for non-commercial use.     

Selective refocusing pulses in magic-angle spinning NMR: characterization and applications to multi-dimensional protein spectroscopy

Band-selective pulses are frequently used in multi-dimensional NMR in solution, but have been used relatively less often in solid-state NMR applications because of the complications imposed by magic-angle spinning. In this work, we examine the frequency profiles and the refocusing efficiency of several commonly employed selective general rotation pi pulses through experiments and numerical simulations. We demonstrate that highly efficient refocusing of transverse magnetization can be achieved, with experiments that agree well with numerical simulations. We also show that the rotational echo is shifted by a half rotor period if a selective pulse is applied over an integer number of rotor periods. Appropriately synchronizing indirect evolution periods with selective pulses ensures proper phasing of cross peaks in 2D spectra. The improved performance of selective pulses in multi-dimensional protein spectroscopy is demonstrated on the 56-residue beta1 immunoglobulin binding domain of protein G (GB1).     

X-ray photoelectron and X-ray Auger electron spectroscopy studies of heavy ion irradiated C60 films

The influence of 200 MeV Au ion irradiation on the surface properties of polycrystalline fullerene films has been investigated. The X-ray photoelectron and X-ray Auger electron spectroscopies are employed to study the ion-induced modification of the fullerene, near the surface region. The shift of C 1s core level and decrease in intensity of shake-up satellite were used to investigate the structural changes (like sp² to sp³ conversion) and reduction of π electrons, respectively, under heavy ion irradiation. Further, X-ray Auger electron spectroscopy was employed to investigate hybridization conversion qualitatively as a function of ion fluence.     

Near-edge X-ray absorption fine structure spectroscopy and structural properties of Ni-doped CeO2 nanoparticles

Ni-doped CeO₂ nanoparticles were prepared by using the co-precipitation method. The prepared nanoparticles were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, field emission scanning electron microscopy (FE-SEM) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The XRD results infer that Ni-doped CeO₂ nanoparticles have single phase nature similar to that of pure CeO₂ nanoparticles. We have calculated lattice parameters using Powder-X software, particle size using Scherer’s formula and strain using the Williamson-Hall method for all the synthesized samples. We have observed a systematic decrease in the lattice parameters, particle size and strain with an increase in Ni doping in CeO₂. The FE-SEM micrographs also confirm that Ni-doped CeO₂ have nanocrystalline behavior and particles are spherical shaped. From the Raman spectra, it is observed that the intensity of classical CeO₂ vibration modes first increases then decreases with Ni doping. The NEXAFS spectra measured at Ce M_4,5 and Ni L_3,2 edges clearly indicate that Ce ions are in the +4 valence state and Ni ions are in the +2 valence state.     

Feasibility of arsenic and antimony NMR spectroscopy in solids: An investigation of some group 15 compounds

The feasibility of obtaining ⁷⁵As and ^121/123Sb NMR spectra for solids at high and moderate magnetic field strengths is explored. Arsenic-75 nuclear quadrupolar coupling constants and chemical shifts have been measured for arsenobetaine bromide and tetraphenylarsonium bromide. Similarly, ^121/123Sb NMR parameters have been measured for tetraphenylstibonium bromide and potassium hexahydroxoantimonate. The predicted pseudo-tetrahedral symmetry at arsenic and the known trigonal bipyramidal symmetry at antimony in their respective tetraphenyl-bromide “salts” are reflected in the measured ⁷⁵As and ¹²¹Sb nuclear quadrupole coupling constants, C_Q(⁷⁵As)=7.8 MHz and C_Q(¹²¹Sb)=159 MHz, respectively. Results of density functional theory quantum chemistry calculations for isolated molecules using ADF and first-principles calculations using CASTEP, a gauge-including projector augmented wave method to deal with the periodic nature of solids, are compared with experiment. Although the experiments can be time consuming, measurements of ⁷⁵As and ¹²¹Sb NMR spectra (at 154 and 215 MHz, respectively, i.e., at B₀=21.14 T) with linewidths in excess of 1 MHz are feasible using uniform broadband excitation shaped pulse techniques (e.g., WURST and WURST-QCPMG).     

Sensitivity and resolution in proton solid-state NMR at intermediate deuteration levels: quantitative linewidth characterization and applications to correlation spectroscopy

We present a systematic study of proton linewidths in rigid solids as a function of sample spinning frequency and proton density, with the latter controlled by the ratio of protonated and perdeuterated model compounds. We find that the linewidth correlates more closely with the overall proton density (rho(H)) than the size of local clusters of (1)H spins. At relatively high magic-angle spinning (MAS) rates, the linewidth depends linearly upon the inverse MAS rate. In the limit of infinite spinning rate and/or zero proton concentration, the linewidth extrapolates to a non-zero value, owing to contributions from scalar couplings, chemical shift dispersion, and B(0) field inhomogeneity. The slope of this line depends on the overall concentration of unexchangeable protons in the sample and the spinning rate. At up to 30% protonation levels ( approximately 2 (1)H/100A(3)), proton detection experiments are demonstrated to have a substantial (2- to 3-fold) sensitivity gain over corresponding (13)C-detected experiments. Within this range, the absolute sensitivity increases with protonation level; the optimal compromise between sensitivity and resolution is in the range of 20-30% protonation. We illustrate the use of dilute protons for polarization transfer to and from low-gamma spins within 5A, and to be utilized as both magnetization source and detection spins. The intermediate protonation regime enhances relaxation properties, which we expect will enable new types of (1)H correlation pulse sequences to be implemented with improved resolution and sensitivity.     

Studies on the structures and interactions of glutathione in aqueous solution by molecular dynamics simulations and NMR spectroscopy

All-atom molecular simulations and temperature-dependent NMR have been used to investigate the conformations and hydrogen bonds of glutathione (GSH) in aqueous solution. The simulations start from three different initial conformations. The properties are characterized by intramolecular distances, radius of gyration, root-mean-square deviation, and solvent-accessible surface. GSH is highly flexible in aqueous solutions in the simulations. Moreover, conformations can covert between “extended” and “folded” states. Interestingly, the two different hydrogen atoms in cysteine (H_N2) and glycin (H_N3) show different capabilities in forming NH?OW hydrogen bonds. The temperature-dependent NMR results of the different amide hydrogen atoms also show agreements with the MD simulations. Competing formation of GSH hydrogen-bonding interactions in aqueous solutions leads to hydrogen-bonding networks and the distribution of conformations. These changes will affect the activity of GSH under physiological conditions.     

Valence state alternation of copper species doped in HY zeolite as revealed by paramagnetic relaxation enhancement NMR spectroscopy

Paramagnetic relaxation enhancement (PRE) solid-state NMR (ssNMR) was used to monitor the valence state alternation of copper species doped in HY zeolite during catalytic reaction processes. The combination of PRE ssNMR and in-situ NMR spectroscopy facilitates the detection of copper species as well as the monitoring of evolution from reactants, intermediates to products in heterogeneously catalyzed processes, which is of great importance for elucidating the detailed catalytic reaction mechanism.