Conventional Nuclear Magnetic Resonance Methods Research Articles

Nuclear Magnetic Resonance Characterization of Petrophysical Properties in Tight Sandstone Reservoirs

AbstractPermeability and bound water saturation (Swb) are key parameters reflecting the petrophysical properties of porous rocks. Nuclear magnetic resonance (NMR) has proved to be effective in investigating the properties of porous media. However, estimating Swb and the permeability in tight sandstone reservoirs based on conventional NMR methods, which requires the inversion of NMR echo data to obtain the transverse relaxation time (T2) distribution, proves to be a challenging task. In this study, a method is proposed to estimate Swb and the permeability in tight sandstone reservoirs based on the direct analysis of NMR echo data, thus avoiding the inversion process. A total of 20 tight sandstone samples from the Ordos Basin in China was taken for laboratory NMR measurements. The kernel function of the echo data was used to characterize the ratio distribution of bound water volume to pore volume for different pore sizes. Following this, an echo data calibration method was applied to estimate Swb without the inversion of the T2 distribution, and the window method was used to reduce the impact of noise in the echo data. Furthermore, models for estimating permeability were proposed based on the determined windows of the NMR echo data in the Swb estimation. The reliability of the proposed method for estimating Swb and permeability was verified by comparing the estimated and experimental results. Our study provides an efficient method for the estimation of petrophysical parameters in porous rocks based on the direct analysis of NMR echo data.

Fast high-resolution nuclear magnetic resonance spectroscopy through indirect zero-quantum coherence detection in inhomogeneous fields

In many cases, high-resolution nuclear magnetic resonance (NMR) spectra are virtually impossible to obtain by conventional nuclear magnetic resonance methods because of inhomogeneity of magnetic field and inherent heterogeneity of sample. Although conventional intramolecular zero-quantum coherence (ZQC) can be used to obtain high-resolution spectrum in inhomogeneous field, the acquisition takes rather long time. In this paper, a spatially encoded intramolecular ZQC technique is proposed to fast acquire high-resolution NMR spectrum in inhomogeneous field. For the first time, the gradient-driven decoding technique is employed to selectively acquire intramolecular ZQC signals. Theoretical analyses and experimental observations demonstrate that high-resolution NMR spectral information can be retrieved within several scans even when the field inhomogeneity is severe enough to erase most spectral information. This work provides a new way to enhance the acquisition efficiency of high-resolution intramolecular ZQC spectroscopy in inhomogeneous fields.

Protein Structure Determination with Paramagnetic Solid-State NMR Spectroscopy

Many structures of the proteins and protein assemblies that play central roles in fundamental biological processes and disease pathogenesis are not readily accessible via the conventional techniques of single-crystal X-ray diffraction and solution-state nuclear magnetic resonance (NMR). On the other hand, many of these challenging biological systems are suitable targets for atomic-level structural and dynamic analysis by magic-angle spinning (MAS) solid-state NMR spectroscopy, a technique that has far less stringent limitations on the molecular size and crystalline state. Over the past decade, major advances in instrumentation and methodology have prompted rapid growth in the field of biological solid-state NMR. However, despite this progress, one challenge for the elucidation of three-dimensional (3D) protein structures via conventional MAS NMR methods is the relative lack of long-distance data. Specifically, extracting unambiguous interatomic distance restraints larger than ∼5 Å from through-space magnetic dipole-dipole couplings among the protein (1)H, (13)C, and (15)N nuclei has proven to be a considerable challenge for researchers. It is possible to circumvent this problem by extending the structural studies to include several analogs of the protein of interest, intentionally modified to contain covalently attached paramagnetic tags at selected sites. In these paramagnetic proteins, the hyperfine couplings between the nuclei and unpaired electrons can manifest themselves in NMR spectra in the form of relaxation enhancements of the nuclear spins that depend on the electron-nucleus distance. These effects can be significant for nuclei located up to ∼20 Å away from the paramagnetic center. In this Account, we discuss MAS NMR structural studies of nitroxide and EDTA-Cu(2+) labeled variants of a model 56 amino acid globular protein, B1 immunoglobulin-binding domain of protein G (GB1), in the microcrystalline solid phase. We used a set of six EDTA-Cu(2+)-tagged GB1 mutants to rapidly determine the global protein fold in a de novo fashion. Remarkably, these studies required quantitative measurements of only approximately four or five backbone amide (15)N longitudinal paramagnetic relaxation enhancements per residue, in the complete absence of the usual internuclear distance restraints. Importantly, this paramagnetic solid-state NMR methodology is general and can be directly applied to larger proteins and protein complexes for which a significant fraction of the signals can be assigned in standard 2D and 3D MAS NMR chemical shift correlation spectra.

CS23D: a web server for rapid protein structure generation using NMR chemical shifts and sequence data

CS23D (chemical shift to 3D structure) is a web server for rapidly generating accurate 3D protein structures using only assigned nuclear magnetic resonance (NMR) chemical shifts and sequence data as input. Unlike conventional NMR methods, CS23D requires no NOE and/or J-coupling data to perform its calculations. CS23D accepts chemical shift files in either SHIFTY or BMRB formats, and produces a set of PDB coordinates for the protein in about 10–15 min. CS23D uses a pipeline of several preexisting programs or servers to calculate the actual protein structure. Depending on the sequence similarity (or lack thereof) CS23D uses either (i) maximal subfragment assembly (a form of homology modeling), (ii) chemical shift threading or (iii) shift-aided de novo structure prediction (via Rosetta) followed by chemical shift refinement to generate and/or refine protein coordinates. Tests conducted on more than 100 proteins from the BioMagResBank indicate that CS23D converges (i.e. finds a solution) for >95% of protein queries. These chemical shift generated structures were found to be within 0.2–2.8 Å RMSD of the NMR structure generated using conventional NOE-base NMR methods or conventional X-ray methods. The performance of CS23D is dependent on the completeness of the chemical shift assignments and the similarity of the query protein to known 3D folds. CS23D is accessible at http://www.cs23d.ca.

Quantitative analysis using steady-state free precession nuclear magnetic resonance

The use of steady-state free precession nuclear magnetic resonance (NMR) for quantitative analysis in low magnetic field is investigated and shown to exhibit substantial advantages compared to more conventional NMR methods. With only minor additional requirements, the technique permits a considerable increase in signal-to-noise ratio for a given acquisition time. The experimental conditions needed for implementation and optimization of the acquisition parameters are explored and shown to be easily accessible with unsophisticated equipment. The applicability of the technique for quantitative analysis of samples with a range of relaxation rates is tested using various examples of practical interest. Highly reproducible results can be obtained much faster and without any special sample-dependent adjustments.

Nuclear magnetic resonance study of a high-surface-area aluminum phosphate glass and its thermally reversible sol-gel precursor

A novel reaction, based on sol-gel chemistry using an aluminum alkoxide and phosphoric acid to produce a high-surface-area aluminum phosphate glass, was followed by 27Al and 31P nuclear magnetic resonance (NMR). The gelled portion of the alcoholic solutions could be characterized with solid state 27Al methods. We discovered that the gelation, which is very rapid under normal conditions, can be controlled by the addition of HCl to the solution and that the gelation is thermally reversible in the presence of HCl. The NMR results showed that this modified sol-gel behavior is a result of acid hydrolysis of the AlP bonds, leading to network termination by POH groups and octahedrally solvated Al atoms. Similar local structures persist in the amorphous calcination product. The glass surface contains Al sites with strong quadrupole interaction. These sites could not be detected by conventional 27Al NMR methods but their existence was demonstrated with a static echo experiment. The quadrupole coupling constant was determined with the recently introduced transfer of populations in double resonance (TRAPDOR) method (a sol 1H 27Al double resonance magic-angle spinning experiment). The TRAPDOR results also showed that these sites, which constitute about 30% of the Al in the glass, carry hydroxyl groups. Characterization of the atomic structure of high-surface AlPO 4 is important for its use as catalyst support.

Two-dimensional exchange nuclear magnetic resonance of powder samples. IV. Distribution of correlation times and line shapes in the intermediate dynamic range

The two-dimensional (2D) exchange nuclear magnetic resonance (NMR) experiment is applied to study ultraslow as well as faster chain motions in amorphous polymers in the glass transition range. Acquisition of the time domain data with a four-pulse sequence leads to new characteristics in the corresponding 2H 2D line shapes if the correlation times of the motion are in the intermediate dynamic range. From the asymmetric 2D line shapes, the width of the correlation time distribution can be determined with higher accuracy than through conventional 1D NMR methods. Experimental data are presented on two amorphous polymers—atactic polypropylene and cis-1,4-polyisoprene—and are analyzed in terms of isotropic rotational diffusion. Deviations from this simple model due to the presence of conformational transitions within the polymer backbone are detected. Close to Tg, the mean correlation times extracted from 2D exchange NMR exhibit strongly nonArrhenius behavior usually described by the Williams–Landel–Ferry (WLF) equation. In addition, the width of the correlation time distribution is found to decrease with increasing temperature.

High-resolution proton and carbon-13 NMR of membranes: why sonicate?

We have obtained high-field (11.7-T) proton and carbon-13 Fourier transform (FT) nuclear magnetic resonance (NMR) spectra of egg lecithin and egg lecithin-cholesterol (1:1) multibilayers, using "magic-angle" sample spinning (MASS) techniques, and sonicated egg lecithin and egg lecithin-cholesterol (1:1) vesicles, using conventional FT NMR methods. Resolution of the proton and carbon-13 MASS NMR spectra of the pure egg lecithin samples is essentially identical with that of sonicated samples, but spectra of the unsonicated lipid, using MASS, can be obtained very much faster than with the more dilute, sonicated systems. With the 1:1 lecithin-cholesterol systems, proton MASS NMR spectra are virtually identical with conventional FT spectra of sonicated samples, while with 13C NMR, we demonstrate that most 13C nuclei in the cholesterol moiety can be monitored, even though these same nuclei are essentially invisible, i.e., are severely broadened, in the corresponding sonicated systems. In addition, 13C MASS NMR, spectra can again be recorded much faster than with sonicated samples, due to concentration effects. Taken together, these results strongly suggest there will seldom be need in the future to resort to ultrasonic disruption of lipid bilayer membranes in order to obtain high-resolution proton or carbon-13 NMR spectra.

Use of fully deuterated micelles for conformational studies of membrane proteins by high resolution 1H nuclear magnetic resonance

Micellar complexes of melittin with fully deuterated detergents have been studied by high resolution 1H nuclear magnetic resonance (NMR). The synthesis of deuterated micelles is described and it is shown that the 1H NMR spectrum of micelle-bound melittin is well resolved and suitable for detailed analysis by conventional high-resolution NMR methods. A preliminary characterization of micelle-bound melittin shows that interaction with the micelle results in different conformational and dynamic features for the hydrophobic and hydrophilic regions of the melittin amino acid sequence. The present experiments on melittin and preliminary results with other polypeptides and proteins demonstrate that in favourable cases high-resolution 1H NMR studies of the complexes formed between membrane proteins and deuterated micelles provides a viable method for conformational studies of membrane-bound proteins.