Analysis of multisite 2D relaxation exchange NMR (original) (raw)
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Spatially-resolved 1H NMR relaxation-exchange measurements in heterogeneous media
Journal of Magnetic Resonance, 2019
In the last decades, the 1 H NMR relaxation-exchange (REXSY) technique has become an essential tool for the molecular investigation of simple and complex fluids in heterogeneous porous solids and soft matter, where the mixing-time-evolution of crosscorrelated peaks enables a quantitative study of diffusive exchange kinetics in multicomponent systems. Here, we present a spatially-resolved implementation of the correlation technique, named , based on one-dimensional spatial mapping along using a rapid frequency-encode imaging scheme. Compared to other phase-encoding methods, the adopted MRI technique has two distinct advantages: (i) is has the same experimental duration of a standard (bulk) measurement, and (ii) it provides a high spatial resolution. The proposed method is first validated against bulk measurements on homogeneous phantom consisting of cyclohexane uniformly imbibed in finely-sized α-Al 2 O 3 particles at a spatial resolution of 0.47 mm; thereafter, its performance is demonstrated, on a layered bed of multi-sized α-Al 2 O 3 particles, for revealing spatiallydependent molecular exchange kinetics properties of intra-and inter-particle cyclohexane as a function of particle size. It is found that localised spectra provide well resolved cross peaks whilst such resolution is lost in standard bulk data. Future prospective applications of the method lie, in particular, in the local characterisation of mass transport phenomena in multi-component porous media, such as rock cores and heterogeneous catalysts.
The Journal of Physical Chemistry A, 1997
We present a new method, the least-error matrix analysis (LEMA), to quantify the dynamic matrix from a series of 2D NMR exchange spectra. The method is based on a weighted averaging of individual dynamic matrices. The matrices are obtained by full-matrix analysis (FMA) from a series of 2D exchange spectra recorded at different mixing times. The weights, calculated by error propagation analysis, are explicit functions of the mixing time. The principal advantage of LEMA in comparison to FMA is that it uses all the known relationships between the spectral peaks: the peak correlations within 2D spectra, and the mixing time evolution among the spectra. We tested LEMA by analyzing a series of 10 cross-relaxation spectra (NOESY, τ m ) 60 µs-1.28 s) in a rigid 10-spin system (cyclo(L-Pro-Gly) in 3:1 v/v H 2 O/DMSO). At 233 K the dipeptide has a mobility like a small protein with a correlation time of 3.8 ns. While FMA at τ m ) 30 ms could extract only 14 distances in a range 1.75-3 Å, LEMA provided 22 distances, of which the longest was 4 Å. The extension of the available interproton distances from 3 to 4 Å afforded by LEMA is caused by a 10-fold decrease of the lower limit of measurable cross-relaxation rates, from -0.59 to -0.06 s -1 . The most important property of LEMA, provision of accurate average values of magnetization exchange rates from a given set of peak volumes, is verified experimentally on a model system.
Journal of The American Chemical Society, 1993
A pulse sequence for a two-dimensional inverse Laplace transform N M R experiment is proposed and demonstrated. The experiment is analogous to the two-dimensional Fourier transform protocol called EXSY, but detects exchange by monitoring alterations in the transverse relaxation time rather than the N M R frequency. The sequence may be useful for measurement of exchange and diffusion of water in vivo and for detecting slow exchange phenomena in glassy polymers.
Relaxation Exchange in Nanoporous Silica by Low-Field NMR
nano Online, 2016
Investigating the 2D-T 2-T 2-relaxation exchange of interstitial water in a packing of sedimented Stöber-silicate spheres, we come the conclusion that contrary to its behaviour in macro-pores, water confined in nano-pores of silica exhibits enhanced diffusivity. The 2D-experiments, performed at different temperatures, reveal a temperature-dependent bimodal relaxation distribution and two-site relaxation exchange. Our recently introduced kinetic multi-site exchange model is applied to derive the according exchange rates. The resulting Arrhenius plot produces an exchange activation energy of 7 kJ/mol, which is well below the hydrogen bond energy or the activation energy for self-diffusion of water in the bulk. A possible hopping-mechanism as the source of enhanced proton-diffusion in nanoporous silica is discussed, as well as its significance to mass transfer in porous media.
Monte-Carlo Simulations of the Two-Dimensional NMR T2-T2 Exchange of Fluids in Porous Media
2009
The effects of molecular exchange processes on the two-dimensional (2D) NMR T2-T2 distributions obtained by Laplace inversion were studied by numerical simulations. The Monte–Carlo technique is used to generate free random walks of a large number of molecules within space regions characterized by different relaxation times. Molecular exchange processes are considered during CPMG encoding periods as well as during the storage period, τstore. Systematic simulations were performed as function of NMR parameters like the storage period, τstore and geometric or physical system properties.
Journal of Magnetic Resonance, 2016
We investigate a way one can achieve good spectral resolution in 2 H MAS NMR experiments. The goal is to be able to distinguish between and study sites in various deuterated materials with small chemical shift dispersion. We show that the 2 H MAS NMR spectra recorded during a spin-relaxation experiment are amenable to spectral decomposition because of the different evolution of spectral components during the relaxation delay. We verify that the results are robust by global least-square fitting of the spectral series both under the assumption of specific line shapes and without such assumptions (COmponent-REsolved spectroscopy, CORE). In addition, we investigate the reliability of the developed protocol by analyzing spectra simulated with different combinations of spectral parameters. The performance is demonstrated in a model material of deuterated poly(methacrylic acid) that contains two 2 H spin populations with similar chemical shifts but different quadrupole splittings. In 2 H-exchanged cellulose containing two 2 H spin populations with very similar chemical shifts and quadrupole splittings, the method provides new site-selective information about the molecular dynamics.
A Markov Model for Relaxation and Exchange in NMR Spectroscopy
The Journal of Physical Chemistry B, 2005
A two-state Markov noise process for lattice fluctuations and chemical exchange dynamics is used to derive a stochastic Liouville equation describing the evolution of the spin-density operator in nuclear magnetic resonance spectroscopy. Relaxation through lattice fluctuations and chemical exchange processes is incorporated into the theory at the same fundamental level, and the results are valid for all time scales provided that lattice fluctuations are much faster than chemical exchange kinetics. Timescale separation emerges as an essential feature from the lowest-order perturbation expansion of the average resolvent in the Laplace domain.
The effect of spatial variation in surface relaxivity on nuclear magnetic resonance relaxation rates
GEOPHYSICS, 2012
Nuclear magnetic resonance (NMR) relaxation measurements are sensitive to the physiochemical environment of water in saturated porous media and can provide information about the properties of geologic material. Interpretation of NMR data typically relies on three assumptions: that pores within the geologic material are effectively isolated such that the diffusion of a proton between pores is limited (i.e., there is weak coupling); that relaxation occurs in the fast-diffusion regime; and that surface relaxivity [Formula: see text] is uniform throughout the measured volume. We investigated the effect of spatial variation in [Formula: see text] on the NMR relaxation measurement and evaluated two equations relating [Formula: see text] to the NMR relaxation rate for samples containing two types of surfaces, each with a different surface relaxivity. One equation was valid when there is weak diffusional coupling between pores, the other is valid when there is strong diffusional coupling. W...
Journal of the American Chemical Society, 2012
A Carr−Purcell−Meiboom−Gill relaxation dispersion experiment is presented for quantifying millisecond time-scale chemical exchange at side-chain 1 H positions in proteins. Such experiments are not possible in a fully protonated molecule because of magnetization evolution from homonuclear scalar couplings that interferes with the extraction of accurate transverse relaxation rates. It is shown, however, that by using a labeling strategy whereby proteins are produced using { 13 C, 1 H}-glucose and D 2 O a significant number of 'isolated' side-chain 1 H spins are generated, eliminating such effects. It thus becomes possible to record 1 H dispersion profiles at the β positions of Asx, Cys, Ser, His, Phe, Tyr, and Trp as well as the γ positions of Glx, in addition to the methyl side-chain moieties. This brings the total of amino acid side-chain positions that can be simultaneously probed using a single 1 H dispersion experiment to 16. The utility of the approach is demonstrated with an application to the four-helix bundle colicin E7 immunity protein, Im7, which folds via a partially structured low populated intermediate that interconverts with the folded, ground state on the millisecond time-scale. The extracted 1 H chemical shift differences at side-chain positions provide valuable restraints in structural studies of invisible, excited states, complementing backbone chemical shifts that are available from existing relaxation dispersion experiments.