J-modulation effects in DOSY experiments and their suppression: the Oneshot45 experiment

Diffusion-ordered spectroscopy (DOSY) is a powerful NMR method for identifying compounds in mixtures. DOSY experiments are very demanding of spectral quality; even small deviations from expected behaviour in NMR signals can cause significant distortions in the diffusion domain. This is a particular problem when signals overlap, so it is very important to be able to acquire clean data with as little overlap as possible. DOSY experiments all suffer to a greater or lesser extent from multiplet phase distortions caused by J-modulation, requiring a trade-off between such distortions and gradient pulse width. Multiplet distortions increase spectral overlap and may cause unexpected and misleading apparent diffusion coefficients in DOSY spectra. These effects are described here and a simple and effective remedy, the addition of a 45° purging pulse immediately before the onset of acquisition to remove the unwanted anti-phase terms, is demonstrated. As well as affording significantly cleaner results, the new method allows much longer diffusion-encoding pulses to be used without problems from J-modulation, and hence greatly increases the range of molecular sizes that can be studied for coupled spin systems. The sensitivity loss is negligible and the added phase cycling is modest. The new method is illustrated for a widely-used general purpose DOSY pulse sequence, Oneshot.     

High resolution 2D 1H-13C correlation of cholesterol in model membrane

High resolution 2D NMR MAS spectra of liposomes, in particular 1H-13C chemical shifts correlations have been obtained on fluid lipid bilayers made of pure phospholipids for several years. We have investigated herein the possibility to obtain high resolution 2D MAS spectra of cholesterol embedded in membranes, i.e. on a rigid molecule whose dynamics is characterized mainly by axial diffusion without internal segmental mobility. The efficiency of various pulse sequences for heteronuclear HETCOR has been compared in terms of resolution, sensitivity and selectivity, using either cross polarization or INEPT for coherence transfer, and with or without MREV-8 homonuclear decoupling during t1. At moderately high spinning speed (9 kHz), a similar resolution is obtained in all cases (0.2 ppm for 1H(3,4), 0.15 ppm for 13C(3,4) cholesterol resonances), while sensitivity increases in the order: INEPT < CP(x4) < CP + MREV. At reduced spinning speed (5 kHz), the homonuclear dipolar coupling between the two geminal protons attached to C(4) gives rise to spinning sidebands from which one can estimate a H-H dipolar coupling of 10 kHz which is in good agreement with the known dynamics of cholesterol in membranes.     

Improved resolution in (13)C solid-state spectra through spin-state-selection

The application of a spin-state-selective coherence transfer experiment (INADEQUATE-SSS) to solid-state NMR spectroscopy is described. Two-dimensional (13)C double-quantum/single-quantum spectra without J splittings in both dimensions lead to enhanced spectral resolution. The method is demonstrated to significantly improve the spectral resolution of the crowded C'-C(alpha) region of two proteins.     

The 2D MAS NMR spin-echo experiment: the determination of 13C-13C J couplings in a solid-state cellulose sample

A simple 13C MAS spin-echo experiment is applied to a partially 13C-labelled cellulose sample extracted from wood. 13C-13C J couplings are determined even though considerable chemical disorder leads to observed linewidths in the normal 1D 13C CP MAS spectrum which far exceed the J couplings. The fitting of the experimental data also allows the quantification of the degree of isotopic enrichment.     

Compressed sensing for resolution enhancement of hyperpolarized 13C flyback 3D-MRSI

High polarization of nuclear spins in liquid state through dynamic nuclear polarization has enabled the direct monitoring of 13C metabolites in vivo at very high signal-to-noise, allowing for rapid assessment of tissue metabolism. The abundant SNR afforded by this hyperpolarization technique makes high-resolution 13C 3D-MRSI feasible. However, the number of phase encodes that can be fit into the short acquisition time for hyperpolarized imaging limits spatial coverage and resolution. To take advantage of the high SNR available from hyperpolarization, we have applied compressed sensing to achieve a factor of 2 enhancement in spatial resolution without increasing acquisition time or decreasing coverage. In this paper, the design and testing of compressed sensing suited for a flyback 13C 3D-MRSI sequence are presented. The key to this design was the undersampling of spectral k-space using a novel blipped scheme, thus taking advantage of the considerable sparsity in typical hyperpolarized 13C spectra. Phantom tests validated the accuracy of the compressed sensing approach and initial mouse experiments demonstrated in vivo feasibility.     

Two-dimensional DOSY experiment with Excitation Sculpting water suppression for the analysis of natural and biological media

The Bipolar Pulse Pair Stimulated Echo NMR pulse sequence was modified to blend the original Excitation Sculpting water signal suppression. The sequence is a powerful tool to generate rapidly, with a good spectrum quality, bidimensional DOSY experiments without solvent signal, thus allowing the analysis of complex mixtures such as plant extracts or biofluids. The sequence has also been successfully implemented for a protein at very-low concentration in interaction with a small ligand, namely the salivary IB5 protein binding the polyphenol epigallocatechine gallate. The artifacts created by this sequence can be observed, checked and removed thanks to NPK and NMRnotebook softwares to give a perfect bidimensional DOSY spectrum.     

Applications of DOSY NMR in the Chemical IndustryApplications of DOSY NMR in the Chemical Industry

DOSY(diffusion ordered spectroscopy)NMR is a technique that separates NMR signals of different species according to their diffusion coefficients.It is a powerful technique to provide solid characterization evidence for various applications in the chemical industry.Encapsulation and functional polymer grafting are two important capabilities in the chemical industry for new product development with challenging characterization requirements.Model systems of encapsulation and grafted polymer were studied and the characterization methods were set up by DOSY NMR.     

High resolution 13C NMR in undoped cis- and trans-polyacetylene

We have applied ¹³C cross-polarization (CP) powder- and MAS-NMR to cis- and trans-polyacetylene. All three elements of the chemical shift tensor (σ₁₁, σ₂₂, σ₃₃) were determined in p.p.m. with respect to TMS as: (a) Cis; σ₁₁ = ?228, σ₂₂ = ?139, σ₃₃ = ?17; (b) Trans; σ₁₁ = ?234, σ₂₂ = ?146, σ₃₃ = ?34.     

High resolution infrared spectra of carbon disulfide: 13CS2

By using a “dual-grille” spectrometer, twelve transitions of ¹³CS₂ have been observed between 1400 and 3000 cm^?1. The position of eleven vibration levels have been fixed accurately. The molecular constants of ¹³CS₂ determined by vibrational and vibrorotational analysis are in good agreement with the constants issued from the ¹²CS₂ analysis.     

13C-1H HSQC experiment of probe molecules aligned in thermotropic liquid crystals: sensitivity and resolution enhancement in the indirect dimension

The spectra of molecules oriented in liquid crystalline media are dominated by partially averaged dipolar couplings. In the 13C-1H HSQC, due to the inefficient hetero-nuclear dipolar decoupling in the indirect dimension, normally carried out by using a pi pulse, there is a considerable loss of resolution. Furthermore, in such strongly orienting media the 1H-1H and 13C-1H dipolar couplings leads to fast dephasing of transverse magnetization causing inefficient polarization transfer and hence the loss of sensitivity in the indirect dimension. In this study we have carried out 13C-1H HSQC experiment with efficient polarization transfer from 1H to 13C for molecules aligned in liquid crystalline media. The homonuclear dipolar decoupling using FFLG during the INEPT transfer delays and also during evolution period combined with the pi pulse heteronuclear decoupling in the t1 period has been applied. The studies showed a significant reduction in partially averaged dipolar couplings and thereby enhancement in the resolution and sensitivity in the indirect dimension. This has been demonstrated on pyridazine and pyrimidine oriented in the liquid crystal. The two closely resonating carbons in pyrimidine are better resolved in the present study compared to the earlier work [H.S. Vinay Deepak, Anu Joy, N. Suryaprakash, Determination of natural abundance 15N-1H and 13C-1H dipolar couplings of molecules in a strongly orienting media using two-dimensional inverse experiments, Magn. Reson. Chem. 44 (2006) 553-565].     

Imaging of the (1)H NMR second moment with (13)C chemical-shift resolution

A method of (13)C chemical-shift-resolved (1)H second moment imaging is proposed for molecular mobility imaging of heterogeneous materials. For evaluating the (1)H second moment, the method relies on the curve fitting procedure using spin-echo shapes indirectly: The information of (1)H echo shapes is transferred to the (13)C signal amplitude through (1)H-(13)C cross polarization and then the curve fitting is made using the (13)C signal amplitude. The (13)C signal is detected under (1)H dipolar decoupling and magic angle spinning, resulting in the incorporation of (13)C chemical-shift resolution. Imaging information is included in the (13)C signal by application of phase-encoding gradients. The second moment images obtained can reflect the molecular mobility at every molecular site separated by (13)C chemical shifts, yielding detailed information on the molecular mobility. The method is demonstrated by spatially 1D experiments performed on a model sample. Copyright 2000 Academic Press.     

Detection of natural abundance 1H-13C correlations of cholesterol in its membrane environment using a gradient enhanced HSQC experiment under high resolution magic angle spinning

The quality and signal to noise ratio of a J-based HETCOR performed on a standard MAS probe have been compared with a gradient enhanced HSQC performed on a HR-MAS probe at 500 MHz. The sample selected was cholesterol, inserted at 30 mol% in acyl chain deuterated phospholipids (DMPC-d54), at a temperature where the bilayer is in a liquid crystalline phase (310 K). It is representative of any rigid molecule undergoing fast axial diffusion in a bilayer as the main movement. After optimization of the spinning rate and carbon decoupling conditions, it is shown that the ge-HSQC/MAS approach is far superior to the more conventional J-HETCOR/MAS in terms of signal to noise ratio, and that it allows the detection of all the natural abundance cross peaks of cholesterol in a membrane environment. Clear differences between the 1H and 13C chemical shifts of cholesterol in a membrane and in chloroform solution were thus revealed.     

High resolution C nuclear magnetic resonance spectra for symmetrical orthodihalobenzenes including long-range CH couplings

Analysis of high resolution ¹³C NMR spectra for symmetrical orthodihalobenzenes have provided all long-range ¹³CH coupling values in orthodichloro-, dibromo-, and diiodobenzene. Furthermore, since the analyses were sensitive to the sign of the coupling constants, the relative signs of these long-range ¹³CH couplings have been determined from unique spectral fits. The substituent effects on the chemical shifts in this series of compounds appear to be additive. The ¹³CH couplings are compared with coupling values in other compounds and are shown to be related to substituent electronegativity. The absolute magnitude of the ¹³CH couplings for these halogen-substituted compounds are larger than those observed in benzene, with but one exception. In all cases, three-bonded ¹³CH couplings are found to be larger than the two-bond values.     

The 13C2 and 12C13C Ballik-Ramsay system

The infrared spectrum of the b³Σ⁻ → a³Π Ballik-Ramsay system has been observed for both ¹³C₂ and ¹²C¹³C isotopic species using high-resolution Fourier transform spectroscopy. Twelve bands: 0 → 0, 1 → 0, 1 → 1, 2 → 0, 2 → 1, 3 → 1, 3 → 2, 4 → 2, 4 → 3, 5 → 2, 5 → 3, 6 → 3 of ¹³C₂ and seven bands: 0 → 0, 1 → 0, 1 → 1, 2 → 0, 2 → 1, 3 → 1, 3 → 2 of ¹²C¹³C were identified in the spectral range 3500–12 000 cm⁻¹. Perturbations in the b³Σ⁻ (v = 0, 1, 2, 3) levels were reduced and accurate perturbation parameters derived from the analysis. A comparison with ab initio calculated spin-orbit coupling constants is presented.     

High resolution digital holography

In digital Fresnel holography, present specifications of charge-coupled device cameras require the incident beams to be quasi-parallel. That implies large speckle grain size and low lateral resolution in reconstructed images. Better lateral resolutions are demonstrated in our work, down to 8 μm, allowing the observation of sub-millimetre objects by digital holography. The experimental set-up built samples the incident light distribution with a definition of 500 pixels mm⁻¹. The maximum acceptable angle is then widened and the hologram recording and reconstruction distances are drastically reduced, as well as the speckle size. Interferometric holography was implemented for the measurement of the deformation of a sub-millimetre silicon cantilever and results in phase-contrasts imaging are also reported. The design of dedicated complementary metal-oxide-semiconductor active pixel sensor cameras is also discussed.     

Processing DOSY spectra using the regularized resolvent transform

A new method for processing diffusion ordered spectroscopy (DOSY) data is presented. This method, the regularized resolvent transform (iRRT-the i denoting the adaptation of the method to evaluate the inverse Laplace transform), is better than conventional processing techniques for generating 2D DOSY spectra using data that has poor chemical shift resolution. From the same data, it is possible to use the iRRT to generate 1D subspectra corresponding to different components of the sample mixture; these subspectra compare favorably to 1D spectra of the pure substances. Both the 2D spectra and the 1D subspectra offer a vast improvement over results generated using a conventional processing technique (non-linear least-squares fitting). Consequently, we present the iRRT as a stable and reliable tool for solving the inverse Laplace transform problem present in experiments such as DOSY.     

Improved DOSY NMR data processing by data enhancement and combination of multivariate curve resolution with non-linear least square fitting

The quality of DOSY NMR data can be improved by careful pre-processing techniques. Baseline drift, peak shift, and phase shift commonly exist in real-world DOSY NMR data. These phenomena seriously hinder the data analysis and should be removed as much as possible. In this paper, a series of preprocessing operations are proposed so that the subsequent multivariate curve resolution can yield optimal results. First, the baseline is corrected according to a method by Golotvin and Williams. Next, frequency and phase shift are removed by a new combination of reference deconvolution (FIDDLE), and a method presented by Witjes et al. that can correct several spectra simultaneously. The corrected data are analysed by the combination of multivariate curve resolution with non-linear least square regression (MCR-NLR). The MCR-NLR method turns out to be more robust and leads to better resolution of the pure components than classic MCR.     

High resolution photoabsorption spectroscopy at the carbonK-Edge

Using plane grating, ellipsoidal mirror, grazing incidence monochromators at the storage ring BESSY, a resolution of 150meV was achieved for a photon energy of 285 eV. This high energy resolution considerably extends the range of possible studies using core level Spectroscopy. Some of the new opportunities are illustrated at the CK edge by resolving the vibrational fine structure of condensed C₂H₄ multilayers in the (1s^–1, ^*) state by means of photoabsorption spectroscopy. For the sake of comparison with other high resolution instruments, the vibrational fine structure of condensed N₂ multilayers at the NK edge ( 400eV) was also measured, yielding a resolving power of 3000.