Gradients For Coherence Selection Research Articles

Single-shot single-voxel lactate measurements using FOCI-LASER and a multiple-quantum filter.

Measurement of tissue lactate using 1H MRS is often confounded by overlap with intense lipid signals at 1.3 ppm. Single‐voxel localization using PRESS is also compromised by the large chemical shift displacement between voxels for the 4.1 ppm (–CH) resonance and the 1.3 ppm –CH3 resonance, leading to subvoxels with signals of opposite phase and hence partial signal cancellation. To reduce the chemical shift displacement to negligible proportions, a modified semi‐LASER sequence was written (“FOCI‐LASER”, abbreviated as fLASER) using FOCI pulses to permit high RF bandwidth even with the limited RF amplitude characteristic of clinical MRI scanners. A further modification, MQF‐fLASER, includes a selective multiple‐quantum filter to detect lactate and reject lipid signals.The sequences were implemented on a Philips 3 T Achieva TX system. In a solution of brain metabolites fLASER lactate signals were 2.7 times those of PRESS. MQF‐fLASER lactate was 47% of fLASER (the theoretical maximum is 50%) but still larger than PRESS lactate. In oil, the main 1.3 ppm lipid peak was suppressed to less than 1%. Enhanced suppression was possible using increased gradient durations. The minimum detectable lactate concentration was approximately 0.5 mM. Coherence selection gradients needed to be at the magic angle to avoid large water signals derived from intermolecular multiple‐quantum coherences. In pilot patient measurements, lactate peaks were often observed in brain tumours, but not in cervix tumours; lipids were effectively suppressed.In summary, compared with PRESS, the fLASER sequence yields greatly superior sensitivity for direct detection of lactate (and equivalent sensitivity for other metabolites), while the single‐voxel single‐shot MQF‐fLASER sequence surpasses PRESS for lactate detection while eliminating substantial signals from lipids. This sequence will increase the potential for in vivo lactate measurement as a biomarker in targeted anti‐cancer treatments as well as in measurements of tissue hypoxia. Copyright © 2015 John Wiley & Sons, Ltd.

Simulation of coherence selection by pulsed field gradients in liquid-state NMR using an auxiliary matrix formalism

An algorithm for simulating coherence selection due to a pulse sequence element consisting of two pulsed field gradients separated by a short collection of pulses and delays is introduced. This algorithm involves computation of the matrix exponential of an auxiliary matrix twice the size of the system Liouvillian, a dimensional increase smaller than is required with other known computational methods. Approximations valid for most simulations of liquid-state NMR spectra are involved in the derivation. Diffusion is omitted, but could be treated in an approximate way as a damping over the pulse sequence element. Several NMR pulse sequences using gradients for coherence selection have been implemented, making use of the functionality of Spinach (http://spindynamics.org/Spinach.php). Example simulations testing these implementations are presented, and the extent to which the formal dimensional reduction can lead to a speedup in simulation time discussed. It is found that the previously known methods can be made competitive with the auxiliary matrix method by making use of their embarrassingly parallel nature. Cases where the relative dimensional reduction of the auxiliary matrix method is very large, or where efficient parallelization of the simulation independent of the nature of the algorithm used exists, are found to lead to situations beneficial for the auxiliary matrix algorithm in this comparison.

Intermolecular double-quantum coherence imaging without coherence selection gradients and its application in in vivo MRI

In the COSY Revamped with Asymmetric Z-gradient Echo Detection (CRAZED) experiments, magnetization is modulated by the distant dipolar field (DDF) generated by coherence selection gradient (CSG) commonly in sinusoidal wave-form and results in detectable intermolecular multiple-quantum coherence (iMQC) signal. IMQCs have some attractive features, but their intrinsic weak signal intensity prevents their widespread applications. In this paper, a new phase cycling scheme was applied to obtain intermolecular double-quantum coherence (iDQC) signal. It is found that DDF can arise from nonspherical sample geometry or background inhomogeneous field in the absence of CSGs, which is more efficient than that created from CSGs. The experimental results show that the resulting DDF can refocus the ±iDQC signals simultaneously and thus enhance the signal intensity to about two folds of that from the conventional CRAZED sequence. Theoretical prediction and experiments give coincident results.

HyperSPASM NMR: A new approach to single-shot 2D correlations on DNP-enhanced samples

Dissolution DNP experiments are limited to a single or at most a few scans, before the non-Boltzmann magnetization has been consumed. This makes it impractical to record 2D NMR data by conventional, t1-incremented schemes. Here a new approach termed HyperSPASM to establish 2D heteronuclear correlations in a single scan is reported, aimed at dealing with this kind of challenge. The HyperSPASM experiment relies on imposing an amplitude-modulation of the data by a single Δt1 indirect-domain evolution time, and subsequently monitoring the imparted encoding on separate echo and anti-echo pathway signals within a single continuous acquisition. This is implemented via the use of alternating, switching, coherence selection gradients. As a result of these manipulations the phase imparted by a heteronucleus over its indirect domain evolution can be accurately extracted, and 2D data unambiguously reconstructed with a single-shot excitation. The nature of this sequence makes the resulting experiment particularly well suited for collecting indirectly-detected HSQC data on hyperpolarized samples. The potential of the ensuing HyperSPASM method is exemplified with natural-abundance hyperpolarized correlations on model systems.

Apparent diffusion behaviors of spins in the presence of distant dipolar field in two-component solution NMR

The diffusion behaviors of spins in the presence of distant dipolar field in two-component spin systems during the second evolution period of a modified CRAZED sequence before acquisition were investigated. Theoretical formulas were deduced based on the distant dipolar field model. The simulation results and experimental observations are consistent with the theoretical predictions. This study shows that the relative intensities of signals from intermolecular zero-quantum coherences (iZQCs) and intermolecular double-quantum coherences (iDQCs) have the same diffusion attenuation characteristic under the combined effect of diffusion weighting gradients and distant dipolar field during the second evolution period. This diffusion attenuation may be different from that of conventional single-quantum coherence signal, depending on the relative orientation of the diffusion weighting gradients to the coherence selection gradients. The results presented herein are helpful for understanding the effect of distant dipolar field from a spin system on the diffusion behavior of other spin system and the signal properties in the iZQC or iDQC magnetic resonance imaging.

2D-NMR reveals different populations of exposed lysine residues in the apoB-100 protein of electronegative and electropositive fractions of LDL particles

Several potentially atherogenic LDL subfractions present low affinity for the LDL receptor, which result in impaired plasma clearance. Electronegative LDL [LDL(-)] is one of these minor subfractions and the molecular basis for its reduced receptor affinity is not well understood. In the present study, high-resolution 2D-NMR spectroscopy has been employed to characterize the surface-exposed lysine residues of the apolipoprotein (apo)B-100 protein in both LDL(-) and LDL(+) subfractions. LDL(+) showed two populations of lysine residues, similar to those previously described in total LDL. "Normal" Lys have a pk(a) of 10.4 whereas "active" Lys have a pk(a) of 8.8 and have been suggested to be involved in receptor binding. In contrast to LDL(+), the LDL(-) subfraction presented a third type of Lys, named as "intermediate" Lys, with a different microenvironment and higher basicity (pk(a) 10.7). These intermediate Lys cannot be reliably identified by 1D-NMR. Because the abundance of normal Lys is similar in LDL(+) and LDL(-), the intermediate Lys in the apoB-100 molecule of LDL(-) should come from a group of active Lys in LDL(+) particles that have a less basic microenvironment in the LDL(-) particle. These differences between LDL(+) and LDL(-) are indicative of a distinct conformation of apoB-100 that could be related to loss of affinity of LDL(-) for the LDL receptor.

Apparent diffusion behaviour of intermolecular double-quantum coherence modulated by a distant dipolar field in solution NMR

A modified correlated spectroscopy (COSY) revamped with asymmetric Z-gradient echo detection sequence was designed to investigate the influence of diffusion behaviour on intermolecular double-quantum coherence signal attenuation during the pre-acquisition period. Theoretical formulas were deduced and experimental measurements and simulations were performed. It is found that the diffusion behaviour of intermolecular double-quantum coherence in the pre-acquisition period may be different from that of conventional single-quantum coherence, depending on the relative orientation of diffusion weighting gradients to coherence selection gradients. When the orientation of the diffusion weighting gradients is parallel or anti-parallel to the orientation of the coherence selection gradients, the diffusion is modulated by the distant dipolar field. This study is helpful for understanding the signal properties in intermolecular double-quantum coherence magnetic resonance imaging.

High-resolution NMR spectra in inhomogeneous fields utilizing the CRAZED sequence without coherence selection gradients

Coherence selection gradients have been considered as indispensable for high-resolution NMR spectroscopy in inhomogeneous fields utilizing the CRAZED-type sequences. However, our experimental results demonstrate that these gradients can be omitted if an appropriate phase cycling is applied. The measured linewidth of reconstructing 1D high-resolution spectral peaks does not depend on the dipolar correlation distance determined by the coherence selection gradients, but is only affected by diffusion and T 2 relaxation. This finding suggests the need to reconsider the mechanism for the iMQC-based high-resolution spectroscopy.

SESAME‐HSQC for simultaneous measurement of NH and CH scalar and residual dipolar couplings

We present a novel pulse sequence, SESAME-HSQC, for the simultaneous measurement of several NH and CH scalar and residual dipolar couplings in double labeled proteins. The proposed Spin-statE Selective All Multiplicity Edited (SESAME)-HSQC combines gradient selected and sensitivity enhanced (15)N- and constant-time (13)C-HSQC experiments with the recently introduced spin-state selective method (Nolis et al., J. Magn. Reson. 180 (2006) 39-50) for measuring couplings simultaneously at amide and aliphatic regions. Excellent resolution and high sensitivity is warranted by removing all coupling interactions during the indirectly detected t(1) period, and by employing pulsed field gradients for coherence selection and utilizing coherence order selective spin-state selection. The scalar and residual dipolar couplings can be readily measured from a two-dimensional (15)N/(13)C-HSQC spectrum without additional spectral crowding. SESAME-HSQC can be used for epitope mapping by observing chemical shift changes in both amide and aliphatic regions. Simultaneously, potential conversion in protein conformation can be probed by analyzing changes in residual dipolar couplings induced by ligand binding. The pulse sequence is experimentally verified with a sample of (15)N/(13)C enriched human ubiquitin. The internuclear vector directions determined from the residual dipolar couplings are found to be in excellent correlation with those predicted from ubiquitin's refined solution structure.

Gradient‐enhanced 1D HMQC‐ and HSQC‐relayed experiments with maximum sensitivity

AbstractThe incorporation of pulsed‐field gradients for coherence selection into 1D proton‐detected pulse sequences provides clean spectra free of cancellation artifacts but, frequently, at the expense of a sensitivity loss by a theoretical factor of 2 due to the rejection of another contributed pathway. A series of slightly modified gradient‐enhanced 1D carbon‐selective HMQC‐ and HSQC‐relayed experiments are proposed to obtain maximum signal‐to‐noise ratios in the same overall acquisition time. In contrast to the implementation of this so‐called PEP approach in multi‐dimensional experiments, the proposed 1D experiments do not have extra requirements for data acquisition or data processing with respect to the original ones and the excellent spectral quality due to of gradient‐selected experiments is preserved. Copyright © 2001 John Wiley & Sons, Ltd.

A novel acylated flavonol glycoside isolated from Brunfelsia grandiflora ssp. grandiflora. Structure elucidation by gradient accelerated NMR spectroscopy at 14T

Kaempferol-3-O-[6Glc-O-feruloyl]-β-D-glucopyranosyl-(1→2)-O-[α-L-rhamnopyranosyl-(1→6)]-β-D-galactopyranoside (1) was isolated from the aerial parts of Brunfelsia grandiflora ssp. grandiflora. The structure of this novel flavonol glycoside was established by fast atom bombardment-mass spectrometry and by means of NMR spectroscopy at 14T, using two-dimensional NMR experiments with pulsed-field gradients for coherence selection. The presence of the constituent aglycones and sugar units of 1 was confirmed independently by acid hydrolysis. Copyright © 2000 John Wiley & Sons, Ltd.

Long-range proton-carbon coupling constants in conformational analysis of oligosaccharides

Long-range heteronuclear coupling constants, 3J(C,H), were measured across glycosidic linkages in β-linked disaccharides and also for vicinally disubstituted trisaccharides. One-dimensional multiple-site 13C excitation experiments using Hadamard spectroscopy and band-selective 1H decoupling during the acquisition period were used in conjunction with pulsed field gradients for coherence selection in the 1H detected experiment. For one dihedral angle the 3J(C,H) value could only be unambiguously determined utilizing the band-selective 1H decoupling. The 2D EXSIDE was also used for the measurement of 3J(C,H) values and the results showed excellent agreement with those of the 1D measurements. The influence of solvent, water and water–dimethyl sulfoxide (7:3), on conformation was investigated, but showed only small changes in 3J(C,H) values. In general, the long-range heteronuclear coupling constants related to the ϕ dihedral angles (3.6–4.3 Hz) were smaller than those of the ψ dihedral angles (4.3–5.3 Hz). A comparison with 3J(C,H) values from Monte Carlo or Langevin dynamics simulations showed better agreement for the ψ dihedral angles than for the ϕ dihedral angles as calculated using a Karplus relationship. © 1998 John Wiley & Sons Ltd.

H(C)CH-COSY and (H)CCH-COSY Experiments for13C-Labeled Proteins in H2O Solution

We present three experiments which serve to identify carbon and proton sidechain resonances in13C-labeled proteins. The first is an improvement on the previously published H(C)CH-COSY experiment and comprises the application of gradients for coherence selection and a reduction in the phase cycle. The second experiment is a new (H)CCH-COSY with two carbon dimensions. The (H)CCH-COSY presents several advantages over the H(C)CH-COSY experiment in terms of better sensitivity, improved resolution and easier identification of amino acid spins systems. The third experiment is a 2D proton-edited (H)C(C)H-COSY that allows suppression of methylene resonances. All three HCCH-COSY experiments show good sensitivity and excellent solvent suppression. The 2D version can be acquired in as little as 45 minutes and the 3D versions acquired overnight. The experiments are demonstrated on a13C-labeled sample of the second PDZ domain from human phosphatase PTP1E in H2O solution.

Structure Elucidation of Secalosides A and B by NMR Spectroscopy

The constitution of secalosides A and B was elucidated through NMR spectroscopy using fast 2D-NMR experiments with pulsed-field gradients for coherence selection. Secalosides A and B are epimers at center C-3‘ of aglycon I and have the constitution shown as 1. The presence of a saccharose unit was confirmed independently by mild basic hydrolysis and enzymatic quantification. The relative configuration of the stereogenic centers of aglycon II could be deduced from ROESY data. Assignments of the two epimers to the R- and S- configurations at C-3‘ in aglycon I, as well as the absolute configuration of aglycon II remain to be determined.

Selective Transient Heteronuclear Cross Relaxation in a Selectively13Cα-Labeled Peptide

A new pulse sequence for the direct measurement of heteronuclear cross-relaxation rates is presented. The pulse sequence uses proton detection of transient carbon magnetization with sensitivity-enhanced transfer of magnetization and pulsed field gradients for coherence selection and water suppression. The heteronuclear cross-relaxation rate is measured using either nonselective or selective inversion of all or particular protons. A technique for the suppression of spin diffusion is also applied. The pulse sequence is tested on a peptide with a selectively13C-labeled α-carbon. The directly measured rate is compared with and agrees well with the cross-relaxation rates as traditionally calculated from the steady-state NOE and carbon longitudinal-relaxation rate.

Gradient-Enhanced One-Dimensional Heteronuclear NOE Experiment with1H Detection

A new experiment, HETGOESY, is described in which 13C–1H NOE enhancements are detected directly on 1H. The experiment uses pulsed field gradients for coherence selection, thus completely avoiding the problems associated with difference spectroscopy. It is shown, both theoretically and experimentally, that the new experiment has sensitivity advantages over the conventional 13C{1H} steady-state NOE difference experiment. The new experiment also has the advantage that it uses selective excitation in the generally better resolved decoupled 13C spectrum rather than the 1H spectrum.

New pulsed field gradient NMR experiments for the detection of bound water in proteins

New H(2)O-selective homonuclear and heteronuclear 2D NMR experiments have been designed for the observation of protein hydration (PHOGSY, Protein Hydration Observed by Gradient Spectroscop Y). These experiments utilize selective excitation of the H(2)O resonance and pulsed field gradients for coherence selection and efficient H(2)O suppression. The method allows for a rapid and sensitive detection of H(2)O molecules in labelled and unlabelled proteins. In addition it opens a way to measure the residence time of water bound to proteins. Its application to uniformly (15)N-labelled FKBP-12 (FK-506 binding protein) is demonstrated.

Proton-Detected 15N NMR-Spectroscopy and Imaging

Proton detection of nitrogen-15, using gradients for coherence selection, was determined to be an effective method for obtaining spectra of 15N-labeled metabolites from extracts and biopsies of tissue infused with [15N] ammonium chloride. The advantage of gradient selection of coherence was best demonstrated by the almost complete single-shot elimination of solvent water in extracts and tissue water in biopsies. As a single-acquisition editing method in which only protons attached to 15N are detected, the potential limitations of dynamic range and motion are also reduced. Gradient-enhanced heteronuclear multiple-quantum coherence (1H[15N] HMQC) was compared with conventional HMQC, and despite selection of only one of the two heteronuclear pathways, GE-HMQC was found to be more effective for resolving the desired signal for dilute solutions; and with a single scan. In addition, effective water elimination made it possible to use the resolution advantage of a frequency-encoding dimension in proton-detected 15N imaging experiments. The limit of detection of the method at 500 MHz was 0.7 mM in 16 scans from a total volume of 400 μl. Signals from tissue extracts were observable in less than one minute for kidney, heart, brain, and muscle. Proton-detected 15N GE-HMQC images with a voxel size of 39 × 78 × 625 μm were obtained at 600 MHz from a 4 mM (1.6 μmol) 15N urea sample in less than four hours. Distribution of [15N] urea in the kidney was observed in a 600 MHz GE-HMQC image of the papilla and some cortical structures.

Effective combination of gradients and crafted RF pulses for water suppression in biological samples

The selection of multiple-quantum coherence wiith field gradients has been shown to be an effective method for suppressing the water resonance in aqueous solutions and for reducing t, noise and other artifacts associated with the phase-cycle selection of coherence (l-3). Recently, some of these advantages have been demonstrated for a phase-sensitive, double-quantum-filtered COSY (DQF COSY) experiment (3). However, for large, dilute biomolecules in water, .the gradient pulse durations and amplitudes must be large to eliminate the water signal. In situations where the gradient amplitude is limited, the gradient duration must be increased, which invariably leads to loss of signal due to spin-spin relaxation. The requirement on the gradient duration may be reduced by combining coherence selection with some other technique which achieves partial water suppression. Traditional presaturation methods require irradiation periods on the order of seconds, which may result in the loss or attenuation of a desired signal due to magnetization transfer. A succe:ssful strategy for water elimination in volume-localized spectroscopy has been the use of selective water excitation followed by gradient dephasing (4, 5). The selectivity of water elimination using this approach can be improved by the use of crafted RF pulse design. In this Communication, we have used this selective water-elimination strategy to attenuate the water signal prior to the coherence-selection sequence, thereby allowing the use of shorter gradient pulses. A phase-sensitive, gradient-enhanced, DQF COSY experiment is demonstrated here using a medium-sized protein in water. The pulse sequence and the corresponding coherence-level diagram are shown in Fig. 1. The water resonance is selectively excited, followed by a gradient pulse along the x axis which dephases the transverse magnetization. This is repeated a second time to remove any remaining magnetization with the exception that the dephasing gradient is applied along the y axis to avoid gradient-recalled echoes. The flip angle of the second selective pulse is adjusted so that the water magnetization is partially inverted to allow for T, recovery during the dephase gradients. This delay also allows the fasterrelaxing protein signals at the water chemical shift to recover. The water-elimination sequence is followed by a phase-sensitive, DQF COSY sequence with short, doublequantum, coherence-selection gradients G, and GZ along the x, y, and z axes. Multiplequantum coherence orders labeled by the gradient G, are then converted into observable antiphase magnetization by the last RF pulse. The relative ratios of the gradient pulses