Off-resonance effects and selectivity profiles in pulsed nitrogen-14 nuclear quadrupole resonance.

In order to alleviate base-line distortions in nitrogen-14 NQR spectra originating from pulse breakthrough, low power radio-frequency (rf) pulses were applied. It is recalled that the required power is four times lower than that for an equivalent NMR experiment. This is easily explained by the fact that, in NMR, half the amplitude of the rf field is active. Moreover, the selectivity profile (i.e. the peak amplitude as a function of the difference between the carrier frequency and the resonance frequency) exhibits a shape which is, in most cases, more favorable in NQR than in NMR. An appropriate theory has been developed for explaining these experimental observations. It is concluded that low power NQR is perfectly feasible and should even be recommended for most applications, provided that the line-width of the NQR signal is not too large.

Diffusive diffraction phenomenon in a porous polymer material observed by NMR using radio-frequency field gradients.

Pulsed field gradient NMR diffusion experiments can, in principle, lead to the "diffusive diffraction" phenomenon. In practice, its observation by gradients of the static magnetic field is difficult in real systems because they involve internal gradients due to the static magnetic field (necessary for polarizing nuclear spins). This latter drawback can be circumvented by using gradients of the radio-frequency (rf) field (the other magnetic field used in any NMR experiment). For the first time, by means of rf gradients, a so-called diffusive diffraction peak has been observed in a real porous system and its position provides a value of the mean distance between pores; this can be further complemented by the mean pore size determined from the dependence of the apparent diffusion coefficient with respect to the diffusion interval.

NMR measurement of self-diffusion coefficients by slice selection.

Most of the time, so-called inversion-recovery experiments concern longitudinal nuclear magnetization of the whole sample, the region of interest being limited by the transmitting-receiving coil. Here we address the question of what occurs if the region of interest is purposely limited to a thin slice selected by means of procedures employing magnetic field gradients. Gradients of both magnetic fields (B(0), the static magnetic field, and B(1), the radio-frequency magnetic field) can be used. In this study we resorted essentially to B(1) gradients and novel procedures, based on the natural inhomogeneity of the B(1) field delivered by a saddle coil, are described. It is obvious that molecules leaving and entering the slice during the evolution (recovery) period should influence the magnetization recovery. Molecular self-diffusion is responsible for such effects, experimentally visible and accounted for by an appropriate theory which has been approximated for by permitting an easy physical assessment. This approach should lead to alternative methods for measuring self-diffusion coefficients.

Rotating-frame spin-lattice relaxation time imaging by radio-frequency field gradients: visualization of strained crosslinked natural rubbers.

NMR imaging by radio-frequency field gradients (B1 gradients) is especially convenient for heterogeneous samples and/or in the case of relatively short transverse relaxation times. The method has been combined with the application of two spin-lock periods of different duration so as to produce rotating-frame spin-lattice relaxation time (T1rho) images. In the case of natural rubber samples with different crosslink densities, such images are not only characteristic of the crosslink density but also reveal the way in which the material has been stressed. The strained parts can be visualized either directly or through histograms showing the T1rho distribution over the whole sample.

Determination of carbon-13 chemical shielding tensor in the liquid state by combining NMR relaxation experiments and quantum chemical calculations.

Based on multifield NMR relaxation measurements and quantum chemistry calculations, a strategy aiming at the determination of the chemical shielding tensor (CST) in the liquid state is described. Brownian motions in the liquid state restrict the direct observation of CST to a third of its trace (isotropic shift), and even if CST can be probed indirectly through some spin relaxation rates (specific longitudinal relaxation rates, dipolar chemical shift anisotropy (CSA) cross-correlation rates), an insufficient number of experimental parameters prevents its complete determination. This lack of information can be compensated by using quantum chemical calculations so as to obtain the molecular CST orientation even if a relatively modest level of computation is used. As relaxation parameters involve a dynamic part, a prerequisite is the determination of the molecular anisotropic reorientation which can be obtained independently from dipolar cross-relaxation rates. A polycyclic molecule exhibiting a well-characterized anisotropic reorientation serves as an example for such a study, and some (but not all) carbon-13 chemical shielding tensors can be accurately determined. A comparison with solid-state NMR data and numerous chemical quantum calculations are presented.

A comprehensive analysis of multifield 15N relaxation parameters in proteins: determination of 15N chemical shift anisotropies.

This study deals with the exploitation of the three classical 15N relaxation parameters (the longitudinal relaxation rate, R1, the transverse relaxation rate, R2, and the 1H-15N cross-relaxation rate, sigmaNH) measured at several magnetic fields in uniformly 15N-labeled proteins. Spectral densities involved in R1, R2 and sigmaNH are analyzed according to the functional form A + B/(1 + omega(2) taus(2)), where taus is the correlation time associated with slow motions sensed by the NH vector at the level of the residue to which it belongs. The coefficient B provides a realistic view of the backbone dynamics, whereas A is associated with fast local motions. According to the "model free approach", B can be identified with 2tausS(2) where S is the generalized order parameter. The correlation time taus is determined from the field dependency of the relaxation parameters while A and B are determined through linear equations. This simple data processing is needed for obtaining realistic error bars based on a statistical approach. This proved to be the key point for validating an extended analysis aiming at the determination of nitrogen chemical shift anisotropy. The protein C12A-p8(MTCP1) has been chosen as a model for this study. It will be shown that all data (obtained at five magnetic field strengths corresponding to proton resonance of 400, 500, 600, 700, and 800 MHz) are very consistently fitted provided that a specific effective correlation time associated with slow motions is defined for each residue. This is assessed by small deviations between experimental and recalculated values, which, in all cases, remain within experimental uncertainty. This strategy makes needless elaborate approaches based on the combination of several slow motions or their possible anisotropy. Within the core of the protein taus fluctuates in a relatively narrow range (with a mean value of 6.15 ns and a root-mean-square deviation of 0.36 ns) while it is considerably reduced at the protein extremities (down to approximately 3 ns). To a certain extent, these fluctuations are correlated with the protein structure. A is not obtained with sufficient accuracy to be valuably discussed. Conversely, order parameters derived from B exhibit a significant correlation with the protein structure. Finally, the multi-field analysis of the evolution of longitudinal and transverse relaxation rates has been refined by allowing the 15N chemical shift anisotropy (csa) to vary residue by residue. Within uncertainties (derived here on a statistical basis) an almost constant value is obtained. This strongly indicates an absence of correlation between the experimental value of this parameter obtained for a given residue in the protein, the nature of this residue, and the possible involvement of this residue in a structured area of the protein.

Nuclear relaxation time images by radiofrequency field gradients applied to the study of solvent permeation into polymeric materials.

T(2) images are obtained by two interleaved B(1)-gradient imaging experiments preceded by CPMG trains of different lengths. The method is assessed by means of a phantom involving compartments of different, though relatively close, T(2) values. T(1) images arise from a previously published procedure also based on two interleaved B(1)-gradient imaging experiments involving different evolution of the longitudinal magnetization. Both types of image appear to be useful in view of the structural characterization of polymer samples through the T(2) and T(1) distribution of a solvent embedded in the material.

High-sensitivity fluorescence anisotropy detection of protein-folding events: application to alpha-lactalbumin.

An experimental procedure has been devised to record simultaneously fluorescence intensity and fluorescence anisotropy. A photoelastic modulator on the excitation beam enables the anisotropy signal to be recorded in one pass using a single photomultiplier tube and eliminates the need for a polarizer on the emission path. In conjunction with a stopped-flow mixer, providing a time-resolved capability, this procedure was used to study the refolding of apo alpha-lactalbumin following dilution from guanidinium chloride. Although the fluorescence intensity does not change detectably, the fluorescence anisotropy was found to resolve the conformational changes occurring between the initial unfolded state and the molten globule state formed either kinetically during refolding at pH 7.0 or at equilibrium at pH 2.0 (A-state). This result provides further evidence that fluorescence anisotropy is a valuable probe of protein structural transitions and that the information it provides concerning the rotational mobility of a fluorophore can be complementary to the information about the local environment provided by fluorescence intensity.

Extending the excitation sculpting concept for selective excitation.

Nowadays, excitation sculpting is probably the most efficient way to achieve selectivity in an NMR experiment, since it associates very clean frequency selection with "user-friendliness." In the present report, it is shown that the excitation sculpting concept, originally based on a double pulse field gradient echo acting on a selected transverse magnetization, can be extended through new experiments designed to act on longitudinal magnetization. This leads to outstanding performances, especially when the transverse relaxation rate is a limiting factor as, for example, in the case of biological macromolecules. Several new sequences are proposed, aiming at the selection of magnetization aligned either/both on a transverse axis or/and on the z-axis. Their potentialities are illustrated in light of different applications including multiplet-selective excitation, band-selective excitation, and water suppression.

A procedure for obtaining pure absorption 2D J-spectra: application to quantitative fully J-decoupled homonuclear NMR spectra.

The phase-twisted lines, resulting from a double complex Fourier transform, can greatly alter the quality of two-dimensional spectra. This problem, which is generally overcome by experimental procedures (requiring twice as much time as a standard measurement), is inevitable in the two-dimensional J-resolved experiment. This experiment is revisited theoretically and a postprocessing treatment, leading to pure 2D J-spectra, is deduced. Quantitative fully J-decoupled homonuclear spectra are accordingly obtained by means of a projection onto the F2 axis after a 45 degrees tilt.

Maps of self-diffusion coefficients by radiofrequency field gradient NMR microscopy

The methods of measurement of spatially resolved diffusion coefficients using radiofrequency field gradient (E. Mischler et al., J. Magn. Reson. B 106, 32, 1995; R. Kimmich et al., J. Magn. Reson. A 112, 7, 1995) produce 1D profiles whose amplitude is not only a function of the local self-diffusion coefficient but also is modulated by cosine functions of spatial coordinates. Due to this modulation diffusion-weighted images cannot be obtained unless cumbersome data processing is used. Here, we present a new sequence which avoids this modulation and yields in a straightforward manner true self-diffusion coefficient maps; this is in contrast with conventional methods which use static field gradients and which are therefore altered by background gradients. The feasibility and the reliability of the method are demonstrated with phantoms; it is also applied to different systems of interest such as solvent swelled rubber, membranes, and plants. Copyright 1999 Academic Press.

Measurement of longitudinal and rotating frame relaxation times through fully J-decoupled homonuclear spectra.

It is shown that fully J-decoupled homonuclear spectra involving Lorentzian lines can be readily obtained by straightforward processing of the 2D data arising from a conventional spin echo sequence (pi/2-t(1)/2-pi-t(1)/2-Acq(t(2))) used in the so-called J-resolved experiment. The method simply rests on power spectra with the drawback of lines having meaningless relative intensities. In principle, the experiment should also yield transverse relaxation times. Several tests demonstrate that this is not so, due to pulse imperfections and nonresolved long-range J couplings. Conversely, longitudinal and rotating frame relaxation times can be easily determined by means of an appropriate preparation period (for instance, a saturation-recovery period in the case of longitudinal relaxation) inserted before the 2D spin echo sequence. Since one is dealing with a single line per nucleus, relaxation measurements become reliable and accurate.

Mechanistic studies of the folding of human lysozyme and the origin of amyloidogenic behavior in its disease-related variants.

The unfolding and refolding properties of human lysozyme and two amyloidogenic variants (Ile56Thr and Asp67His) have been studied by stopped-flow fluorescence and hydrogen exchange pulse labeling coupled with mass spectrometry. The unfolding of each protein in 5.4 M guanidine hydrochloride (GuHCl) is well described as a two-state process, but the rates of unfolding of the Ile56Thr variant and the Asp67His variant in 5.4 M GuHCl are ca. 30 and 160 times greater, respectively, than that of the wild type. The refolding of all three proteins in 0.54 M GuHCl at pH 5.0 proceeds through persistent intermediates, revealed by multistep kinetics in fluorescence experiments and by the detection of well-defined populations in quenched-flow hydrogen exchange experiments. These findings are consistent with a predominant mechanism for refolding of human lysozyme in which one of the structural domains (the alpha-domain) is formed in two distinct steps and is followed by the folding of the other domain (the beta-domain) prior to the assembly of the two domains to form the native structure. The refolding kinetics of the Asp67His variant are closely similar to those of the wild-type protein, consistent with the location of this mutation in an outer loop of the beta-domain which gains native structure only toward the end of the refolding process. By contrast, the Ile56Thr mutation is located at the base of the beta-domain and is involved in the domain interface. The refolding of the alpha-domain is unaffected by this substitution, but the latter has the effect of dramatically slowing the folding of the beta-domain and the final assembly of the native structure. These studies suggest that the amyloidogenic nature of the lysozyme variants arises from a decrease in the stability of the native fold relative to partially folded intermediates. The origin of this instability is different in the two variants, being caused in one case primarily by a reduction in the folding rate and in the other by an increase in the unfolding rate. In both cases this results in a low population of soluble partially folded species that can aggregate in a slow and controlled manner to form amyloid fibrils.

Nuclear longitudinal relaxation time images by radiofrequency field gradients.

Combination of the Super Fast Inversion Recovery (SUFIR) method (D. Canet, J. Brondeau, and K. Elbayed, J. Magn. Reson. 77, 483 (1988)) and imaging procedures by radiofrequency field gradients (P. Maffei, P. Mutzenhardt, A. Retournard, B. Diter, R. Raulet, J. Brondeau, and D. Canet, J. Magn. Reson. A 107, 40 (1994)) provides spatially resolved maps of longitudinal relaxation times (T1). In addition to accurate T1 values, enhanced spatial resolution is obtained.

Diffusion measurements using radiofrequency field gradient: artifacts, remedies, practical hints

The two major advantages of experiments carried out with radiofrequency (RF) field-gradient NMR are the instrumental simplicity and the insensitivity to background static magnetic field gradients. These features combined with large RF gradients, which became available only recently, should make this technique especially attractive for molecular translational diffusion studies. However, a critical evaluation of the method shows that under some circumstances (small and/or heterogeneous samples, weak diffusion coefficients, very short relaxation times) the quality of measurements may be affected by a number of artifacts. Their origin has been investigated and several remedies have been considered; in particular, a new improved sequence is presented. The success of various experimental tests demonstrates the efficiency of the proposed solutions which thus open the way to much wider application fields. Copyright 1998 Academic Press.

Chemical shift imaging in rotating solids by radiofrequency field gradients

A dedicated equipment has been designed for solid state NMR imaging in the spatial direction coinciding with the rotation axis of a 13C CP/MAS probe. It involves a two-turn B1 gradient coil whose plane is perpendicular to that axis and a 1H-13C doubly tuned saddle coil used for normal spectroscopic operations, orthogonal to the latter. The experiment provides a two-dimensional diagram with 13C chemical shift in one dimension whereas the second dimension corresponds to the space variable. Some test experiments demonstrate the validity of this approach. Copyright 1998 Academic Press.

Water-surfactant contact studied by 19F-1H heteronuclear overhauser effect spectroscopy

Intermolecular 19F-1H cross-relaxation is measured using heteronuclear Overhauser effect NMR spectroscopy (HOESY) in the micellar solution of cesium pentadecafluorooctanoate. The results are analyzed in terms of a weak 1H-19F cross-relaxation between the water protons and the fluorines in the fluoroalkyl chain and a strong 19F-19F cross-relaxation within the fluoroalkyl chain. The water-surfactant cross-relaxation indicates a water approach to the first CF2 segment in the order of 2.0 A and a short (<

Self-Diffusion Measurements by Carbon-13 NMR Using Radiofrequency Field Gradients

This paper demonstrates the feasibility of self-diffusion measurements by radiofrequency field gradients with equipment tuned to the carbon-13 resonance frequency (i.e., 50 MHz for a field of 4.7 T). For the first time, self-diffusion coefficients of carbon disulfide and carbon tetrachloride are measured by NMR. The agreement with results from tracer methods is excellent. Copyright 1998 Academic Press. Copyright 1998 Academic Press

Magnitudes and mutual orientations of dipolar and shielding interaction tensors determined from the orientation dependence of spinning sidebands of slowly rotating powder samples.

It is shown that the orientation dependence of spinning sidebands of slowly rotating solids can be exploited to determine with good precision the heteronuclear bond distances and mutual orientation of dipolar and shielding tensors from a single dipolar modulated chemical shift anisotropy spectrum. The method is based on the dynamics of polarization transfer in the initial period of tens of microseconds, and may be useful in solids with relatively isolated groups of spins.

Visualization of residual anisotropic interactions in crosslinked natural rubbers by dipolar local field measurements and 2H natural abundance NMR spectroscopy.

An extension of the exploitation of indirect observation of 1H nuclei through 13C resonances is presented in the case of crosslinked elastomers. It is demonstrated that, by using this method in vulcanized elastomers above Tg, a direct visualization of residual dipolar interactions on different functional groups as well as their dependence on motional constraints is available. It is also shown that 2H natural abundance NMR spectra of elastomers provide similar information on motional constraints by way of residual quadrupolar interactions.