High-Speed Magic-Angle SpinningC MAS NMR Spectra of Adamantane: Self-Decoupling of the Heteronuclear Scalar Interaction and Proton Spin Diffusion

We have investigated the carbon line shape of solid adamantane under high-speed magic-angle sample spinning (MAS) acquired without proton decoupling. The CH-group shows a spinning-speed-dependent line broadening while the CH₂-group consists of a spinning-speed-independent sharp component and a spinning-speed-dependent broader part. These phenomena can be explained by self-decoupling of theJ-interaction due to proton spin diffusion. Such a self-decoupling process can be described by a magnetization exchange process between the multiplet lines. Changing the spin-diffusion rate constant by off-resonance irradiation of the protons allows us to observe the full range from slow exchange to coalescence to fast exchange of the carbon spectra. One of the multiplet components in the CH₂-group corresponds to a group spin of the protons of zero and therefore does not couple to the other protons. This gives rise to the sharp central line. The magnetization exchange rate constant between the different multiplet lines can be determined from the spectra and is a measure for the spinning-speed-dependent proton spin-diffusion rate constant. Even at an MAS speed of 30 kHz, proton spin diffusion is still observable despite the relatively weak intermolecular proton dipolar-coupling network in adamantane which results in a static proton line width of only 14 kHz (full width at half height).     

Selective dephasing of OH and NH proton magnetization based on (1)H chemical-shift anisotropy recoupling

A method for selectively suppressing the signals of OH and NH protons in (1)H combined rotation and multiple-pulse spectroscopy (CRAMPS) and in (1)H-(13)C heteronuclear correlation (HETCOR) solid-state NMR spectra is presented. It permits distinction of overlapping CH and OH/NH proton signals, based on the selective dephasing of the magnetization of OH and NH protons by their relatively large (1)H chemical-shift anisotropies. For NH protons, the (14)N-(1)H dipolar coupling also contributes significantly to this dephasing. The dephasing is achieved by a new combination of heteronuclear recoupling of these anisotropies with (1)H homonuclear dipolar decoupling. Since the 180 degrees pulses traditionally used for heteronuclear dipolar and chemical-shift anisotropy recoupling would result in undesirable homonuclear dephasing of proton magnetization, instead the necessary inversion of the chemical-shift Hamiltonian every half rotation period is achieved by inverting the phases of all the pulses in the HW8 multiple-pulse sequence. In the HETCOR experiments, carefully timed (13)C 180 degrees pulses remove the strong dipolar coupling to the nearby (13)C spin. The suppression of NH and OH peaks is demonstrated on crystalline model compounds. The technique in combination with HETCOR NMR is applied to identify the CONH and NH-CH groups in chitin and to distinguish NH and aromatic proton peaks in a peat humin.     

A 1D sensitivity-enhanced 1H spin diffusion experiment for determining membrane protein topology

A sensitivity-enhanced 1D (1)H spin diffusion experiment, CHH, for determining membrane protein topology is introduced. By transferring the magnetization of the labeled protein (13)C to lipid and water protons for detection, the CHH experiment reduces the time of the original 2D (13)C-detected experiment by two orders of magnitude. The sensitivity enhancement results from (1)H detection and the elimination of the (13)C dimension. Consideration of the spin statistics of the membrane sample indicates that the CHH sensitivity depends on the (13)C labeling level and the number of protein protons relative to the mobile protons. 5-35% of the theoretical sensitivity was achieved on two extensively (13)C labeled proteins. The experimental uncertainties arise from incomplete suppression of the equilibrium (1)H magnetization and the magnetization of lipid protons directly bonded to natural-abundance carbons. The technique, demonstrated on colicin Ia channel domain, confirms the presence of a transmembrane domain and the predominance of surface-bound helices.     

2D CP/MASC Isotropic Chemical Shift Correlation Established byH Spin Diffusion

A new 2D solid-state CP/MAS¹³C NMR exchange experiment for through-space isotropic chemical shift correlation is proposed and demonstrated. Through-space correlation is established via a second cross polarization from¹³C to¹H and subsequent¹H spin diffusion. A third cross polarization results in the final¹³C–¹³C isotropic chemical shift correlation. The¹H spin diffusion time is a variable parameter allowing different mean square magnetization displacements to be probed. Experimental results on mixtures of differently¹³C-labeled alanine and polyethylene indicate that this site-selective 2D technique can be used to characterize domain sizes and proximities over a wide range of length scales (1–200 nm) in solids such as polymers or biological materials.     

Relaxation-induced dipolar exchange with recoupling-An MAS NMR method for determining heteronuclear distances without irradiating the second spin

A new magic-angle spinning NMR method for distance determination between unlike spins, where one of the two spins in question is not irradiated at all, is introduced. Relaxation-induced dipolar exchange with recoupling (RIDER) experiments can be performed with conventional double-resonance equipment and utilize the familiar π-pulse trains to recouple the heteronuclear dipolar interaction under magic-angle spinning conditions. Longitudinal relaxation of the passive spin during a delay between two recoupling periods results in a dephasing of the heteronuclear coherence and consequently a dephasing of the magnetization detected after the second recoupling period. The information about the dipolar coupling is obtained by recording normalized dephasing curves in a fashion similar to the REDOR experiment. At intermediate mixing times, the dephasing curves also depend on the relaxation properties of the passive spin, i.e., on single- and double-quantum longitudinal relaxation times for the case of I = 1 nuclei, and these relaxation times can be estimated with this new method. To a good approximation, the experiment does not depend on possible quadrupolar interactions of the passive spin, which makes RIDER an attractive method when distances to quadrupolar nuclei are to be determined. The new method is demonstrated experimentally with ¹⁴N and ²H as heteronuclei and observation of ¹³C in natural abundance.     

Cross-correlations between low-γ nuclei in solids via a common dipolar bath

Correlation of chemical shifts of low-γ nuclei (such as 15N) is an important method for assignment of resonances in uniformly-labeled biological solids. Under static experimental conditions, an efficient mixing of low-γ nuclear spin magnetization can be achieved by a thermal contact to the common reservoir of dipole-dipole interactions in order to create 15N-15N, 13C-13C, or 15N-13C cross-peaks in a 2D correlation spectrum. A thermodynamic approach can be used to understand the mechanism of magnetization mixing in various 2D correlation pulse sequences. This mechanism is suppressed under magic-angle spinning, when mixing via direct cross-polarization with protons becomes more efficient. Experimental results are presented for single-crystalline and powder samples of 15N-labeled N-acetyl-L-15N-valyl-L-15N-leucine (NAVL). In addition to the thermodynamic analysis of mixing pulse sequences, two different new mixing sequences utilizing adiabatic pulses are also experimentally demonstrated.     

Spin-diffusion NMR at low field for the study of multiphase solids

The use of spin-diffusion NMR for the measurement of domain sizes in multiphase materials is becoming increasingly popular, in particular for the study of heterogeneous polymers. Under conditions where T(1) relaxation can be neglected, which is mostly the case at high field, analytical and approximate solutions to the evolution of spin diffusion are available. In order to extend the technique to more general conditions, we performed a comprehensive study of the diffusion of magnetization in a model copolymer at low field, where T(1) tends to be of the same order of magnitude as the typical spin-diffusion time. In order to study the effects of T(1) and to delineate the optimal T(1) values for back correction prior to applying the initial-rate approximation, we developed a numerical simulation based on the diffusion equation and including longitudinal relaxation. We present and discuss the limits of simple correction strategies for initial-slope analysis based on apparent relaxation times from saturation-recovery experiments or the spin-diffusion experiments themselves. Our best strategy faithfully reproduces domain sizes obtained by both TEM investigations and full simultaneous fitting of spin-diffusion and saturation-recovery curves. Full fitting of such independent data sets not only yields correct domain sizes, but also the true longitudinal relaxation times, as well as spin-diffusion coefficients. Effects of interphases with distinct mobility on spin-diffusion curves, as well as practical hints concerning the reliable component decomposition of the detected low-resolution FID signal by help of different magnetization filters are also discussed in detail.     

Binuclear spin state selective detection of 1H single quantum transitions using triple quantum coherence: a novel method for enantiomeric discrimination

In the present work a novel methodology is developed for the unambiguous discrimination of enantiomers aligned in chiral liquid crystalline media and the simultaneous determination of 1H-1H and 13C-1H couplings in a single experiment. An INEPT transfer and back transfer of magnetization to protons retain the 13C edited 1H magnetization which is utilized to generate spin selective homonuclear triple quantum coherence of dipolar coupled methyl protons. Spin selective correlation of triple quantum to single quantum coherence results in spin state selective detection by 13C spin and the remaining passive protons. The difference between the successive transitions in the triple quantum dimension pertains to sum of the passive couplings and results in enhanced resolution by a factor of three. This results in unambiguous chiral visualization. The masked 13C satellite transitions in the single quantum spectrum are extracted for chiral discrimination. The technique retains all the passive homo- and heteronuclear couplings in the triple quantum dimension by the application of non-selective refocusing pulse on 1H as well as on 13C spins. This, however, refocuses the chemical shift evolution in the triple quantum dimension, and also overcomes the problem of field inhomogeneity. The method enables the determination of spectral information which is otherwise not possible to derive from the broad and featureless proton spectra. The elegant experimental technique has been demonstrated on different chiral molecules.     

Local monitoring of proton spin diffusion in static and rotating samples via spy detection

A method is described for investigating local proton “spin diffusion” by means of a ¹³C spin probe. The procedure does not require spectral resolution of proton resonance lines and can be applied in the laboratory frame of reference as well as in the rotating frame. Experimental results are presented for a static single crystal of ferrocene and for a powder sample under magic-angle spinning. The spin-diffusion rate constant is found to be proportional to the spinning speed in the range from 1 to 8 kHz.     

Sensitivity and resolution in proton solid-state NMR at intermediate deuteration levels: quantitative linewidth characterization and applications to correlation spectroscopy

We present a systematic study of proton linewidths in rigid solids as a function of sample spinning frequency and proton density, with the latter controlled by the ratio of protonated and perdeuterated model compounds. We find that the linewidth correlates more closely with the overall proton density (rho(H)) than the size of local clusters of (1)H spins. At relatively high magic-angle spinning (MAS) rates, the linewidth depends linearly upon the inverse MAS rate. In the limit of infinite spinning rate and/or zero proton concentration, the linewidth extrapolates to a non-zero value, owing to contributions from scalar couplings, chemical shift dispersion, and B(0) field inhomogeneity. The slope of this line depends on the overall concentration of unexchangeable protons in the sample and the spinning rate. At up to 30% protonation levels ( approximately 2 (1)H/100A(3)), proton detection experiments are demonstrated to have a substantial (2- to 3-fold) sensitivity gain over corresponding (13)C-detected experiments. Within this range, the absolute sensitivity increases with protonation level; the optimal compromise between sensitivity and resolution is in the range of 20-30% protonation. We illustrate the use of dilute protons for polarization transfer to and from low-gamma spins within 5A, and to be utilized as both magnetization source and detection spins. The intermediate protonation regime enhances relaxation properties, which we expect will enable new types of (1)H correlation pulse sequences to be implemented with improved resolution and sensitivity.     

Heteronuclear spin decoupling in solid-state NMR under magic-angle sample spinning

Achieving high spectral resolution is an important prerequisite for the application of solid-state NMR to biological molecules. Higher spectral resolution allows to resolve a larger number of resonances and leads to higher sensitivity. Among other things, heteronuclear spin decoupling is one of the important factors which determine the resolution of a spectrum. The process of heteronuclear spin decoupling under magic-angle sample spinning is analyzed in detail. Continuous-wave RF irradiation leads only in a zeroth-order approximation to a full decoupling of heteronuclear spin systems in solids under magic-angle spinning (MAS). In a higher-order approximation, a cross-term between the dipolar-coupling tensor and the chemical-shielding tensor is reintroduced, providing a scaled coupling term between the heteronuclear spins. In strongly coupled spin systems this second-order recoupling term is partially averaged out by the proton spin-diffusion process, which leads to exchange-type narrowing of the line by proton spin flips. This process can be described by a spin-diffusion type superoperator, allowing the efficient simulation of strongly coupled spin systems under heteronuclear spin decoupling. Low-power continuous-wave decoupling at fast MAS frequencies offers an alternative to high-power irradiation by reversing the order of the averaging processes. At fast MAS frequencies low-power continuous-wave decoupling leads to significantly narrower lines than high-power continuous-wave decoupling while at the same time reducing the power dissipated in the sample by several orders of magnitude. The best decoupling is achieved by multiple-pulse sequences at high RF fields and under fast MAS. Two such sequences, two-pulse phase-modulated decoupling (TPPM) and X-inverse-X decoupling (XiX), are discussed and their properties analyzed and compared.     

Basic principles of static proton low-resolution spin diffusion NMR in nanophase-separated materials with mobility contrast

We review basic principles of low-resolution proton NMR spin diffusion experiments, relying on mobility differences in nm-sized phases of inhomogeneous organic materials such as block-co- or semicrystalline polymers. They are of use for estimates of domain sizes and insights into nanometric dynamic inhomogeneities. Experimental procedures and limitations of mobility-based signal decomposition/filtering prior to spin diffusion are addressed on the example of as yet unpublished data on semicrystalline poly(ϵ-caprolactone), PCL. Specifically, we discuss technical aspects of the quantitative, dead-time free detection of rigid-domain signals by aid of the magic-sandwich echo (MSE), and magic-and-polarization-echo (MAPE) and double-quantum (DQ) magnetization filters to select rigid and mobile components, respectively. Such filters are of general use in reliable fitting approaches for phase composition determinations. Spin diffusion studies at low field using benchtop instruments are challenged by rather short ¹H T₁ relaxation times, which calls for simulation-based analyses. Applying these, in combination with domain sizes as determined by small-angle X-ray scattering, we have determined spin diffusion coefficients D for PCL (0.34, 0.19 and 0.032 nm²/ms for crystalline, interphase and amorphous parts, respectively). We further address thermal-history effects related to secondary crystallization. Finally, the state of knowledge concerning the connection between D values determined locally at the atomic level, using ¹³C detection and CP- or REDOR-based “¹H hole burning” procedures, and those obtained by calibration experiments, is summarized. Specifically, the non-trivial dependence of D on the magic-angle spinning (MAS) frequency, with a minimum under static and a local maximum under moderate-MAS conditions, is highlighted.     

Kinetics of 1H→13C NMR cross-polarization in polymorphs and solvates of the antipsychotic drug olanzapine

The (1)H→(13)C NMR cross-polarization (CP) was studied under magic-angle spinning at 7.5 kHz in various crystal forms of the antipsychotic drug olanzapine: two polymorphs (metastable I and stable II) and eight solvates containing organic solvent and water molecules. The CP kinetics followed the non-classical I-I(*)-S model, in which CP begins in a spin cluster of proximate abundant spins I(*) and rare spins S, then is controlled by spin diffusion of the abundant spins I from bulk to the I(*) spins of the spin cluster and finally is governed by spin-lattice relaxation of the abundant spins in the rotating frame. The corresponding CP kinetics parameters were determined and analyzed. It was demonstrated that the, λ and T(df) values (the CP time constant, the cluster composition parameter and the (1)H spin-diffusion constant, respectively) were very useful to discriminate the functional groups, especially in the 3D parameter space. In order to conveniently analyze the large amount (175) of the collected CP parameters, the number of the observed variables was reduced using the principal component (PC) analysis. The 2D plot of PC2 vs. PC1 showed adequate separation of the CH(3), CH(2), CH and C cases (C stands for carbons without adjacent hydrogens). It was demonstrated that those cases were located along the PC1 axis in the order of increasing (1)H-(13)C dipolar couplings: C相似文献   

Methylene spectral editing in solid-state 13C NMR by three-spin coherence selection

A robust new solid-state nuclear magnetic resonance (NMR) method for selecting CH2 signals in magic-angle spinning (MAS) 13C NMR spectra is presented. Heteronuclear dipolar evolution for a duration of 0.043 ms, under MREV-8 homonuclear proton decoupling, converts 13C magnetization of CH2 groups into two- and three-spin coherences. The CH2 selection in the SIJ (C H H) spin system is based on the three-spin coherence S(x)I(z)J(z), which is distinguished from 13C magnetization (S(x)) by a 1H 0 degrees/90 degrees pulse consisting of two 45 degrees pulses. The two-spin coherences of the type S(y)I(z) are removed by a 13C 90 degrees x-pulse. The three-spin coherence is reconverted into magnetization during the remainder of the rotation period, still under MREV-8 decoupling. The required elimination of 13C chemical-shift precession is achieved by a prefocusing 180 degrees pulse bracketed by two rotation periods. The selection of the desired three-spin coherence has an efficiency of 13% theoretically and of 8% experimentally relative to the standard CP/MAS spectrum. However, long-range couplings also produce some three-spin coherences of methine (CH) carbons. Therefore, the length of the 13C pulse flipping the two-spin coherences is increased by 12% to slightly invert the CH signals arising from two-spin coherences and thus cancel the signal from long-range three-spin coherences. The signal intensity in this cleaner spectrum is 6% relative to the regular CP/TOSS spectrum. The only residual signal is from methyl groups, which are suppressed at least sixfold relative to the CH2 peaks. The experiment is demonstrated on cholesteryl acetate and applied to two humic acids.     

Spin-state-selective excitation in selective 1D inverse NMR experiments

A general and very simple strategy for achieving clean spin-state-selective excitation with full sensitivity in carbon-selective gradient-enhanced 1D HMQC and HSQC pulse schemes is presented. The incorporation of an additional hard 90 degrees (13)C pulse applied along a specific orthogonal axis just prior to acquisition into the conventional sequences allows us to select a simultaneous coherence transfer pathway which usually is not detected. The superimposition of this resulting antiphase magnetization to the conventional in-phase magnetization gives the exclusive excitation of the directly attached proton showing only the alpha or beta spin state of the passive (13)C nucleus. The propagation of this particular spin state to other protons can be accomplished by adding any homonuclear mixing process just after this supplementary pulse. Such an approach affords a suite of powerful selective 1D (13)C-edited NMR experiments which are helpful for resonance assignment purposes in overcrowded proton spin systems and also for the accurate determination of the magnitude and sign of long-range proton-carbon coupling constants in CH spin sytems for samples at natural abundance. Such measurements are performed by measuring the relative displacement of relayed signals in the corresponding alpha and beta 1D subspectra.     

Multi-scale NMR characterisation of mesostructured materials using 1H-->13C through-bond polarisation transfer,fast MAS,and 1H spin diffusion

A NMR method for the characterisation of materials at different length scales, robust and simple to implement, is presented. It combines selection of 1H-13C pairs by a through-bond polarisation transfer (INEPT here) and exploration of larger distances by the introduction of 1H spin diffusion. This characterisation method is well adapted to the highest MAS rates and takes benefits of it. The effect of 1H dephasing on the efficiency of the 1H-->13C through-bond polarisation transfer is determined. This allows consecutively the quantification of signals. Mesostructured spherical silica-based particles containing CTA+ cations were studied by this multi-scale characterisation method. Contrasted spin diffusion curves were found and qualitatively explained by differences in terms of mobility and spatial distributions.     

3D HCCH(3)-TOCSY for resonance assignment of methyl-containing side chains in (13)C-labeled proteins

Two 3D experiments, (H)CCH(3)-TOCSY and H(C)CH(3)-TOCSY, are proposed for resonance assignment of methyl-containing amino acid side chains. After the initial proton-carbon INEPT step, during which either carbon or proton chemical shift labeling is achieved (t(1)), the magnetization is spread along the amino acid side chains by a carbon spin lock. The chemical shifts of methyl carbons are labeled (t(2)) during the following constant time interval. Finally the magnetization is transferred, in a reversed INEPT step, to methyl protons for detection (t(3)). The proposed experiments are characterized by high digital resolution in the methyl carbon dimension (t(2max) = 28.6 ms), optimum sensitivity due to the use of proton decoupling during the long constant time interval, and an optional removal of CH(2), or CH(2) and CH, resonances from the F(2)F(3) planes. The building blocks used in these experiments can be implemented in a range of heteronuclear experiments focusing on methyl resonances in proteins. The techniques are illustrated using a (15)N, (13)C-labeled E93D mutant of Schizosacharomyces pombe phosphoglycerate mutase (23.7 kDa).     

Fast proton spin-lattice relaxation time in the rotating frame during the application of time averaged precession frequency

A relatively rapid phase alternation of the effective field in the time averaged precession frequency (TAPF) sequence results in averaging of the proton RF spin-lock field. The spin-locking of the proton magnetization becomes less efficient and thus shortens T(1rho)(H), the proton spin-lattice relaxation time in the rotating frame. The relaxation time also depends on the ratio of tau(1) and tau(2) intervals i.e. tau(1)/tau(2) and not only on the number of tau(c)=tau(1)+tau(2) blocks, i.e. the number of the phase transients. Experiments are performed on solid samples of ferrocene and glycine and for some time intervals, T(1rho)(H) is shortened by factors of 9-100 compared to the relaxation times obtained in the standard experiment.     

Recoupled long-range C-H dipolar dephasing in solid-state NMR,and its use for spectral selection of fused aromatic rings

This work introduces a simple new solid-state 13C NMR method for distinguishing various types of aromatic residues, e.g. those of lignin from fused rings of charcoal. It is based on long-range dipolar dephasing, which is achieved by recoupling of long-range C-H dipolar interactions, using two 1H 180 degrees pulses per rotation period. This speeds up dephasing of unprotonated carbon signals approximately threefold compared to standard dipolar dephasing without recoupling and thus provides much more efficient differential dephasing. It also reduces the effects of spinning-speed dependent effective proton-proton dipolar couplings on the heteronuclear dephasing. Signals of unprotonated carbons with two or more protons at a two-bond distance dephase to <3% within less than 0.9 ms, significantly faster than those of aromatic sites separated from the nearest proton by three or more bonds. Differential dephasing among different unprotonated carbons is demonstrated in a substituted anthraquinone and 3-methoxy benzamide. The data yield a calibration curve for converting the dephasing rates into estimated distances from the carbon to the nearest protons. This can be used for peak assignment in heavily substituted or fused aromatic molecules. Compared to lignin, slow dephasing is observed for the aromatic carbons in wood charcoal, and even slower for inorganic carbonate. Direct 13C polarization is used on these structurally complex samples to prevent loss of the signals of interest, which by design originate from carbons that are distant from protons and therefore crosspolarize poorly. In natural organic matter such as humic acids, this combination of recoupled dipolar dephasing and direct polarization at 7-kHz MAS enables selective observation of signals from fused rings that are characteristic of charcoal.     

Slow-down of 13C spin diffusion in organic solids by fast MAS: a CODEX NMR Study.

One- and two-dimensional 13C exchange nuclear magnetic resonance experiments under magic-angle spinning (MAS) can provide detailed information on slow segmental reorientations and chemical exchange in organic solids, including polymers and proteins. However, observations of dynamics on the time scale of seconds or longer are hampered by the competing process of dipolar 13C spin exchange (spin diffusion). In this Communication, we show that fast MAS can significantly slow down the dipolar spin exchange effect for unprotonated carbon sites. The exchange is measured quantitatively using the centerband-only detection of exchange technique, which enables the detection of exchange at any spinning speed, even in the absence of changes of isotropic chemical shifts. For chemically equivalent unprotonated 13C sites, the dipolar spin exchange rate is found to decrease slightly less than proportionally with the sample-rotation frequency, between 8 and 28 kHz. In the same range, the dipolar spin exchange rate for a glassy polymer with an inhomogeneously broadened MAS line decreases by a factor of 10. For methylene groups, no or only a minor slow-down of the exchange rate is found.