Obtaining molecular and structural information from 13C-14N systems with 13C FIREMAT experiments

The effect of dipolar coupling to 14N on 13C FIREMAT (five pi replicated magic angle turning) experiments is investigated. A method is developed for fitting the 13C FIREMAT FID employing the full theory to extract the 13C-14N dipolar and 13C chemical shift tensor information. The analysis requires prior knowledge of the electric field gradient (EFG) tensor at the 14N nucleus. In order to validate the method the analysis is done for the amino acids alpha-glycine, gamma-glycine, l-alanine, l-asparagine, and l-histidine on FIREMAT FIDs recorded at 13C frequencies of 50 and 100 MHz. The dipolar and chemical shift data obtained with this analysis are in very good agreement with the previous single-crystal 13C NMR results and neutron diffraction data on alpha-glycine, l-alanine, and l-asparagine. The values for gamma-glycine and l-histidine obtained with this new method are reported for the first time. The uncertainties in the EFG tensor on the resultant 13C chemical shift and dipolar tensor values are assessed.     

13C-(27)Al TRAPDOR and REDOR experiments for the detection of (13)C-(27)Al dipolar interactions in solids.

We report (13)C-(27)Al double resonance experiments (REDOR and TRAPDOR) on several aluminum organic compounds with the aim of detecting (13)C-(27)Al dipolar couplings and distances in solids. The (13)C and (27)Al pulses are applied to the same probe channel because their resonance frequencies are in close proximity. The different possibilities of controlling the efficiency of the TRAPDOR approach (by varying the (27)Al RF amplitude and the MAS frequency) are investigated. The results indicate that TRAPDOR is superior to REDOR in resolving differences in (13)C-(27)Al distances when choosing the proper experimental conditions. Where known, the crystal structure data are in qualitative agreement with the distance information extracted from our experiments. The experiment should be very valuable in different fields of solid state chemistry, where the interaction of organic and inorganic sample fractions is of fundamental importance.     

Combination of (13)C-(13)C COSY and DARR (COCODARR) in solid-state NMR

In this work, we describe a new 2D (13)C-(13)C correlation experiment in solids, in which (13)C-(13)C J-correlation (COSY) and dipolar correlation (DARR) are recorded at the same time. The sequence is similar to COCONOSY in the liquid-state NMR, in which (1)H-(1)H COSY and NOESY spectra are obtained in a single experiment. The combined COSY and DARR experiment facilitates assignment of (13)C signals in solids.     

碳苷化合物的13C-NMR定量研究

本文报道了通过计算碳苷α,β端基异构体糖环上相同碳原子在¹³C-NMR全去偶谱和NNE谱中峰强度或峰面积之比进行定量研究的方法.结果表明,如果碳苷化合物具有合适的溶解度,可以得到很准确的结果,相对误差小于1%.     

Heteronuclear Double-Quantum MAS NMR Spectroscopy in Dipolar Solids

A new pulse sequence for high-resolution solid-state heteronuclear double-quantum MAS NMR spectroscopy of dipolar-coupled spin-12 nuclei is introduced. It is based on the five-pulse sequence known from solution-state NMR, which is here applied synchronously to both spin species. The heteronuclear double-quantum (HeDQ) spinning-sideband patterns produced by this experiment are shown to be sensitive to the heteronuclear distance, as well as the relative orientations of the chemical-shift and dipolar tensors. In particular, it is shown that the HeDQ patterns exhibit an enhanced sensitivity to the chemical shielding tensors as compared with the single-quantum spinning-sideband patterns. The detection of HeDQ patterns via the I and S spins is discussed. The isolated (13)C-(1)H spin pair in deuterated ammonium formate with (13)C in natural abundance was chosen as a model system, and the perturbing influence of dipolar couplings to surrounding protons on the (13)C-(1)H DQ coherence is discussed. The pulse sequence can also be used as a heteronuclear double-quantum filter, hence providing information about heteronuclear couplings, and thus allowing the differentiation of quaternary and CH(n) bonded carbons. The elucidation of (13)C-(1)H dipolar proximities is presented for a sample of bisphenol A polycarbonate with (13)C in natural abundance, recorded with a broadband version of the synchronized five-pulse sequence.     

C-13 NMR Spectra of N-Aminopyridinium Perchlorates

C-13 NMR spectra of fifteen N-aminopyridinium perchlorates show the effect of amination of the ring nitrogen atom to be similar to that observed in the pyridine N-oxide spectra, particularly for the 2(6) and 3(5) positions. Benzene and pyridine increments predict well the carbon chemical shifts for the title compounds unless both positions 2 and 6 are occupied. The chemical shifts of some carbon atoms in the compounds studied are linearly dependent on those in the respective benzene derivatives as well as on the Hammett σ constants.     

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].     

(13)C chemical shift and (13)C-(14)N dipolar coupling tensors determined by (13)C rotary resonance solid-state NMR

This work explores the utility of simple rotary resonance experiments for the determination of the magnitude and orientation of (13)C chemical shift tensors relative to one or more (13)C--(14)N internuclear axes from (13)C magic-angle-spinning NMR experiments. The experiment relies on simultaneous recoupling of the anisotropic (13)C chemical shift and (13)C--(14)N dipole--dipole coupling interactions using 2D rotary resonance NMR with RF irradiation on the (13)C spins only. The method is demonstrated by experiments and numerical simulations for the (13)C(alpha) spins in powder samples of L-alanine and glycine with (13)C in natural abundance. To investigate the potential of the experiment for determination of relative/absolute tensor orientations and backbone dihedral angles in peptides, the influence from long-range dipolar coupling to sequential (14)N spins in a peptide chain ((14)N(i)--(13)C(alpha)(i)--(14)N(i+1) and (14)N(i+1)--(13)C'(i)--(14)N(i) three-spin systems) as well as residual quadrupolar-dipolar coupling cross-terms is analyzed numerically.     

Proton MRI of (13)C distribution by J and chemical shift editing.

The sensitivity of (13)C NMR imaging can be considerably favored by detecting the (1)H nuclei bound to (13)C nuclei via scalar J-interaction (X-filter). However, the J-editing approaches have difficulty in discriminating between compounds with similar J-constant as, for example, different glucose metabolites. In such cases, it is almost impossible to get J-edited images of a single-compound distribution, since the various molecules are distinguishable only via their chemical shift. In a recent application of J-editing to high-resolution spectroscopy, it has been shown that a more efficient chemical selectivity could be obtained by utilizing the larger chemical shift range of (13)C. This has been made by introducing frequency-selective (13)C pulses that allow a great capability of indirect chemical separation. Here a double-resonance imaging approach is proposed, based on both J-editing and (13)C chemical shift editing, which achieves a powerful chemical selectivity and is able to produce full maps of specific chemical compounds. Results are presented on a multicompartments sample containing solutions of glucose and lactic and glutamic acid in water.     

2D(13)C-(13)C MAS NMR correlation spectroscopy with mixing by true (1)H spin diffusion reveals long-range intermolecular distance restraints in ultra high magnetic field

An improved 2D (13)C-(13)C CP(3) MAS NMR correlation experiment with mixing by true (1)H spin diffusion is presented. With CP(3), correlations can be detected over a much longer range than with direct (1)H-(13)C or (13)C-(13)C dipolar recoupling. The experiment employs a (1)H spin diffusion mixing period tau(m) sandwiched between two cross-polarization periods. An optimized CP(3) sequence for measuring polarization transfer on a length scale between 0.3 and 1.0 nm using short mixing times of 0.1 ms < tau(m) < 1 ms is presented. For such a short tau(m), cross talk from residual transverse magnetization of the donating nuclear species after a CP can be suppressed by extended phase cycling. The utility of the experiment for genuine structure determination is demonstrated using a self-aggregated Chl a/H(2)O sample. The number of intramolecular cross-peaks increases for longer mixing times and this obscures the intermolecular transfer events. Hence, the experiment will be useful for short mixing times only. For a short tau(m) = 0.1 ms, intermolecular correlations are detected between the ends of phytyl tails and ring carbons of neighboring Chl a molecules in the aggregate. In this way the model for the structure, with stacks of Chl a that are arranged back to back with interdigitating phytyl chains stretched between two bilayers, is validated.     

Pursuing structure in microcrystalline solids with independent molecules in the unit cell using 1H-13C correlation data

The (1)H-(13)C solid-state NMR heteronuclear correlation (HETCOR) experiment is demonstrated to provide shift assignments in certain powders that have two or more structurally independent molecules in the unit cell (i.e. multiple molecules per asymmetric unit). Although this class of solids is often difficult to characterize using other methods, HETCOR provides both the conventional assignment of shifts to molecular positions and associates many resonances with specific molecules in the asymmetric unit. Such assignments facilitate conformational characterization of the individual molecules of the asymmetric unit and the first such characterization solely from solid-state NMR data is described. HETCOR offers advantages in sensitivity over prior methods that assign resonances in the asymmetric unit by (13)C-(13)C correlations and therefore allows shorter average analysis times in natural abundance materials. The (1)H-(13)C analysis is demonstrated first on materials with known shift assignments from INADEQUATE data (santonin and Ca(OAc)(2) phase I) to verify the technique and subsequently is extended to a pair of unknown solids: (+)-catechin and Ca(OAc)(2) phase II. Sufficient sensitivity and resolution is achieved in the spectra to provide assignments to one of the specific molecules of the asymmetric unit at over 54% of the sites.     

Resolution of 13C-19F interactions in the 13C NMR of spinning solids and liquid crystals

Anomalous line-broadenings of carbon resonances close to 19F have commonly been reported in the 13C NMR of liquid crystals and solids. We have previously shown that these effects in static liquid-crystal samples are related to the difficulty of 1H decoupling in the presence of strong 1H-19F dipolar interactions. We here extend this work to spinning samples (both liquid crystals and solids). A number of different line-broadening mechanisms are elucidated: analogous decoupling effects, magic angle misset, and 19F lifetime-broadening. In relatively mobile systems, such as liquid crystals or soft solids, the limiting factor on 13C resolution (and the ability to directly quantify the 13C-19F interactions) is found to be the efficiency and robustness of the 1H-decoupling. In rigid solids, the lifetime of the 19F spin-states is found to be an additional critical factor.     

Measuring (13)C-(2)D dipolar couplings with a universal REDOR dephasing curve

A (13)C-observe REDOR experiment is described which allows (13)C-(2)D dipolar couplings to be obtained by a universal dipolar dephasing curve. Previous (13)C-observe REDOR experiments on (13)C-(2)D spin pairs generally relied on numerical simulations to obtain the dipolar coupling. The REDOR experiment described in this article is based on a deuterium composite pulse, and the data analysis eliminates the need for numerical simulations and is the same as the traditional REDOR analysis performed on pairs of spin-12 nuclei. Copyright 2000 Academic Press.     

Broadband 13C-13C adiabatic mixing in solution optimized for high fields

Experiments which require mixing among spins with large frequency differences are generally performed with sequences based on composite pulses or computer-optimized cycles. Adiabatic pulses generally offer several advantages over other approaches, including greater single spin inversion bandwidths and tolerance to RF inhomogeneity. Here, a novel theoretical framework is presented in order to understand how spin-spin interactions are influenced by adiabatic inversion pulses, and insights from this approach are used to design more efficient adiabatic coherence exchange experiments. For very large frequency differences, this new approach generally offers improved results over previously applied mixing sequences, as applied to 13C-13C experiments which are the basis of modern sidechain assignment techniques in proteins. It is also anticipated that the approach presented here will be applicable to the analysis of various alternative approaches to adiabatic mixing.     

Double-quantum filtered rotational-echo double resonance

The homonuclear scalar coupling of a directly bonded 13C-13C pair has been used to create a double-quantum filter (DQF) to remove the natural-abundance 13C background in 13C{15N} rotational-echo double-resonance (REDOR) experiments. The DQF scalar and REDOR dipolar evolution periods are coincident which is important for sensitivity in the event of weak 13C-15N dipolar coupling. Calculated and observed 13C{15N} DQF-REDOR dephasings were in agreement for a test sample of mixed recrystallized labeled alanines. Glycine metabolism in a single uniform-15N soybean leaf labeled for 6 min by 13CO2 was measured quantitatively by 13C{15N} DQF-REDOR with no background interferences.     

Multidimensional dipolar exchange-assisted recoupling measurements in solid-state NMR

Aseries of uni- and multidimensional variants of the dipolar exchange-assisted recoupling (DEAR) NMR experiment is described and applied to determinations of (13)C-(14)N dipolar local field spectra in amino acids and dipeptides. The DEAR protocol recouples nearby nuclei by relying on differences in their relative rates of longitudinal relaxation, and has the potential to give quantitative geometric results without requiring radiofrequency pulsing on both members of a coupled spin pair. One- and two-dimensional variants of this recoupling strategy on generic I-S pairs are discussed, and measurements of (13)C-(14)N distances and 2D local field experiments sensitive to the relative orientation of CN vectors with respect to the (13)C shielding tensor are presented. Since these measurements did not involve pulsing on the broad nitrogen resonance, their results were independent of the quadrupolar parameters of this nucleus. High-resolution 3D NMR versions of the 2D experiments were also implemented in order to separate their resulting local field patterns according to the isotropic shifts of inequivalent (13)C sites. These high-resolution 3D acquisitions involved collecting a series of 2D DEAR NMR data sets on rotating samples as a function of spinning angle, and then subjecting the resulting data to a processing akin to that involved in variable-angle correlation NMR. Once successfully tested on l-alanine this experiment was applied to the analysis of a series of dipeptides, allowing us to extract separate local field (13)C-(14)N spectra from this type of multisite systems.     

C-13 and H-1 NMR Study of Alkyl and Aryl-Substituted 1-(4′-Dimethylaminobenzylideneamino)pyridinium Perchlorates

C-13 and H-1 NMR spectra of some 1-(4′-dimethylaminobenzylideneamino)pyridinium perchlorates show that the angle of twist of the pyridine ring in unsubstituted, 1, and 2-alkyl substituted compounds 2–5 is comparable. However, it is considerably increased in 2,6-dialkyl derivatives. As seen from the spectral data, pyridine and phenyl rings in 2,6-diphenyl derivative 15 are not coplanar. The effect of 4-alkyl substituent is of hyperconjugative chacter. In general, the amount of the positive charge at C-4 in 2,6-dialkyl substituted salts is higher as compared to 2-monosubstituted compounds.     

Rotational-echo double resonance of uniformly labeled 13C clusters

The use of rotational-echo double resonance NMR to measure distances from an observed tightly coupled cluster of 13C spins to a distant 15N, 31P, or 19F is practical if 13C chemical shifts and homonuclear 13C-13C isotropic J interactions are refocused by a combination of rotor-synchronized 13C pi and pi/2 pulses. This scheme is illustrated by experiments performed on diluted and recrystallized L-[13C(3),15N]alanine and L-[13C(6),alpha-15N]histidine.     

High-resolution solid-state 13C NMR studies of poly[(R)-3-hydroxybutyric] acid

Solid poly[(R)-3-hydroxybutyric] acid was examined by high-resolution ¹³C MAS NMR, differential scanning calorimetry and infrared spectroscopy. The ¹³C methyl group resonance consists of three components: the rigid amorphous phase, the crystalline phase and the mobile amorphous phase. Spectral deconvolution, using the Lorentz function, reveals the relative amounts as 62% crystalline and 38% amorphous at 333 K, and 42% crystalline, 40% pure amorphous and 18% β-orthorhombic at 413 K. NMR indicates a large difference in molecular mobility between the crystalline and amorphous regions of the sample. Infrared spectroscopy shows that the stretching at 1725 cm⁻¹ (characteristic of the -form) comes from the crystalline region, and the bands at 1744 cm⁻¹ (characteristic of the β-form) and 1800 cm⁻¹ come from the amorphous region.     

Estimation of the lateral dimensions of cellulose crystallites using 13C NMR signal strengths

Differences in proton rotating-frame spin relaxation rates were exploited to edit the 13C NMR spectra of solid lignocellulosics, separating signals assigned to cellulose crystallites from signals assigned to amorphous material. Clusters of signals at 89 and 85 ppm were assigned to C-4 in the interiors and on the surfaces of cellulose crystallites, respectively. Relative signal areas were used to estimate the weight-averaged lateral dimensions of crystallites, using a model in which crystallites have approximately square cross sections. The same 10 samples were also characterized by wide-angle X-ray scattering (WAXS). There was a strong correlation (r2 = 0.988) between the two sets of lateral dimensions, but those estimated by WAXS were typically 10% lower than those estimated by NMR. The deviations were attributed to differences in molecular conformations between interior and surface chains, causing broadening of the WAXS peaks. In the case of an eleventh sample containing well-ordered xylan, the NMR and WAXS methods were in good agreement only after exclusion of a xylan signal at 82.6 ppm from the NMR data.