Frequency-selective homonuclear dipolar recoupling in solid state NMR

We introduce a new approach to frequency-selective homonuclear dipolar recoupling in solid state nuclear magnetic resonance (NMR) with magic-angle spinning (MAS). This approach, to which we give the acronym SEASHORE, employs alternating periods of double-quantum recoupling and chemical shift evolution to produce phase modulations of the recoupled dipole-dipole interactions that average out undesired couplings, leaving only dipole-dipole couplings between nuclear spins with a selected pair of NMR frequencies. In principle, SEASHORE is applicable to systems with arbitrary coupling strengths and arbitrary sets of NMR frequencies. Arbitrary MAS frequencies are also possible, subject only to restrictions imposed by the pulse sequence chosen for double-quantum recoupling. We demonstrate the efficacy of SEASHORE in experimental (13)C NMR measurements of frequency-selective polarization transfer in uniformly (15)N, (13)C-labeled L-valine powder and frequency-selective intermolecular polarization transfer in amyloid fibrils formed by a synthetic decapeptide containing uniformly (15)N, (13)C-labeled residues.     

Efficient symmetry-based homonuclear dipolar recoupling of quadrupolar spins: double-quantum NMR correlations in amorphous solids

We report novel symmetry-based pulse sequences for exciting double-quantum (2Q) coherences between the central transitions of half-integer spin quadrupolar nuclei in the NMR of rotating solids. Compared to previous 2Q-recoupling techniques, numerical simulations and 23Na and 27Al NMR experiments on Na2SO4 and the open-framework aluminophosphate AlPO-CJ19 verify that the new dipolar recoupling schemes display higher robustness to both radio-frequency field inhomogeneity and to spreads in resonance frequencies. These advances allowed for the first demonstration of 2Q-recoupling in an amorphous solid for revealing its intermediate-range structural features, in the context of mapping 27Al-27Al connectivities between the aluminium polyhedra (AlO4, AlO5 and AlO6) of a lanthanum aluminate glass (La0.18Al0.82O1.5).     

Towards measurement of homonuclear dipolar couplings in 1H solid-state NMR: recoupling with a rotor-synchronized decoupling scheme

We describe a magic-angle spinning NMR experiment for (1)H-(1)H homonuclear dipole-dipole coupling estimations in organic solids. The methodology involves reintroducing dipolar interactions with rotor-synchronized homonuclear decoupling pulse sequences. Frequency-selective DANTE pulses are used to isolate a specific spin pair from a natural isotopic abundance sample. The coupling of interest, between the selected spin pair, may be extracted by a non linear least-squares fit of the experimentally observed modulation of the signal intensity to an exact analytical formula. The experiment is demonstrated on natural isotopic abundance glycine and alanine powder samples.     

A study of homonuclear dipolar recoupling pulse sequences in solid-state nuclear magnetic resonance

Dipolar recoupling pulse sequences are of great importance in magic angle spinning solid-state NMR. Recoupling sequences are used for excitation of double-quantum coherence, which, in turn, is employed in experiments to estimate internuclear distances and molecular torsion angles. Much effort is spent on the design of recoupling sequences that are able to produce double-quantum coherence with high efficiency in demanding spin systems, i.e., spin systems with small dipole-dipole couplings and large chemical-shift anisotropies (CSAs). The sequence should perform robustly under a variety of experimental conditions. This paper presents experiments and computer calculations that extend the theory of double-quantum coherence preparation from the strong coupling/small CSA limit to the weak coupling limit. The performance of several popular dipole-dipole recoupling sequences-DRAWS, POST-C7, SPC-5, R1, and R2-are compared. It is found that the optimum performance for several of these sequences, in the weak coupling/large CSA limit, varies dramatically, with respect to the sample spinning speed, the magnitude and orientation of the CSAs, and the magnitude of dipole-dipole couplings. It is found that the efficiency of double-quantum coherence preparation by gamma-encoded sequences departs from the predictions of first-order theory. The discussion is supported by density-matrix calculations.     

Improving solid-state NMR dipolar recoupling by optimal control

We present the first solid-state NMR experiments developed using optimal control theory. Taking heteronuclear dipolar recoupling in magic-angle-spinning NMR as an example, it proves possible to significantly improve the efficiency of the experiments while introducing robustness toward instrumental imperfections such as radio frequency inhomogeneity. The improvements are demonstrated by numerical simulations as well as practical experiments on a 13Calpha,15N-labeled powder of glycine. The experiments demonstrate a gain of 53% in the efficiency for 15N to 13Calpha coherence transfer relative to the typically double-cross-polarization experiments.     

Symmetry-based 29Si dipolar recoupling magic angle spinning NMR spectroscopy: a new method for investigating three-dimensional structures of zeolite frameworks

A new 29Si solid-state MAS NMR experiment is described for investigating the framework structures of pure silica zeolites. The symmetry-based homonuclear dipolar recoupling sequence SR26411 has been incorporated into a two-dimensional NMR experiment to probe the Si-O-Si bonding connectivities and long-range Si-Si distances in zeolite frameworks. This dipolar recoupling sequence is shown to have a number of advantages over the J-coupling-based INADEQUATE experiment. For the clathrasil Sigma-2, it is demonstrated that there is excellent agreement between experimental double-quantum build-up curves obtained from a series of two-dimensional double-quantum correlation spectra and simulated curves which consider all Si-Si distances out to 8 A. This result suggests that this experiment could be used to solve zeolite frameworks with unknown structures.     

Determination of peptide backbone torsion angles using double-quantum dipolar recoupling solid-state NMR spectroscopy

Several approaches for utilizing dipolar recoupling solid-state NMR (ssNMR) techniques to determine local structure at high resolution in peptides and proteins have been developed. However, many of these techniques measure only one torsion angle or are accurate for only certain classes of secondary structure. Additionally, the efficiency with which these dipolar recoupling experiments suppress the deleterious effects of chemical shift anisotropy (CSA) at high magnetic field strengths varies. Dipolar recoupling with a windowless sequence (DRAWS) has proven to be an effective pulse sequence for exciting double-quantum (DQ) coherences between adjacent carbonyl carbons along the peptide backbone. By allowing this DQ coherence to evolve, it is possible to measure the relative orientations of the CSA tensors and subsequently use this information to determine the Ramachandran torsion angles phi and psi. Here, we explore the accuracies of the assumptions made in interpreting DQ-DRAWS data and demonstrate their fidelity in measuring torsion angles corresponding to a variety of secondary structures irrespective of hydrogen-bonding patterns. It is shown how a simple choice of isotopic labels and experimental conditions allows accurate measurement of backbone secondary structures without any prior knowledge. This approach is considerably more sensitive for determining structure in helices and has comparable accuracy for beta-sheet and extended conformations relative to other methods. We also illustrate the ability of DQ-DRAWS to distinguish between structures in heterogeneous samples.     

Measuring distances between half-integer quadrupolar nuclei and detecting relative orientations of quadrupolar and dipolar tensors by double-quantum homonuclear dipolar recoupling nuclear magnetic resonance experiments

We studied the possibility of using double-quantum homonuclear dipolar recoupling magic angle spinning nuclear magnetic resonance experiments for structural analysis of systems of half-integer quadrupolar nuclei. We investigated symmetry-based recoupling schemes R2(2) (1) and R2(2) (1)R2(2) (-1) and showed that the obtained double-quantum filtered signals depend substantially on magnitudes and relative orientations of dipolar and quadrupolar tensors. Experimental results measured on aluminophosphate molecular sieve AlPO(4)-14, containing dipolar-coupled spin-52 aluminum nuclei, were compared to results of time-consuming numerical simulations. The comparison for short mixing times allowed us to roughly measure internuclear Al-Al distances, if constraints about relative tensor orientations were available. Inspection of relative orientations of dipolar and quadrupolar tensors, using known distances between nuclei, required experimental and simulated data for long mixing times and yielded less accurate results. Two experimental protocols were employed for measuring double-quantum filtered curves, the symmetric protocol, in which excitation and reconversion periods are incremented simultaneously, and the asymmetric protocol, in which only the length of the excitation period is incremented and the length of the reconversion period is kept constant. The former experimental protocol was more convenient for the detection of internuclear distances, and the latter one was more appropriate for the inspection of relative orientations of interaction tensors.     

Spin dynamics in the modulation frame: application to homonuclear recoupling in magic angle spinning solid-state NMR

We introduce a family of solid-state NMR pulse sequences that generalizes the concept of second averaging in the modulation frame and therefore provides a new approach to perform magic angle spinning dipolar recoupling experiments. Here, we focus on two particular recoupling mechanisms-cosine modulated rotary resonance (CMpRR) and cosine modulated recoupling with isotropic chemical shift reintroduction (COMICS). The first technique, CMpRR, is based on a cosine modulation of the rf phase and yields broadband double-quantum (DQ) (13)C recoupling using >70 kHz omega(1,C)/2pi rf field for the spinning frequency omega(r)/2=10-30 kHz and (1)H Larmor frequency omega(0,H)/2pi up to 900 MHz. Importantly, for p>or=5, CMpRR recouples efficiently in the absence of (1)H decoupling. Extension to lower p values (3.5相似文献   

Determining hydrogen-bond strengths in the solid state by NMR: the quantitative measurement of homonuclear J couplings

Hydrogen-bonding strengths in the solid state are quantitatively determined by the accurate measurement of 15N-15N J couplings using a straightforward 2D MAS NMR spinecho approach.     

Chemical bonding in silicophosphate gels: Contribution of dipolar and J-derived solid state NMR techniques

A representative silicophosphate gel was synthesized, starting from orthophosphate groups and pyrophosphate species. At 136°C, a complex mixture of crystalline phases and amorphous components was obtained. A new panel of solid state NMR techniques was implemented, including dipolar based experiments (CP MAS), as well as J-derived techniques, in both homonuclear (³¹P INADEQUATE-MAS) and heteronuclear (³¹P/²⁹Si HMQC-MAS) versions. These experiments are suitable for the fine characterization of P–O–P, P–O–Si, P–OH…linkages in silicophosphate gels and materials.     

Effective Hamiltonians by optimal control: solid-state NMR double-quantum planar and isotropic dipolar recoupling

We report the use of optimal control algorithms for tailoring the effective Hamiltonians in nuclear magnetic resonance (NMR) spectroscopy through sophisticated radio-frequency (rf) pulse irradiation. Specifically, we address dipolar recoupling in solid-state NMR of powder samples for which case pulse sequences offering evolution under planar double-quantum and isotropic mixing dipolar coupling Hamiltonians are designed. The pulse sequences are constructed numerically to cope with a range of experimental conditions such as inhomogeneous rf fields, spread of chemical shifts, the intrinsic orientation dependencies of powder samples, and sample spinning. While the vast majority of previous dipolar recoupling sequences are operating through planar double-or zero-quantum effective Hamiltonians, we present here not only improved variants of such experiments but also for the first time homonuclear isotropic mixing sequences which transfers all I(x), I(y), and I(z) polarizations from one spin to the same operators on another spin simultaneously and with equal efficiency. This property may be exploited to increase the signal-to-noise ratio of two-dimensional experiments by a factor of square root 2 compared to conventional solid-state methods otherwise showing the same efficiency. The sequences are tested numerically and experimentally for a powder of (13)C(alpha),(13)C(beta)-L-alanine and demonstrate substantial sensitivity gains over previous dipolar recoupling experiments.     

Radio frequency-driven recoupling at high magic-angle spinning frequencies: homonuclear recoupling sans heteronuclear decoupling

We describe solid-state NMR homonuclear recoupling experiments at high magic-angle spinning (MAS) frequencies using the radio frequency-driven recoupling (RFDR) scheme. The effect of heteronuclear decoupling interference during RFDR recoupling at high spinning frequencies is investigated experimentally and via numerical simulations, resulting in the identification of optimal decoupling conditions. The effects of MAS frequency, RF field amplitude, bandwidth, and chemical shift offsets are examined. Most significantly, it is shown that broadband homonuclear correlation spectra can be efficiently obtained using RFDR without decoupling during the mixing period in fully protonated samples, thus considerably reducing the rf power requirements for acquisition of (13)C-(13)C correlation spectra. The utility of RFDR sans decoupling is demonstrated with broadband correlation spectra of a peptide and a model protein at high MAS frequencies and high magnetic field.     

Measurements of carbon to amide-proton distances by C-H dipolar recoupling with 15N NMR detection

A new magic-angle spinning NMR method for measuring internuclear distances between a 13C-labeled site and amide protons is described. The magnetization of the protons evolves under homonuclear decoupling and the recoupled 13C-1H dipolar interaction, which provides simple spin-pair REDOR curves if only one 13C-labeled site is present. The modulation of the amide proton HN is detected via short 1H-15N cross polarization followed by 15N detection. The method is demonstrated on two specifically 13C- and 15N-labeled peptides, with 13C-HN distances from 2.2 to ca. 6 A. This technique promises to be particularly useful for measuring distances between 13C=O and H-15N groups, to identify hydrogen bonds in peptides and proteins.     

Determination of long-range distances and dynamic order parameters by dipolar recoupling in high-resolution magic-angle spinning NMR spectroscopy

By introducing dipolar recoupling methods to high-resolution magic-angle spinning (HRMAS) NMR spectroscopy, a class of experiments has been delevoped that allows the measurement of residual dipole-dipole couplings of approximately 1 Hz in weakly immobilized molecules. Using homonuclear 1H-1H recoupling, distances of up to approximately 8 A can be selectively determined, while heteronuclear 1H-13C recoupling provides access to dynamic order parameters of individual molecular segments on the order of approximately 10-3. The experiments are demonstrated on functionalized oligopeptides that are attached to polymer resins.     

Supercycled homonuclear dipolar decoupling in solid-state NMR: toward cleaner 1H spectrum and higher spinning rates

A homonuclear dipolar decoupling scheme based on windowed phase-modulated Lee-Goldburg (wPMLG) pulse sequences that causes a"z-rotation" of the spins for high-resolution proton NMR spectroscopy of solids is described and analyzed. This supercycled scheme suppresses the effect of pulse imperfections on the spectra and significantly relaxes the off-resonance dependence of the line-narrowing efficiency and scale factor. This leads to a broad spectral window that is free of artifacts such as zero lines, image peaks, and localized rotor-radio-frequency resonances. High-resolution (1)H spectra and two-dimensional homonuclear (1)H-(1)H correlation spectra of standard amino acids, obtained by a combination of this supercycled scheme with magic angle spinning frequencies up to 25 kHz, are demonstrated.     

Theory of stochastic dipolar recoupling in solid-state nuclear magnetic resonance

Dipolar recoupling techniques in solid-state nuclear magnetic resonance (NMR) consist of radio frequency (rf) pulse sequences applied in synchrony with magic-angle spinning (MAS) that create nonzero average magnetic dipole-dipole couplings under MAS. Stochastic dipolar recoupling (SDR) is a variant in which randomly chosen rf carrier frequency offsets are introduced to cause random phase modulations of individual pairwise couplings in the dipolar spin Hamiltonian. Several aspects of SDR are investigated through analytical theory and numerical simulations: (1) An analytical expression for the evolution of nuclear spin polarization under SDR in a two-spin system is derived and verified through simulations, which show a continuous evolution from coherent, oscillatory polarization exchange to incoherent, exponential approach to equilibrium as the range of random carrier offsets (controlled by a parameter f(max)) increases; (2) in a many-spin system, polarization transfers under SDR are shown to be described accurately by a rate matrix in the limit of large f(max), with pairwise transfer rates that are proportional to the inverse sixth power of pairwise internuclear distances; (3) quantum mechanical interferences among noncommuting pairwise dipole-dipole couplings, which are a complicating factor in solid-state NMR studies of molecular structures by traditional dipolar recoupling methods, are shown to be absent from SDR data in the limit of large f(max), provided that coupled nuclei have distinct NMR chemical shifts.     

Quadrupolar coupling selective cross-polarization in solid state NMR

A new strategy is presented for achieving selective heteronuclear polarization transfers from half-integer quadrupolar spins in magic-angle spinning (MAS) NMR. By combining cross-polarization with a recently introduced RAPT pulse sequence that selectively excites the signal of a half-integer quadrupolar nucleus based on its quadrupolar coupling constant magnitude, we demonstrate that hetero-nuclei in its close proximity may be selectively excited. Selective 23Na --> 1H polarization transfers are demonstrated in Na2MoO4 x 2 H2O, Na2HPO4 x 2 H2O and a mixture of NaHCO3 and Na2HPO4 x 2 H2O.     

Broadband rotational resonance in solid state NMR spectroscopy

A new technique for restoring nuclear magnetic dipole-dipole couplings under magic-angle spinning (MAS) in solid state nuclear magnetic resonance (NMR) spectroscopy is described and demonstrated. In this technique, called broadband rotational resonance (BroBaRR), the coupling between a pair of nuclear spins with NMR frequency difference close (but not necessarily equal) to the MAS frequency is restored by the application of a train of weak radio-frequency pulses at a carrier frequency close to the average of the two NMR frequencies. Phase or amplitude modulation of the pulse train at half the MAS frequency splits the carrier into sidebands close to the two NMR frequencies. The pulse train then removes offsets from the exact rotational resonance condition, leading to dipolar recoupling over a bandwidth controlled by the amplitude of the pulse train. (13)C NMR experiments on uniformly (15)N,(13)C-labeled L-valineHClH(2)O powder validate the theoretical analysis. BroBaRR will be useful in studies of molecular structures by solid state NMR, for example in the detection of long-range couplings between carbons in uniformly labeled organic and biological materials.