Double-quantum dipolar recoupling at high magic-angle spinning rates

A full investigation of the possible homonuclear double-quantum recoupling sequences, based on the RN family of sequences with N < or = 20, is given. Several new RN sequences, R16(6)(5), R18(8)(5), and R18(10)(5), were applied at high magic-angle spinning rates and compared with theory. The R18(10)(5) technique can be used to recouple dipolar couplings at spinning rates up to 39 kHz, and the application of the sequence in an INADEQUATE experiment is shown for a spinning rate of 30 kHz.     

Switching-angle sample spinning NMR probe with a commercially available 20 kHz spinning system

A switching-angle sample spinning (SASS) probe workable at high spinning speeds was developed using a commercially available rotor/housing system. Details of the construction are described. As application examples of the SASS probe, we report experiments of powder pattern separation at the spinning speed of 20 kHz and broadband 13C-13C polarization transfer at 16 kHz.     

Biomolecular solid state NMR with magic-angle spinning at 25K

A magic-angle spinning (MAS) probe has been constructed which allows the sample to be cooled with helium, while the MAS bearing and drive gases are nitrogen. The sample can be cooled to 25 K using roughly 3 L/h of liquid helium, while the 4-mm diameter rotor spins at 6.7 kHz with good stability (±5 Hz) for many hours. Proton decoupling fields up to at least 130 kHz can be applied. This helium-cooled MAS probe enables a variety of one-dimensional and two-dimensional NMR experiments on biomolecular solids and other materials at low temperatures, with signal-to-noise proportional to 1/T. We show examples of low-temperature ¹³C NMR data for two biomolecular samples, namely the peptide Aβ_14–23 in the form of amyloid fibrils and the protein HP35 in frozen glycerol/water solution. Issues related to temperature calibration, spin–lattice relaxation at low temperatures, paramagnetic doping of frozen solutions, and ¹³C MAS NMR linewidths are discussed.     

Fast magic-angle spinning proton NMR studies of polymers at surfaces and interfaces

Solid-state proton NMR with fast magic-angle sample spinning has been used to study the structure and dynamics of polymers and the water interface in porous glass composites. The composites were prepared by photopolymerization of poly(ethyl acrylate) and other acrylate formulations in a high surface-area rigid glass matrix with 40-A interconnected pores. High resolution solid-state proton spectra were obtained for polymer films and composites with 15 kHz magic-angle sample spinning at temperatures above the polymer glass transition temperature. The solid-state proton spectra can be detected with high sensitivity and used to determine the composition of polymer and water filling the pores. These results and spin diffusion studies using 1H-29Si 2D heteronuclear correlation and wideline separation NMR show that the polymer fills the central 30 A of the pore, and that the remaining volume is filled with surface hydroxyl groups and water.     

Large field permittivity of TGS at 20 kHz

The paper gives the results of complex permittivity measurements of triglycine sulphate at large electric field amplitudes (36.6 V/m–386 kV/m) and a frequency of 20 kHz in a temperature range including the Curie temperature (22–53°C). An experimental arrangement for measuring by means of the pulse method is described which was used in order to prevent heating of the sample due to dielectric losses. The relations established between the permittivity and the amplitude of the electric field and temperature are in good qualitative agreement with the corresponding results obtained by calculation in the paper of J. Fousek [J. Appl. Phys. 36 (1965), 588].The authors are indebted to Dr. Z. Málek, Dr. J. Fousek, Ing. J. Janta, M. Marvan, Ing. J. Mastner and J. Fiala for valuable discussions and comments, to Ing. Z. Vojta for his help in realizing the electronic apparatus and to K. Müller for the careful preparation of the figures for publication.     

Multiple-quantum magic-angle spinning spectroscopy using nonlinear sampling

NMR spectroscopy is a relatively insensitive technique and many biomolecular applications operate near the limits of sensitivity and resolution. A particularly challenging example is detection of the quadrupolar nucleus 17O, due to its low natural abundance, large quadrupole couplings, and low gyromagnetic ratio. Yet the chemical shift of 17O spans almost 1000 ppm in organic molecules and it serves as a potentially unique reporter of hydrogen bonding in peptides, nucleic acids, and water, and as a valuable complement to 13C and 15N NMR. Recent developments including the multiple-quantum magic-angle spinning (MQMAS) experiment have enabled the detection of 17O in biological solids, but very long data acquisitions are required to achieve sufficient sensitivity and resolution. Here, we perform nonlinear sampling in the indirect dimension of MQMAS experiments to substantially reduce the total acquisition time and improve sensitivity and resolution. Nonlinear sampling prevents the use of the discrete Fourier transform; instead, we employ maximum entropy (MaxEnt) reconstruction. Nonlinearly sampled MQMAS spectra are shown to provide high resolution and sensitivity in several systems, including lithium sulfate monohydrate (LiSO(4)-H(2)17O) and L-asparagine monohydrate (H(2)17O). The combination of nonlinear sampling and MaxEnt reconstruction promises to make the application of 17O MQMAS practical in a wider range of biological systems.     

Recoupling of heteronuclear dipolar interactions with rotational-echo double-resonance at high magic-angle spinning frequencies

Heteronuclear dipolar recoupling with rotational-echo double-resonance (REDOR) is investigated in the rapid magic-angle spinning regime, where radiofrequency irradiation occupies a significant fraction of the rotor period (10-60%). We demonstrate, in two model (13)C-(15)N spin systems, [1-(13)C, (15)N] and [2-(13)C, (15)N]glycine, that REDOR DeltaS/S(0) curves acquired at high MAS rates and relatively low recoupling fields are nearly identical to the DeltaS/S(0) curve expected for REDOR with ideal delta-function pulses. The only noticeable effect of the finite pi pulse length on the recoupling is a minor scaling of the dipolar oscillation frequency. Experimental results are explained using both numerical calculations and average Hamiltonian theory, which is used to derive analytical expressions for evolution under REDOR recoupling sequences with different pi pulse phasing schemes. For xy-4 and extensions thereof, finite pulses scale only the dipolar oscillation frequency by a well-defined factor. For other phasing schemes (e.g., xx-4 and xx-4) both the frequency and amplitude of the oscillation are expected to change.     

Powder pattern recoupling at 10 kHz spinning speed applied to cellulose

Effective powder pattern recoupling by pi-pulses at spinning speeds up to 10kHz has been introduced. In a 2D experiment, the static chemical shift spectra of the indirect dimension were separated by the isotropic values of the direct dimension. Sufficient high spinning speeds ensured optimal exploitation of spectral intensities. This experiment was used to extract the 13C chemical shift tensor values of native Cellulose I and regenerated Cellulose II.     

Heating of samples induced by fast magic-angle spinning

Intense sample heating through high-speed magic-angle spinning (MAS; up to 58 K temperature difference) is demonstrated. The role of probehead and spinner design, as well as that of the temperature of the bearing air on the heating of a rotating sample, is examined. MAS-induced heating can affect the accurate determination of the isotropic value of the chemical shift as well as the principal values, asymmetry and anisotropy parameters of the chemical shift tensor. In some cases, a very large temperature gradient (12 K) within the fast rotating sample was found, which may limit the resolution of high-speed 1H MAS nuclear magnetic resonance (NMR) spectra.     

Non-contact electrodynamic measurement on metallic vibrators at 20kHz

Measurement of vibration amplitude on metallic rods by a contactless electrodynamic method is described. The sensitivity of a typical pick-up is investigated     

Effects of magic-angle spinning on spin-lattice relaxations in talc

The spin-lattice relaxation times T₁ of ¹H and ²⁹Si spins in talc have been measured at room temperature with and without magic-angle spinning (MAS) of the sample. Paramagnetic impurities work as relaxation centers. ¹H T₁ depends on the spinning rate, whereas ²⁹Si T₁ is independent of the spinning rate. These facts demonstrate that spin diffusion plays an important role in ¹H relaxation but not in ²⁹Si relaxation. ²⁹Si spins relax through dipole-dipole interactions with electron spins directly, which mechanism is not affected by spinning. The relaxation rates have been analyzed theoretically.     

Correlation of fast and slow chemical shift spinning sideband patterns under fast magic-angle spinning

A new two-dimensional solid-state NMR experiment, which correlates slow and fast chemical shift anisotropy sideband patterns is proposed. The experiment, dubbed ROSES, is performed under fast magic-angle spinning and leads to an isotropic spectrum in the directly detected omega(2) dimension. In the evolution dimension omega(1), the isotropic chemical shift is reduced by a factor S, and spinning sidebands are observed spaced by a scaled effective spinning speed omega(R)/S. These spinning sidebands patterns are not identical to those observed with standard slow magic-angle spinning experiments. Chemical shift anisotropy parameters can be accurately extracted with standard methods from these spinning sideband patterns. The experiment is demonstrated with carbon-13 experiments on powdered samples of a dipeptide and a cyclic undecapeptide, cyclosporin-A.     

Comparison of ultrasound effects in different reactors at 20 kHz

To compare the performances of three power ultrasonic devices at 20 kHz: a horn, a cup horn and a tube, the local intensity distributions of local effects of cavitation have been investigated. The sensor is an electrochemical probe, measuring the solid-liquid mass transfer rate, related to the cavitation intensity. The axial and radial profiles of mass transfer coefficients have been investigated in three devices, at various power inputs. In all these equipments very strong heterogeneities have been characterized, whether a standing wave appears or not.     

Cavitation damage on metallic plate surfaces oscillating at 20 kHz

Plates of aluminium (UNI 4507) of about 2.55 cm2 and 1.0 mm thick, of different shapes, vibrating in a liquid at a frequency of about 20 kHz and subjected to strong cavitation damage on their central parts exhibit, on the zone near the edge, a reduction of cavitative effect up to inhibition. We named this phenomenon, the "border effect". Evidence is reported which demonstrates that the border effect, depending on the plate velocity, is related to the angle between the plate surface and its direction displacement, that is to the value of the component of the liquid velocity tangent to the plate.     

Nuclear spin-lattice relaxation mechanisms in kaolinite confirmed by magic-angle spinning

Spin-lattice relaxation mechanisms in kaolinite have been reinvestigated by magic-angle spinning (MAS) of the sample. MAS is useful to distinguish between relaxation mechanisms: the direct relaxation rate caused by the dipole-dipole interaction with electron spins is not affected by spinning while the spin diffusion-assisted relaxation rate is. Spin diffusion plays a dominant role in ¹H relaxation. MAS causes only a slight change in the relaxation behavior, because the dipolar coupling between ¹H spins is strong. ²⁹Si relaxes directly through the dipole-dipole interaction with electron spins under spinning conditions higher than 2 kHz. A spin diffusion effect has been clearly observed in the ²⁹Si relaxation of relatively pure samples under static and slow-spinning conditions. ²⁷Al relaxes through three mechanisms: phonon-coupled quadrupole interaction, spin diffusion and dipole-dipole interaction with electron spins. The first mechanism is dominant, while the last is negligibly small. Spin diffusion between ²⁷Al spins is suppressed completely at a spinning rate of 2.5 kHz. We have analyzed the relaxation behavior theoretically and discussed quantitatively. Concentrations of paramagnetic impurities, electron spin-lattice relaxation times and spin diffusion rates have been estimated.     

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.     

A solid-state N magic-angle spinning NMR study of some amino acids

Experimental strategies for the acquisition of high-quality 14N magic-angle spinning (MAS) NMR spectra of the simple amino acids, which exhibit 14N quadrupole coupling constants (C(Q)) on the order of 1.2 MHz, are devised. These are the first useful 14N MAS spectra reported for nitrogen compounds having a C(Q)(14N) value in excess of 1 MHz. The complete manifolds of spinning sidebands (ssbs), i.e., about 300 ssbs for a spinning frequency of 6.0 kHz, have been observed in the 14N MAS NMR spectra of a series of amino acids. In their crystal structure these amino acids all exhibit the zwitterionic form and thus the 14N MAS NMR spectra represent those of a rotating -NH(3)(+) group and not of an amino (-NH(2)) group. Computer simulations combined with fitting of simulated to the experimental ssb intensities result in the determination of precise values for the 14N quadrupole coupling (C(Q)) and its associated asymmetry parameter (eta(Q)) for the nitrogen sites in these molecules. For some of the amino acids the 14N MAS NMR spectra exhibit overlap between the manifolds of ssbs from two different nitrogen sites in accordance with their crystal structures. Computer analysis of these spectra results in two different sets of (C(Q), eta(Q)) values which mainly differ in the magnitudes for eta(Q).     

Comparison of enhancement of pentachlorophenol sonolysis at 20 kHz by dual-frequency sonication

The comparison of enhancement effect of pentachlorophenol sonolysis at 20 kHz by different dual-frequency ultrasonic irradiations has been investigated. Dual-frequency (20 kHz/40 kHz, 20 kHz/530 kHz, 20 kHz/800 kHz and 20 kHz/1040 kHz) ultrasounds have been used. It has been found that the rate of pentachlorophenol degradation at dual-frequency ultrasonic irradiation is the highest compared to mono-frequency ultrasonic systems. The combination of dual-frequency systems has synergistic effect and the enhancement effect of sonochemical degradation of pentachlorophenol at 20 kHz by dual-frequency systems appears to be remarkable frequency sensitive. The order of contribution to the enhancement effect of sonochemical degradation of pentachlorophenol at 20 kHz is as follows: 530 kHz > 800 kHz > 40 kHz > 1040 kHz.