A general strategy for the NMR observation of half-integer quadrupolar nuclei in dilute environments.

A general strategy for the observation of low gamma half-integer quadrupolar nuclides in biological systems is presented. The methodology combines low-temperature (4-100 K) techniques with cross-polarization (CP) experiments while employing a so-called Carr-Purcell-Meiboom-Gill spin-echo sequence (CPMG). This combined approach is termed CP/QCPMG. Also discussed are data processing issues that are unique to the induced signals that result from the QCPMG pulse sequence. Central to this strategy is the development of a stable low-temperature (4 to 250 K) NMR double-resonance probe. The probe is robust enough to handle multiple contact experiments and long acquisition periods with 1H decoupling. This approach is illustrated with low-temperature solid-state 67Zn and 25Mg NMR CP/QCPMG experiments on model compounds. The conclusion reached is that the strategy affords sufficient sensitivity to examine Zn2+ and/or Mg2+ binding sites in metalloproteins.     

Experimental method for low-temperature reaction studies in sealed samples by 13C CP/MAS NMR

A new, simple, and inexpensive technique is presented for monitoring high-resolution solid-state NMR of ¹³C at temperatures ranging between 85 and 450 K. In this procedure, the reaction conditions are controlled by preparing samples at 77 K in 5 mm NMR tubes, while attached to a vacuum system. The NMR tubes are prefitted with a rotor for spinning. After preparation, the samples are sealed, transferred to the double-resonance MAS NMR probe, and analyzed, all while the sample temperature is maintained as low as 85 K. The spinning rates vary from 3.0 kHz at 85 K to 5.2 kHz at 300 K using nitrogen drive gas. Probe design and performance, sample-preparation procedure, and details of the low-temperature experiment are described. In general, the technique may be applied in studies of low-temperature reaction mechanisms and kinetics. ¹³C CP/MAS spectra of ethylene adsorbed on silica-supported ruthenium catalyst are presented to illustrate its performance and possible application.     

13C-14N REAPDOR and 13C-2D theta-REDOR NMR on a blend of tri-p-tolylamine and bisphenol-A-polycarbonate

Solid-state NMR experiments are performed on amorphous blends of tri-p-tolylamine (TTA) and polycarbonate. Amines that are similar to TTA are blended with polycarbonates and used as photoconducting components in xerography. Because of its simple molecular structure, TTA is a charge-transporting molecule that is used as a model to understand charge transport in photoconducting materials. Charge transfer between amine molecules must depend on the separation and relative orientation between amine molecules. However, little information is known about the structural relationship between the amine molecules and the polymer host, presumably because of the difficulties associated with characterizing amorphous materials. We report the results of 13C-14N rotational-echo, adiabatic-passage, double-resonance (REAPDOR) and 13C-2D theta-rotational-echo, double-resonance (theta-REDOR) magic-angle spinning NMR experiments on a TTA/polycarbonate blend. The experiments provide information about the organization of TTA relative to polycarbonate.     

Recollections of REDOR

Rotational-echo, double-resonance NMR (REDOR) is an experiment designed to measure heteronuclear dipolar couplings in solids and is most often used to obtain structural details in solids. A brief history of its inception and development is presented.     

Low temperature probe for dynamic nuclear polarization and multiple-pulse solid-state NMR

Here, we describe the design and performance characteristics of a low temperature probe for dynamic nuclear polarization (DNP) experiments, which is compatible with demanding multiple-pulse experiments. The competing goals of a high-Q microwave cavity to achieve large DNP enhancements and a high efficiency NMR circuit for multiple-pulse control lead to inevitable engineering tradeoffs. We have designed two probes-one with a single-resonance RF circuit and a horn-mirror cavity configuration for the microwaves and a second with a double-resonance RF circuit and a double-horn cavity configuration. The advantage of the design is that the sample is in vacuum, the RF circuits are locally tuned, and the microwave resonator has a large internal volume that is compatible with the use of RF and gradient coils.     

19F/29Si rotational-echo double-resonance and heteronuclear spin counting under fast magic-angle spinning in fluoride-containing octadecasil

19F/29Si rotational-echo double-resonance (REDOR) and theta-REDOR NMR techniques have been applied under fast magic-angle spinning to a powder sample of fluoride-containing octadecasil. Efficient dipolar recoupling was observed and the effect of finite pulse lengths was found to be negligible using standard radiofrequency field strengths. Moreover, the determined internuclear distance of the 19F-29Si spin pairs formed by the silicons in the D4R units (T-1 site) and the fluoride anions is in very good agreement with previous REDOR and Hartmann-Hahn cross-polarization measurements. Numerical simulation of the REDOR dephasing curves at both the T-1 and T-2 sites considering all fluoride anions in the infinite solid lattice clearly confirm the X-ray crystal structure of octadecasil. Heteronuclear spin-counting theta-REDOR experiments are found to be very useful to obtain direct insight into the local network of dipolar interactions. Indeed, while 19F-29Si pair-like behavior is confirmed at the T-1 site, multiple dipolar interactions are clearly evidenced at the T-2 site.     

An efficient (1)H/(31)P double-resonance solid-state NMR probe that utilizes a scroll coil

The construction and performance of a scroll coil double-resonance probe for solid-state NMR on stationary samples is described. The advantages of the scroll coil at the high resonance frequencies of (1)H and (31)P include: high efficiency, minimal perturbations of tuning by a wide range of samples, minimal RF sample heating of high dielectric samples of biopolymers in aqueous solution, and excellent RF homogeneity. The incorporation of a cable tie cinch for mechanical stability of the scroll coil is described. Experimental results obtained on a Hunter Killer Peptide 1 (HKP1) interacting with phospholipid bilayers of varying lipid composition demonstrate the capabilities of this probe on lossy aqueous samples.     

Chemisorption and surfaces studied by nuclear magnetic resonance spectroscopy

This paper presents an introduction to the study of surfaces and chemically adsorbed species with nuclear magnetic resonance (NMR) spectroscopy. The analysis is based on nuclear magnetic interactions in the solid state: dipole-dipole couplings, chemical shift anisotropy, Knight shifts, and quadrupolar splitting. The physical origins and characteristics of each interaction, as well as relative intensities for different nuclei, are discussed. In particular, emphasis is placed on the relation of these interactions to quantities of interest to studies in adsorption and catalysis: motional properties of the adsorbate, the distribution of adsorption sites, the chemical state of atoms adsorbed at the surface, electrostatic field gradients, and the metallic character of surface atoms. Techniques to observe these interactions are described; subdivided by the type of nucleus: strongly coupled nuclei (e.g. ¹H, ¹⁹F), weakly coupled nuclei (e.g. ¹³C, ¹⁵N, ²⁹Si, ¹⁹⁵Pt), and quadrupolar nuclei (e.g. ²H, ¹⁴N, ²⁷Al). The techniques described to isolate and identify the overlapping effects in the spectra include multiple-pulse spin echoing and decoupling, double-resonance irradiation, multiple-quantum excitation, and mechanical sample spinning. A review of the recent application of these techniques to studies of adsorption and surfaces illustrates the potentials and limitations. Finally, a procedure for formulating a NMR study of surface samples is proposed, with respect to sample composition and character, and the type of information desired.     

A solid-state NMR study of the formation of molecular sieve SAPO-34

This work examined the formation of a catalytically important microporous material, SAPO-34, in the presence of HF under hydrothermal synthesis conditions. The local environments of P, Al, F and Si atoms in several solid phases obtained at different stages of crystallization were characterized by several solid-state NMR techniques including ³¹P, ²⁷Al, ¹⁹F and ²⁹Si MAS, ²⁷Al triple-quantum MAS, ³¹P{²⁷Al} transfer of populations in double-resonance, ²⁷Al{³¹P} rotational-echo double-resonance (REDOR), ²⁷Al→³¹P heteronuclear correlation spectroscopy, ³¹P{¹⁹F} and ²⁷Al{¹⁹F} REDOR as well as ¹H→³¹P cross polarization. The NMR results provide the new insights into the formation of SAPO-34.     

Double-resonance circuit for nuclear magnetic resonance spectroscopy

A new double-resonance probe circuit design is described. The circuit contains no quarter-wavelength elements or equivalents, yet nonetheless achieves adequate isolation between the two input channels. It contains relatively few components, and so is both compact and efficient. It has been incorporated in two solid-state nuclear magnetic resonance (NMR) probes, with excellent results.     

Relaxation effects in weakly-coupled three spin systems by double resonance

Nuclear magnetic double-resonance spectra of weakly-coupled spin systems exhibit several interesting features. The effect of relaxation on the symmetry properties of double resonance spectra of weakly-coupled spin systems is investigated using spin inversion operators. The double-resonance spectra of the AX₂ type systems formed by the protons in (I) 1,1,2-trichloroethane and in (II) 2,2-dichloroethanol (excluding the hydroxyl proton) exhibit these symmetry effects. In both these molecules relaxation is contributed by several mechanisms. In sample II the effect of the exchanging proton in the hydroxyl group is also considered and the exchange time is estimated.     

A strip-shield improves the efficiency of a solenoid coil in probes for high-field solid-state NMR of lossy biological samples

A strip-shield inserted between a high inductance double-tuned solenoid coil and the glass tube containing the sample improves the efficiency of probes used for high-field solid-state NMR experiments on lossy aqueous samples of proteins and other biopolymers. A strip-shield is a coil liner consisting of thin copper strips layered on a PTFE (polytetrafluoroethylene) insulator. With lossy samples, the shift in tuning frequency is smaller, the reduction in Q, and RF-induced heating are all significantly reduced when the strip-shield is present. The performance of 800 MHz ¹H/¹⁵N and ¹H/¹³C double-resonance probes is demonstrated on aqueous samples of membrane proteins in phospholipid bilayers.     

"Development of REDOR rotational-echo double-resonance NMR" by Terry Gullion and Jacob Schaefer [J. Magn. Reson. 81 (1989) 196-200

The popularity of rotational-echo double-resonance (REDOR) NMR arises from its ability to measure weak dipolar couplings and long-range heteronuclear distances accurately. This ability was not anticipated in the first REDOR experiments and resulted from the effectiveness of a simple radiofrequency phase alternation scheme to suppress amplitude and phase distortions in echo trains even after hundreds of pi pulses.     

Field-cycled PEDRI imaging of free radicals with detection at 450 mT

This paper describes the design, construction and use of a field-cycled proton-electron double-resonance imaging (FC-PEDRI) system for the detection and imaging of free radicals. The unique feature of this imager is its use of a 450-mT detection magnetic field in order to achieve good image quality and sensitivity. The detection magnetic field is provided by a superconducting magnet, giving high stability and homogeneity. Field cycling is implemented by switching on and off the current in an internal, coaxial, resistive secondary magnet that partially cancels the superconducting magnet's field at the sample; the secondary magnet is actively shielded to avoid eddy currents. EPR irradiation takes place at approximately 5 mT, following which the field is switched to 450 mT in 40 ms for NMR signal detection. Full details of the imager's subsystems are given, and experiments to image the distribution of stable free radical contrast agents in phantoms and in anesthetized rats are described.     

A low-field, low-cost Halbach magnet array for open-access NMR

A working prototype of a novel low-cost Halbach-array-based NMR system is described. The new design provides open access to the sample relative to conventional NMR magnet designs and this facilitates the simultaneous use of multi-sensor techniques on the same sample, in which NMR/MRI can potentially be combined with other spectroscopies such as impedance spectroscopy, laser scattering and rheological experiments.     

Medium-range order in cesium borate glasses probed by double-resonance NMR

Rotational-echo double-resonance NMR is used to probe the proximity of Cs+ network modifiers to network-forming boron in binary cesium borate glasses. Low- and high-alkali glasses show distinctly different dephasing curves, which indicate preferential association of Cs+ with four-co-ordinate boron ([4]degrees )B) at low-alkali contents only. Different [4]B sites within a given glass appear to be subject to the same 133Cs dipolar field, thus placing constraints on the possible assignments of multiple tetrahedral boron peaks to different types of medium-range order and guiding future structural modeling studies.     

Unambiguously distinguishing Si[3Si,1Al] and Si[3Si,1OH] stuctural units in zeolite by 1H/29Si/27Al triple resonance solid state NMR spectroscopy

We present an experimentally feasible triple-resonance NMR method that establishes the correlation among three different nuclei, avoiding the difficulty to directly explore the weak coupling between two NMR nuclei, such as (29)Si and (27)Al. Using this method, we are able to give an unambiguous assignment to the various peaks in (29)Si CP NMR spectrum of MCM-22 zeolite and discriminate (29)Si signals from SiOHAl and SiOH groups. In addition, in combination with (1)H/(27)Al double-resonance technique, the (1)H/(27)Al/(29)Si triple-resonance experiment suggests the presence of two different kinds of Br?nsted acid sites in H-MCM-22 zeolite.     

Optical rigidity in a stable double-resonance regime

The properties of the optical rigidity created by double pumping in an interferometric gravitational-wave antenna are investigated. The double-resonance regime takes place when two eigenfrequencies of the system coincide with each other. A simple criterion of the stability of the optical rigidity is proposed and is applied to calculations of the double-resonance regime. The schemes of gravitational-wave antennas using such a regime are very promising because they allow overcoming the Standard Quantum Limit. We show that a stable double-resonance regime can be realized in laboratory prototypes before its implementation in full-scale gravitational-wave detectors.     

A Modified Alderman–Grant Coil makes possible an efficient cross-coil probe for high field solid-state NMR of lossy biological samples

The design, construction, and performance of a cross-coil double-resonance probe for solid-state NMR experiments on lossy biological samples at high magnetic fields are described. The outer coil is a Modified Alderman–Grant Coil (MAGC) tuned to the ¹H frequency. The inner coil consists of a multi-turn solenoid coil that produces a B₁ field orthogonal to that of the outer coil. This results in a compact nested cross-coil pair with the inner solenoid coil tuned to the low frequency detection channel. This design has several advantages over multiple-tuned solenoid coil probes, since RF heating from the ¹H channel is substantially reduced, it can be tuned for samples with a wide range of dielectric constants, and the simplified circuit design and high inductance inner coil provides excellent sensitivity. The utility of this probe is demonstrated on two electrically lossy samples of membrane proteins in phospholipid bilayers (bicelles) that are particularly difficult for conventional NMR probes. The 72-residue polypeptide embedding the transmembrane helices 3 and 4 of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) (residues 194–241) requires a high salt concentration in order to be successfully reconstituted in phospholipid bicelles. A second application is to paramagnetic relaxation enhancement applied to the membrane-bound form of Pf1 coat protein in phospholipid bicelles where the resistance to sample heating enables high duty cycle solid-state NMR experiments to be performed.     

A High-Conversion-Factor, Double-Resonance Structure for High-Field Dynamic Nuclear Polarization

This contribution presents a novel design of a double-resonance structure for high-field dynamic nuclear polarization operating at 95 GHz and 144 MHz, in which a miniaturized radiofrequency coil is integrated within a single-mode nonradiative dielectric resonator. After a detailed discussion of the design principles, the conversion factors of this system are determined by means of microwave and radiofrequency measurements. The obtained results, 1.68 mT/W^1/2 for the microwave conversion factor and 0.8 mT/W^1/2 for the radiofrequency conversion factor, represent the state-of-the-art among the double-resonance structures. Simultaneous electron paramagnetic resonance and liquid-state ¹H nuclear magnetic resonance experiments are performed on samples of nitroxide radical 2,2,6,6-tetramethylpiperidine-1-oxyl dissolved in a mixture of water and dioxane. A maximum dynamic nuclear polarization enhancement of about ?16 is obtained at a microwave power of 70 mW with a radical concentration of 10 mM in nanoliter-sized sample volumes. These results are discussed in view of further improvements and applications of the proposed double-resonance structure.