Using a cross-coil to reduce RF heating by an order of magnitude in triple-resonance multinuclear MAS at high fields

Four different coil designs for use with MAS in triple-resonance multi-nuclear experiments at high fields are compared, using a combination of finite element analysis (FEA) software and NMR experiments, with respect to RF field strength per unit power and relative sample heating, as governed by mean E/B(1) within the sample region. A commercial FEA package, Microwave Studio 5.1 by Computer Simulation Technology (CST) is shown to obtain remarkably accurate agreement with the experiments in Q(L), L, B, E, and mode frequencies in all cases. A simplified treatment of RF heating in NMR MAS samples is derived and shown to agree with the NMR experimental results within about 10% for two representative stator designs. The coil types studied include: (1) a variable-pitch solenoid outside a ceramic coilform, (2) a conventional solenoid very closely spaced to the MAS rotor, (3) a scroll coil, and (4) a segmented saddle cross coil (XC) for (1)H with an additional solenoid over it for the two lower-frequency channels. The XC/solenoid is shown to offer substantial advantages in reduced decoupler heating, improved S/N, and improved compatibility with multinuclear tuning and high-power decoupling. This seems largely because the division of labor between two orthogonal coils allows them each, and their associated circuitry, to be separately optimized for their respective regimes.     

Using low-E resonators to reduce RF heating in biological samples for static solid-state NMR up to 900 MHz

RF heating of solid-state biological samples is known to be a destabilizing factor in high-field NMR experiments that shortens the sample lifetime by continuous dehydration during the high-power cross-polarization and decoupling pulses. In this work, we describe specially designed, large volume, low-E 15N-1H solid-state NMR probes developed for 600 and 900 MHz PISEMA studies of dilute membrane proteins oriented in hydrated and dielectrically lossy lipid bilayers. The probes use an orthogonal coil design in which separate resonators pursue their own aims at the respective frequencies, resulting in a simplified and more efficient matching network. Sample heating at the 1H frequency is minimized by a loop-gap resonator which produces a homogeneous magnetic field B1 with low electric field E. Within the loop-gap resonator, a multi-turn solenoid closely matching the shape of the sample serves as an efficient observe coil. We compare power dissipation in a typical lossy bilayer sample in the new low-E probe and in a previously reported 15N-1H probe which uses a double-tuned 4-turn solenoid. RF loss in the sample is measured in each probe by observing changes in the 1H 360 degrees pulse lengths. For the same values of 1H B1 field, sample heating in the new probe was found to be smaller by an order of magnitude. Applications of the low-E design to the PISEMA study of membrane proteins in their native hydrated bilayer environment are demonstrated at 600 and 900 MHz.     

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.     

Capacitively decoupled tunable loop microstrip (TLM) array at 7 T

Microstrip transmission-line loop arrays have been recently proposed for parallel imaging at ultrahigh fields due to their advantages in element decoupling and to their increased coil quality factor. In the microstrip loop array design, interconnecting capacitors become necessary to further improve the decoupling between the adjacent elements when nonoverlapped loops are placed densely. However, at ultrahigh fields, the capacitance required for sufficient decoupling is very small. Hence, the isolations between the elements are usually not optimized and the array is extremely sensitive to the load. In this study, a theoretical model is developed to analyze the capacitive decoupling circuit. Then, a novel tunable loop microstrip (TLM) array that can accommodate capacitive decoupling more easily at ultrahigh fields is proposed. As an example, a four-element TLM array is constructed at 7 T. In this array, the decoupling capacitance is increased to a more reasonable value. Isolation between the adjacent elements is better than -37 dB with the load. The performance of this TLM array is also demonstrated by MRI experiments.     

Progress in correlation spectroscopy at ultra-fast magic-angle spinning: basic building blocks and complex experiments for the study of protein structure and dynamics

Recent progress in multi-dimensional solid-state NMR correlation spectroscopy at high static magnetic fields and ultra-fast magic-angle spinning is discussed. A focus of the review is on applications to protein resonance assignment and structure determination as well as on the characterization of protein dynamics in the solid state. First, the consequences of ultra-fast spinning on sensitivity and sample heating are considered. Recoupling and decoupling techniques at ultra-fast MAS are then presented, as well as more complex experiments assembled from these basic building blocks. Furthermore, we discuss new avenues in biomolecular solid-state NMR spectroscopy that become feasible in the ultra-fast spinning regime, such as sensitivity enhancement based on paramagnetic doping, and the prospect of direct proton detection.     

Current CONtrolled Transmit And Receive Coil Elements (C2ONTAR) for Parallel Acquisition and Parallel Excitation Techniques at High-Field MRI

A novel intrinsically decoupled transmit and receive radio-frequency coil element is presented for applications in parallel imaging and parallel excitation techniques in high-field magnetic resonance imaging. Decoupling is achieved by a twofold strategy: during transmission elements are driven by current sources, while during signal reception resonant elements are switched to a high input impedance preamplifier. To avoid B(0) distortions by magnetic impurities or DC currents a resonant transmission line is used to relocate electronic components from the vicinity of the imaged object. The performance of a four-element array for 3 T magnetic resonance tomograph is analyzed by means of simulation, measurements of electromagnetic fields and bench experiments. The feasibility of parallel acquisition and parallel excitation is demonstrated and compared to that of a conventional power source-driven array of equivalent geometry. Due to their intrinsic decoupling the current-controlled elements are ideal basic building blocks for multi-element transmit and receive arrays of flexible geometry.     

Design of small volume HX and triple-resonance probes for improved limits of detection in protein NMR experiments

Three- and four-frequency nuclear magnetic-resonance probes have been designed for the study of small amounts of protein. Both "HX" (1H, X, and 2H channels) and "triple-resonance" (1H, 15N, 13C, and 2H) probes were implemented using a single transmit/receive coil and multiple-frequency impedance matching circuits. The coil used was a six-turn solenoid with an observe volume of 15 microl. A variable pitch design was used to improve the B1 homogeneity of the coil. Two-dimensional HSQC spectra of approximately 1mM single labeled 15N- and double labeled 15N/13C-proteins were acquired in experimental times of approximately 2h. Triple-resonance capability of the small-volume triple-resonance probe was demonstrated by acquiring three-dimensional HNCO spectra from the same protein samples. In addition to enabling very small quantities of protein to be used, the extremely short pulse widths (1H = 4, 15N = 4, and 13C = 2 micros) of this particular design result in low power decoupling and wide-bandwidth coverage, an important factor for the ever-higher operating frequencies used for protein NMR studies.     

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.     

Resonator with reduced sample heating and increased homogeneity for solid-state NMR

In the application of solid-state NMR to many systems, the presence of radiofrequency (rf) electric fields inside classical solenoidal coils causes heating of lossy samples. In particular, this is critical for proteins in ionic buffers. Rf sample heating increases proportional to frequency which may result in the need to reduce the rf pulse power to prevent partial or total sample deterioration. In the present paper, we propose a multifrequency-tunable NMR resonator where the sample is electrically shielded from the NMR coil by a conductive sheet that increases the magneto-electric ratio. Expressions for the B1 efficiency as function of magnetic and electric filling factors are derived that allow a direct comparison of different resonators. Rf efficiency, homogeneity, signal-to-noise, and rf sample heating are compared. NMR spectra at 700MHz on ethylene glycol, glycine, and a model protein were acquired to compare the resonators under realistic experimental conditions.     

Solid-state variable-temperature H MAS NMR studies on deuterated polyethylene

Solid-state varible-temperature/magic angle spinning(VT/MAS) ¹H NMR measurements were carried out on deuterated polyethylene. From these experimental results it was found that the ¹H chemical shift induced by conformational and morphological changes of the polyethylene sample is within the linewidth of ≈ 0.5 ppm. Furthermore, from MAS/dipolar decoupling experiments it was found that the resonance frequency of the proton varies linearly with the inverse square of the deuterium decoupling power. This experimental finding is discussed theoretically.     

高频单线圈双调谐NMR单晶探头的研制

本文报道了一种测角单晶NMR探头。该探头采用单线圃双调谐电路,工作频率在90MHz-110 MHz连续可调,可进行交叉极化大功率去耦实验。文中提出了一种简单而有效的测角装置,可使单晶绕三个互相垂直轴转动实现单晶的NMR测量。作为典型的应用例子,本文利用该探头实现了单晶DGO(2NH₂CH₂COOH·H₂C₂O₄)屏蔽张量的测量。     

SPINAL modulated decoupling in high field double- and triple-resonance solid-state NMR experiments on stationary samples

Continuous wave irradiation has limited bandwidth for heteronuclear 1H decoupling at high fields and for 13C decoupling in 1H/13C/15N triple-resonance experiments. SPINAL-16 modulation is shown to improve the efficiency of 1H and 13C heteronuclear decoupling on single crystals of peptides and on magnetically aligned samples of membrane proteins in bicelles, which is of particular importance because aqueous samples of biomolecules are lossy at high fields, which limits the strengths of the RF fields that can be applied.     

Selective averaging for high-resolution solid-state NMR spectroscopy of aligned samples

Solid-state NMR experiments benefit from being performed at high fields, and this is essential in order to obtain spectra with the resolution and sensitivity required for applications to protein structure determination in aligned samples. Since the amount of rf power that can be applied is limited, especially for aqueous protein samples, the most important pulse sequences suffer from bandwidth limitations resulting from the same spread in chemical shift frequencies that aids resolution. SAMPI4 is a pulse sequence that addresses these limitations. It yields separated local field spectra with narrower and more uniform linewidths over the entire spectrum than the currently used PISEMA and SAMMY experiments. In addition, it is much easier to set up on commercial spectrometers and can be incorporated as a building block into other multidimensional pulse sequences. This is illustrated with a two-dimensional HETCOR experiment, where it is crucial to transfer polarization from the amide protons to their directly bonded nitrogens over a wide range of chemical shift frequencies. A quantum-mechanical treatment of the spin Hamiltonians under high-power rf pulses is presented which gives the scaling factor for SAMPI4 as well as the durations of the rf pulses to achieve optimal decoupling.     

Hydration-optimized oriented phospholipid bilayer samples for solid-state NMR structural studies of membrane proteins

The preparation of oriented, hydration-optimized lipid bilayer samples, for NMR structure determination of membrane proteins, is described. The samples consist of planar phospholipid bilayers, containing membrane proteins, that are oriented on single pairs of glass slides, and are placed in the coil of the NMR probe with the bilayer plane perpendicular to the direction of the magnetic field. Lipid bilayers provide a medium that closely resembles the biological membrane, and sample orientation both preserves the intrinsic membrane-defined directional quality of membrane proteins, and provides the mechanism for resonance line narrowing. The hydration-optimized samples overcome some of the difficulties associated with multi-dimensional, high-resolution, solid-state NMR spectroscopy of membrane proteins. These samples have greater stability over the course of multi-dimensional NMR experiments, they have lower sample conductance for greater rf power efficiency, and enable greater rf coil filling factors to be obtained for improved experimental sensitivity. Sample preparation is illustrated for the membrane protein CHIF (channel inducing factor), a member of the FXYD family of ion transport regulators.     

A novel low-E field coil to minimize heating of biological samples in solid-state multinuclear NMR experiments

A novel coil, called Z coil, is presented. Its function is to reduce the strong thermal effects produced by rf heating at high frequencies. The results obtained at 500MHz in a 50 microl sample prove that the Z coil can cope with salt concentrations that are one order of magnitude higher than in traditional solenoidal coils. The evaluation of the rf field is performed by numerical analysis based on first principles and by carrying out rf field measurements. Reduction of rf heating is probed with a DMPC/DHPC membrane prepared in buffers of increasing salt concentrations. The intricate correlation that exists between the magnetic and electric field is presented. It is demonstrated that, in a multiply tuned traditional MAS coil, the rf electric field E(1) cannot be reduced without altering the rf magnetic field. Since the detailed distribution differs when changing the coil geometry, a comparison involving the following three distinct designs is discussed: (1) a regular coil of 5.5 turns, (2) a variable pitch coil with the same number of turns, (3) the new Z coil structure. For each of these coils loaded with samples of different salt concentrations, the nutation fields obtained at a certain power level provide a basis to discuss the impact of the dielectric and conductive losses on the rf efficiency.     

"Cooking the sample": radiofrequency induced heating during solid-state NMR experiments

Dissipation of radiofrequency (RF) energy as heat during continuous wave decoupling in solid-state NMR experiment was examined outside the conventional realm of such phenomena. A significant temperature increase could occur while performing dynamic NMR measurements provided the sample contains polar molecules and the sequence calls for relatively long applications of RF power. It was shown that the methyl flip motion in dimethylsulfone (DMS) is activated by the decoupling RF energy conversion to heat during a CODEX pulse sequence. This introduced a significant bias in the correlation time–temperature dependency measurement used to obtain the activation energy of the motion. By investigating the dependency of the temperature increase in hydrated lead nitrate on experimental parameters during high-power decoupling one-pulse experiments, the mechanisms for the RF energy deposition was identified. The samples were heated due to dissipation of the energy absorbed by dielectric losses, a phenomenon commonly known as “microwave” heating. It was thus established that during solid-state NMR experiments at moderate B₀ fields, RF heating could lead to the heating of samples containing polar molecules such as hydrated polymers and inorganic solids. In particular, this could result in systematic errors for slow dynamics measurements by solid-state NMR.     

Analysis of RF heating and sample stability in aligned static solid-state NMR spectroscopy

Sample instability during solid-state NMR experiments frequently arises due to RF heating in aligned samples of hydrated lipid bilayers. A new, simple approach for estimating sample temperature is used to show that, at 9.4 T, sample heating depends mostly on (1)H decoupling power rather than on (15)N irradiation in PISEMA experiments. Such heating for different sample preparations, including lipid composition, salt concentration and hydration level was assessed and the hydration level was found to be the primary parameter correlated with sample heating. The contribution to RF heating from the dielectric loss appears to be dominant under our experimental conditions. The heat generated by a single scan was approximately calculated from the Q values of the probe, to be a 1.7 degrees C elevation per single pulse sequence iteration under typical sample conditions. The steady-state sample temperature during PISEMA experiments can be estimated based on the method presented here, which correlates the loss factor with the temperature rise induced by the RF heating of the sample.     

Cryogenic Q-band (35GHz) probehead featuring large excitation microwave fields for pulse and continuous wave electron paramagnetic resonance spectroscopy: performance and applications

The construction and performance of a Q-band (35GHz) cryogenic probehead for pulse electron paramagnetic resonance and continuous wave electron paramagnetic resonance measurements with down-scaled loop gap resonators (LGRs) is presented. The advantage of the LGR in comparison to TE(012) resonators lies in the large B(1) microwave (mw) fields that can be generated with moderate input mw power. We demonstrated with several examples that this allows optimal performance for double-quantum electron coherence, HYSCORE, and hyperfine decoupling experiments employing matched and high turning angle mw pulses with high B(1)-fields. It is also demonstrated that with very low excitation power (i.e. 10-40 mW), B(1)-fields in LGRs are still sufficient to allow short mw pulses and thus experiments such as HYSCORE with high-spin systems to be performed with good sensitivity. A sensitivity factor Lambda(rs) of LGRs with different diameters and lengths is introduced in order to compare the sensitivity of different resonant structures. The electromagnetic field distribution, the B(1)-field homogeneity, the E(1)-field strength, and the microwave coupling between wave guide and LGRs are investigated by electromagnetic field calculations. The advantage and application range using LGRs for small sample diameters is discussed.     

Cross polarization, radio frequency field homogeneity, and circuit balancing in high field solid state NMR probes

Homogeneous radio frequency (RF) fields are important for sensitivity and efficiency of magnetization transfer in solid state NMR experiments. If the fields are inhomogeneous the cross polarization (CP) experiment transfers magnetization in only a thin slice of sample rather than throughout the entire volume. Asymmetric patterns have been observed in plots of the CP signal versus RF field mismatch for an 800 MHz solid-state NMR probe where each channel is resonated in a single-ended mode. A simple model of CP shows these patterns can be reproduced if the RF fields for the two nuclei are centered at different places in the coil. Experimental measurements using B1 field imaging, nutation arrays on extremely short NMR samples, and de-tuning experiments involving disks of copper incrementally moved through the coil support this model of spatially offset RF fields. We have found that resonating the high frequency channel in a double-ended or "balanced" mode can alleviate this field offset problem, and have implemented this in a three-channel solid state NMR probe of our own design.     

1H/31P双核并行磁共振成像线圈的研究与设计

本文通过对鸟笼线圈原理和阵列线圈去耦原理的分析，提出了一种适用于自主研发的多核并行磁共振成像（MRI）系统的双核并行成像线圈设计方案，并在电感去耦的基础上提出LC并联trap去耦法，提高了去耦方法的可适性．依据设计方案制作了¹H/³¹P双核并行成像线圈，并将其应用于4.7 T磁体系统，利用自主研发的多核并行MRI系统进行了并行成像实验测试，成功获得了¹H和³¹P的并行磁共振图像，验证了设计方案的可行性．