Microcoil High-Resolution Magic Angle Spinning NMR Spectroscopy

Abstract

We present a new technique that combines the versatility of magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy with the superior sensitivity provided by very small detection coils. This opens the way for NMR studies of solid samples with nanoliter volumes. Furthermore, the very strong radio frequency (rf) fields that can be generated by these microcoils facilitate a much broader excitation bandwidth and/or decoupling efficiency.Although solid-state NMR is the method of choice for investigating local structure, alignment, and dynamics of nonsoluble and noncrystalline functional materials, its feasibility for studying chemically and biologically relevant systems is often hampered by sensitivity. For sensitivity reasons, microcoil technology was introduced in liquid-state NMR for mass-limited samples 1 and highresolution, small-volume MRI experiments 2 , based on the observation that the efficiency of a solenoid coil scales inversely with its diameter. 1,3 By reciprocity, this also allows the generation of very high rf fields with limited amount of power, which has been exploited for static wide-line NMR studies in solids. 4 Two important pillars underpin the success of solid-state NMR as an analytical tool in materials science: first, sample rotation about the so-called "magic angle" 5 needed to average anisotropic contributions to the resonance lines and second, polarization transfer from abundant nuclei (e.g., protons) to less-abundant nuclei with low gyromagnetic ratios such as 13 C or 15 N. 6,7 The evolution of a variety of techniques for studying internuclear distances, bond angles, molecular orientation and dynamics, spin diffusion, and chemical exchange processes, is built on these foundations.Here we describe a microMAS design, featuring microcoilbased resonators with either 400/300 µm outer/inner diameter or 335/235 µm outer/inner diameter coils and sample holders down to 170/125 µm outer/inner diameter (10 nL sample volume), mounted on a regular commercially available MAS unit ( Figure 1). The microcoil resonator is a solenoid coil integrated into a capacitor similar to that developed for static experiments 4 leaving a 220 µm opening (30 nL internal volume) for the rotor. The advantage of this design is its mechanical stability and minimization of signal losses. Compared to regular millimeter-sized coils with only a few windings, our microcoil design with over 10 windings allows for a better B 1 homogeneity over the sample volume, thus improving efficiency. Using microcoils, susceptibility broadening can be a serious issue that limits the resolution. In the present design, this is not the case, as these effects are averaged by MAS. The resolution of most solids spectra is, therefore, determined by the intrinsic spin interactions in the materials. Low-rf-power operation combined with the fact that only a small homogeneous B 0 volume is needed may help to fulfill the promise of tabletop NMR equipment. Low-power requirements and the absence of frictional heating in combination with th...

Spinning proteins, the faster, the better?

“…[14] For slower spinning (< 30 kHz for proteins), high-power rf irradiation is typically applied ( ) while at faster spinning frequencies (> 40 kHz), low-power irradiation becomes attractive ( ) [14,15], although high-power decoupling is still possible if the decoupling frequency is increased proportionally to the spinning frequency which is often possible due to the small diameter of the coil in fast MAS stator assemblies [16]. Nevertheless, lowpower decoupling is certainly preferable because of the lower energy deposition in the sample, allowing for faster scan repetition rates and increased sensitivity per unit time.…”

Evaluation of High Resolution Magic-Angle Coil Spinning NMR Spectroscopy for Metabolic Profiling of Nanoliter Tissue Biopsies

“…Signals belonging to the glucose protons present a clear multiplicity with measurable J-couplings (see Table 1 and Figure 2 Figure 2 is a marked improvement over that of the currently available approaches to micro-detection with MAS, either using inductively coupled spinning microcoils (MACS) 11,13 or using a static microcoil in a piggy-back MAS approach. 18 Since the microcoil is static in the latter approach, the coil susceptibility is not averaged by the sample spinning. Thus, good shimming can be difficult to achieve in that case and can hinder high 8 resolution acquisitions.…”

Proton micro-magic-angle-spinning NMR spectroscopy of nanoliter samples

“…[2]: a solenoid microcoil is integrated into a double-sided circuit board that is mounted on a regular 2.5 mm Varian MAS pencil stator as shown in Fig. 1c In order to ensure the complete absence of 1 H background signal, capillaries from fused silica with inner and outer diameters of 320 and 400 m, respectively, are used as sample holders.…”

“…In order to maximize the sensitivity of solid-state NMR experiments under these circumstances the use of microcoils (diameters below 1 mm) for excitation and detection is favorable. Two approaches have been presented recently to combine solenoid microcoils and MAS: Firstly, the nanoliter sample container is 'piggy-backed' onto a regular size solid-state NMR rotor and spun inside the microcoil, which is mounted on top of the regular MAS stator [2] and [3]. Secondly, the microcoil is loaded into a regular MAS rotor and inductively coupled to the regular macroscopic NMR detection coil [4].…”

“…Secondly, the microcoil is loaded into a regular MAS rotor and inductively coupled to the regular macroscopic NMR detection coil [4]. Both approaches have so far been used to obtain 1 H spectra [4], 13 C cross-polarization (CP) MAS spectra [2] and [3], 13 C double-quantum spectra [3], and 29 Si one-dimensional (1D) spectra [4]. In addition, 27 Al 1D MAS spectra were recorded on a single crystal of mesolite [3].…”