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Magic Angle Spinning Spheres

Penelitian - Washington University researchers demonstrate spherical rotors can be spun stably at the magic angle. Spherical rotors conserve valuable space in the probe head and simplify sample exchange and microwave coupling for dynamic nuclear polarization.

Magic angle spinning (MAS) nuclear magnetic resonance (NMR) or MAS NMR experiments partially average anisotropic spin interactions in the magnetic resonance Hamiltonian through mechanical rotation of samples about the magic angle.

Penelitian Magic Angle Spinning Spheres

The spatial averaging extends spin relaxation times and improves the resolution of solid-state NMR spectroscopy. Single resonances can often be assigned to chemically distinct nuclear spins to yield site-specific signatures encoding structural information and molecular dynamics.

MAS NMR is, therefore, a powerful technique to characterize diverse molecular architectures including membrane proteins, amyloid fibrils, bacterial biofilms, and materials and surfaces. Mechanical sample rotation must be comparable to, or greater than, the frequency of the internal anisotropic spin interaction to produce significant averaging.

Over the past 60 years of MAS, samples have typically been packed into hollow cylindrical sample containers (rotors), with turbine inserts to supply drive propulsion. The long, narrow cylindrical shape of these rotors and the necessity for them to rotate at the magic angle complicate the sample exchange process.

Sample exchanges for cylindrical rotors mandate an angle adjustment of either the rotor or the stator for insertion or ejection within the magnet bore. This alignment typically requires considerable space within the magnet.

“We sought to improve MAS instrumentation to simplify sample exchange, improve DNP, reduce cryogen usage, and access spinning frequencies more 150 kHz. Macroscopic metallic spherical rotors have been spun up to 667 kHz using electromagnetic bearings and drive energy,” said Alexander Barnes of the Washington University in St. Louis.

“Although this technology cannot be directly translated into MAS within superconducting magnets, the fact that macroscopic spheres can be spun effectively is important and exploitable. We therefore turned to spherical rotors for MAS,” Barnes said.

Spherical rotors have distinct advantages over cylindrical rotors. Wobbling about the long axis of cylindrical rotors results in bearing collisions that can destroy the sample and stator. The isotropic spherical rotor alleviates these issues, because wobbling about an axis other than the primary spinning axis does not result in stator collisions.

“Our implementation of a spherical rotor enables a simplified vertical sample exchange while providing better access for microwave illumination. Here, we introduce spheres for MAS, demonstrating MAS NMR spectroscopy of samples packed within spherical rotors spinning stably at the magic angle,” Barnes said.

Journal : Pinhui Chen et al. Magic angle spinning spheres, Science Advances, 21 Sep 2018, DOI:10.1126/sciadv.aau1540



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