instrumentals
You spin me round right, baby A new approach to magic-angle spinning boosts signal detection for solid-state NMR.
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DIMITRIOS SAKELLARIOU
ike a Shakespearean hero, NMR both are impacted negatively by the of small-scale NMR probe design so spectroscopy has many sterling hardware necessary for MAS. far.” Particularly given that the apqualities and one tragic flaw. The techSakellariou and colleagues solved proach is widely adaptable to existing nique can be used to study the chemboth problems at once by winding a NMR systems, he says, it should have istry, structure, and dynamics of a broad appeal. “There are a number wide range of liquids, gases, and of parameters which remain to solids down to atomic resolution be optimized,” Webb says, “and without sample destruction. But further gains in sensitivity will, no although the technique is powerdoubt, be made. But the authors ful, its sensitivity can be too low have enabled a very critical NMR to work on very small samples; this technique to be used to study rules out many applications in biomass-limited samples.” logical and materials science, for In one experiment, Sakellariou example. Various sensitivity-boostand co-workers achieved sufficient ing strategies have been develspectral resolution on a ~0.3 mg oped, but none is applied easily to bovine muscle sample to detect the study of solid materials, where and identify the resonances of rapid sample spinning is required. triglycerides, lactate, and phosphoNow, Dimitrios Sakellariou and creatine; the same level of resolucolleagues at the Commissariat à tion with standard high-resolution 10 mm l’Énergie Atomique (France) have MAS required ~365 mg of sample. found a way to bridge that gap by The MACS approach should also using inductive coupling between The schematic shows how MACS works. Sample and lend itself to the study of radioaca standard static coil and a smallmicrocoil (brown) are spun together within a ceramic tive and biohazardous materials, er, rotating microcoil that wraps insert (green) in a standard high-resolution MAS which must be safely contained around the sample (Nature 2007, rotor (blue) that is surrounded by a detector coil (light within protective shielding, says 447, 694–697). “We had an intuiyellow). Sakellariou. “The filling factor tive idea,” Sakellariou recalls. “We is inherently low, but the MACS gave it a quick try, it worked, and detector coil around the sample itself technique can retrieve some of the lost then we explored the details of it.” and placing both coil and sample within sensitivity by using a rotating coil fitted The intuitive idea was to bring a rotor assembly to be spun together. inside the innermost barrier,” he adds. sample spinning and signal detection “To detect magnetic resonance, one Alexander Pines of the University together. Solid-state NMR is done needs a radio-frequency antenna surof California Berkeley points out that typically with a technique called magicrounding the sample,” says Sakellariou. along with solids, “the MACS approach angle spinning (MAS), in which the “We have shown that this antenna can will be equally applicable to any anisosample is spun in a rotor surrounded be incorporated in the sample holder tropic sample in which sample spinning by a static coil at thousands of revoluis of benefit, including liquid crystals, and spun together with it. The micro tions per second at an angle of 54.7° coil is connected wirelessly to the rest membranes, and large proteins partially relative to the main magnetic field. of the detection chain by a standard aligned in orientation-confining solvent Spinning at this “magic angle” causes commercial probe, which surrounds the environments.” Perhaps most signifithe frustratingly broad NMR spectra rotor and has a standard static coil. It’s cantly, MACS achieves improved resolutypical of anisotropic samples to average really a very simple idea.” tion and sensitivity without requiring out into much sharper, more easily anaSakellariou and colleagues call their hardware or software. The result, says lyzed bands. But two problems arise: technique “magic-angle coil spinning” Pines, “is simply beautiful. It’s importhe diameter of the detector coil and (MACS). The design, says Andrew tant, timely, and elegant work, and the the proportion of the space within the Webb of Pennsylvania State University, possible applications leap immediately coil occupied by the sample (the “fill“builds nicely upon previous work with to mind. My reaction was, ‘How lovely. ing factor”) are both important controls liquid and static solid samples and repNow why didn’t I think of that?’” a for the S/N and overall sensitivity, and resents the most challenging application —Thomas Hayden 6438
A n a ly t i c a l C h e m i s t r y / S e p t e m b e r 1 , 2 0 0 7
© 2007 American Chemical Societ y
