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Analytical Chemistry

New Method Improves NMR Spectral Resolution

PSYCHE pulse sequence eliminates spin-spin coupling, yields ‘pure shift’ NMR spectra that contain chemical-shift information only

by Celia Henry Arnaud
June 6, 2014 | A version of this story appeared in Volume 92, Issue 23

An improved technique for ultra-high-resolution NMR spectroscopy may prove useful for applications such as process optimization, metabolomics, and studies of intrinsically disordered proteins.

The technique is a highly sensitive type of “pure shift” proton NMR spectroscopy, which typically has better spectral resolution than conventional proton NMR but often poor sensitivity.

Pure shift NMR obtains spectra that contain only chemical shift information. There is a single peak for each chemically distinct proton. The technique does this by eliminating the effects of spin-spin coupling, which usually splits peaks into multiples.

Gareth A. Morris of the University of Manchester, in England, and coworkers report a pure shift NMR method with much greater sensitivity than the current state of the art (Angew. Chem. Int. Ed. 2014, DOI: 10.1002/anie.201404111).

NMR spectroscopists have been trying to develop pure shift methods for decades, Morris says, “but only in the last 10 years or so have we had techniques that are really practical.”

Morris and coworkers’ new method, PSYCHE (pure shift yielded by chirp excitation), has sensitivity 10 times that of the next-best pure shift method, the Zangger-Sterk experiment. They achieve that improved sensitivity by using radio-frequency pulse sequences that allow them to look at signals from only one spin at a time. They report having used the PSYCHE method to acquire proton NMR spectra of estradiol and cyclosporin A.

“The presented pure shift spectra of estradiol and cyclosporin A are of superb quality,” says Klaus Zangger, an NMR spectroscopist at the University of Graz, in Austria, and one of the inventors of the Zangger-Sterk pure shift method. “Heavily overlapped spectral regions are clearly separated into individual resonances. Without proton-proton decoupling, the complete separation of these signals would require magnetic field strengths of several gigahertz, which won’t be feasible for NMR spectrometers for many years.”

“We still need to explore what the limits are,” Morris says. “We need to find out how close we can allow chemical shifts to come and still get clean singlet signals.”

Conventional, PSYCHE, and Zangger-Sterk NMR spectra of estradiol.
Credit: Angew. Chem. Int. Ed.
In NMR spectra of estradiol, the PSYCHE method (middle) has better resolution than conventional proton NMR (top) and more sensitivity than the Zangger-Sterk method (bottom).


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