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  • Published: Dec 1, 2004
  • Author: David Bradley
  • Channels: NMR Knowledge Base
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Hungarian scientists have shown how homonuclear gradient-enhanced selective NMR experiments can help in the structure determination of plant steroid derivatives. The work could open up new avenues for agrochemical and nutritional research into phytosterols.

The natural derivatives of steroids generally exhibit complex, highly overlapped multiplets at low chemical shift in their proton NMR spectra, explain Péter Forgó of the University of Szeged and Katalin Kövér of the University of Debrecen in the January issue of the journal Steroids. Some characteristic signals in the high chemical shift region, however, if unraveled, can provide a useful starting point for selective NMR experiments that allow us to investigate scalar (J coupling) and dipolar (NOE) interactions, Forgó told Resonants.

The Hungarian team has now carried out such experiments on stigmasterol as a model compound for plant steroids, phytosterols, using a double pulsed-field gradient spin-echo sequence (DPFGSE) for selective excitation. In this sequence, 180 degree Gaussian selective pulses are sandwiched between sine shaped z-gradients.

The team used a range of techniques, homonuclear DPFGSE-COSY, DPFGSE-relay-COSY, and DPFGSE-TOCSY to study the scalar interactions. The DPFGSE-NOESY experiment allowed them to monitor the spatial environment of the selectively excited protons.

The net result is that the team was able to make an unambiguous assignment of the signals due to the main skeleton of stigmasterol and also its side chain. The researchers point out that they could also glean the conformationally important homonuclear proton-proton coupling constants from the same experiments.

The technique could open up the study of phytosterols, which are important physiologically, and their potential as nutraceuticals and other agents. "The full NMR characterization of similar molecules allows one to further investigate their behaviour in biological systems, including ineractions with other bioactive species (macromolecules) resulting in a better understanding of their biological functions," Forgó told us.

Phytosterols, and their hydrogenated equivalents, phytostanols, are being keenly investigated for their role in effectively lowering plasma total and low density lipoprotein (LDL) cholesterol and this occurs by inhibiting the absorption of cholesterol from the small intestine.

Brussels sprouts
Brussels sprouts: a rich dietary source of cholesterol lowering phytosterols (Credit: David Bradley)

Péter Forgó
Péter Forgó

NMR spectrum of stigmasterol
NMR spectrum of stigmasterol

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