Gordon Troup, Laura O’Dea, Steven Langford, and Donald Hutton (a) School of Physics and (b) School of Chemistry, Monash University, Clayton3800, Victoria, Australia.
Resveratrol, the biphenolic stilbene found in wine, and made in the grapeskins, has been studied in a solid powder form by X-band (~9.4 GHz) Electron Paramagnetic Resonance (EPR) at room temperature (~20C) and 120K. A single, almost structureless line of ~ 8 Gauss linewidth between inflection points was observed at both temperatures: the line is not temperature broadened, but does have a short spin lattice relaxation time. Evidenced by a saturation study. The antioxidant efficiency was measure by the standard technique described in AIM-Digest 12 (1), 14 (2003). Since resveratrol is scarcely soluble in water , the measurements were made in methanol solution. Pure methanol has no antioxidant action. Vitamin E gave 91% efficiency, resveratrol 96%, both with uncertainty +/- 3%. These results are in keeping with the reputation of resveratrol as being a very powerful antioxidant
Once a molecule that is deemed ‘good for us’ is discovered in wine, or indeed in plants other than wine grapes, extracts of the molecule(s) are made and put on the ‘dietary supplement’ or ‘alternative medicine’ market, ever since the grapeseed extract Endotelon was put on the French market about 1980 by Prof. Jack Masquelier, the discoverer of the mixture of oligomers of the catechins he named ‘O.P.C.s’. The same is now true of resveratrol, the virtues of which have already been described in AIM-Digest 16 (3), 8, (2005) by Dr. Philip Norrie. Hence it has now become possible to examine this molecule by EPR and to verify its antioxidant efficiency by the standard technique employed by our group.
Materials and Methods
Solid 98% pure resveratrol in powder form was obtained from China. A sample was placed in a standard quartz EPR tube (Wilmad), which was then transferred a Bruker X-band EPR spectrometer. The unsaturated line spectrum obtained at room temperature is shown in Fig.1. At 4mw. power, the spectrum was on the verge of saturation, but saturation broadening only commenced at 100mw., perhaps indicating a short spin-lattice relaxation time: this means that the magnetic moment of the free radical is strongly coupled to the vibrations of the solid lattice. At 120K temperature, however, the unsaturated linewidth did not change, so temperature induced broadening was not occurring. The linewidth of ~ 8 Gauss is twice that of such grapeseed extracts as Prof. Masquelier’s Anthogenol, containing O.P.C.’s.
The antioxidant efficiency measurement was carried out. Usually the material to be tested is dissolved in a phosphate buffer solution, but resveratrol is little soluble in water, so methanol had to be used. Under these conditions, vitamin E gave an efficiency of 91%, and resveratrol, 96%, both uncertain within +/- 3%. The typical antioxidant efficiency of catechin containing products lie in the 75% - 85% range when measured with the phosphate buffer: vitamin E there usually gives 96% - 99%.
The fact that resveratrol radical(s) have a broader linewidth than those on catechins is one reason why the former has a greater antioxidant efficiency than the latter. One would also expect the efficiency of resveratrol to be at least equal to that of vitamin E, given its well established reputation. EPR is a useful and informative way of studying the efficiency of antioxidants.