NMR spectral data - notable testimony in antioxidant interactions research: case studies of some grape metabolites

Abstract

Current research portrays the natural antioxidant systems in wine: trans- resveratrol-L-ascorbic acid and dihydroxyfumaric acid-L-ascorbic acid, in an attempt to model and elucidate plausible interactions between naturally occurring antioxidants in grapes and wine based on of the scavenging activity of 1,1-diphenyl-2-picrylhydrazyl (DPPH•) radicals by the named antioxidants and relying on 13 C and 1 H NMR spectroscopy data as a method of investigation. The evidence provided by the NMR experiments was compared with the UV-VIS spectroscopy data. For the trans- resveratrol-L-ascorbic acid system, based on the 13 C NMR method, the formation of trans-resveratrol oligomers (viniferins) was demonstrated as an effect of oxidation, in a process assisted by DPPH•. By comparing the 13 C NMR spectral results with the UV- VIS data, it was established that pronounced synergistic effects in the mentioned system are present precisely in the experimental cases for which the presence of viniferins was attested. For the dihydroxyfumaric acid-L-ascorbic acid system, the oxidation of dihydroxyfumaric acid to diketosuccinic acid, described in the literature, was not attested. Instead, through 1 H and 13 C NMR spectral studies, it was established that as a result of the interaction of dihydroxyfumaric acid with DPPH•, acid degradation occurs. The exact identification of the products of the transformation of dihydroxyfumaric acid was hindered by the secondary processes involved, with the major involvement of dihydroxyfumaric acid, namely the polymerization of acetone, triggered by dihydroxyfumaric acid, with the participation of methanol. These conclusions are supported on the basis of two-dimensional NMR experiments: 1 H/ 13 C HMBC and 1 H/ 13 C HSQC and DOSY.