C-Glycosidic Ellagitannins and Their Influence on Wine Chemistry - Chemistry and Biology of Ellagitannins - page 352

biochemical) transformation events that are far from having been all
mechanistically elucidated (Ribéreau-Gayon, 1973, Ribéreau-Gayon et
al. , 1983, Singleton, 1987, Cheynier et al. , 1986, 1990, Pontallier, 1992,
Moutounet and Mazauric, 2001).
9.3.4.2 Oxidative conversion of acutissimin A into mongolicain A
Most investigations on the fate of wine phenolics upon oxygenation at
the various stages of the wine making process have concerned phenolic
acids ( e.g. , gallic acid, caftaric acid), anthocyanins, flavanols and their
proanthocyanidic oligomers. Very few studies have addressed what
happens to oak ellagitannins in this context at the molecular level, and
the information glaned from the literature is rather contradictory. Some
authors concluded that oak ellagitannins play a major role as oxidation
regulators in wine, quickly absorbing dissolved oxygen and facilitating
the hydroperoxidation of some wine components ( e.g. , ethanol into
ethanal, vide supra ) (Vivas and Glories, 1993, 1996), whereas others
concluded that the oxidation of ellagitannins, like vescalagin ( 1 ), is a
very slow process (Moutounet et al. , 1992). One possible explanation to
this apparent contradiction is that the galloyl-derived units of oak C -
glycosidic ellagitannins are engaged in fast inter- and/or intramolecular
oxido-reductive processes during which their pyrogallol moieties are
reversibly converted into semiquinone free radicals and/or ortho -
quinones through one- and/or two-electron transfers.
Support for this speculative interpretation can be drawn from a
molecular-level observation made in our laboratory. In an aqueous
solution left under air at 60 °C, pure acutissimin A ( 14 ) was converted
into mongolicain A ( 18 ), which could be isolated by semi-preparative
HPLC in a yield of 22% (Fig. 9.16). This same conversion was also
observed in a wine model solution ( unpublished results , see Lefeuvre,
2006). Interestingly, it is worth recalling here that mongolicain A is
thought to be naturally derived from the oxidation of acutissimin A, as
both molecules usually co-exist in their plant sources (Nonaka et al. ,
1988, see Section 9.1.1).