Flavanol glycosides in grapes and wines : the key missing molecular intermediates in condensed tannin biosynthesis ?

Abstract

Polyphenols are present in a wide variety of plants and foods such as tea, cacao and grape1. An important sub-class of these compounds is the flavanols present in grapes and wines as monomers (e.g (+)-catechin or (-)-epicatechin), or polymers also called condensed tannins or proanthocyanidins. They have important antioxidant properties2 but their biosynthesis remains partly unknown. Some recent studies have focused on the role of glycosylated intermediates that are involved in the transport of the monomers and may serve as precursors in the polymerization mechanism3, 4. The global objective of this work is to identify flavanol glycosides in grapes or wines, describe their structure and determine their abundance during grape development and in wine. Material and methods: Chardonnay and Syrah grapes and a Tannat red wine (Tannat, Languedoc, 2015) were used to make polyphenol extracts by using different preparative gel chromatography techniques (HW40S, LH20 and silica gels). The different fractions obtained were analyzed by UPLC-ESI-IT_MS (Waters Acquity, negative scan and targeted mode). Specific molecular ions corresponding to monomeric and dimeric flavanol glycosides were targeted with specific m/z values: 451 (epi) catechin glucoside, 467 epigallocatechin glucoside, 603 epicatechin gallate glucoside and 739 (epi) catechin glucoside dimer. Results: The existence of glycosylated flavan-3-ol monomers in wine and grape seeds have already been reported based on MS/MS experiments.5 Our results confirmed their presence in Tannat wines and grapes but new MS ions corresponding to glycosylated procyanidin dimers (m/z = 739) were also detected. MS/MS specific ions were also found for these dimers like an ion at m/z = 449 (quinone-methide cleavage (QM)) and at m/z = 587 Da (Retro Diels Alder reaction (RDA)). Further work is on the way to elucidate the exact structure of these compounds (hexose nature and position) by NMR. The complete structural elucidation of these glycosylated dimers will help to determine their exact role in proanthocyanidin biosynthesis.

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