Abstract
Flavanols are an important class of natural products occurring in almost all plants, fruits and vegetables; they have a great influence on wine ageing potential, astringency, colour stability and biological activities. In wine, flavanols react with sulfur dioxide (hbox {SO}_2), the most widely used preservative in oenology, leading to sulfonated products. Here we report a kinetic investigation, through LC-MS quantitative measurements carried out at different pH (3 and 4) and temperature values (23, 30, 40, 50 and 60, ^circ hbox {C}), of the reaction products obtained by hbox {SO}_2 addition to both monomeric (epicatechin and catechin) and dimeric flavanols (procyanidin B2 and procyanidin B3). The results proved that: (a) the major sulfonation route that leads quickly and in good yields to monomeric 4beta-sulfonated derivatives passes through the acid-catalysed depolymerisation of proanthocyanidins; (b) monomeric flavanols lead to the same 4beta-sulfonated products, although in a considerably slower manner, and also to other sulfonated regioisomers; (c) the kinetic data in our hands, in particular the temperature dependence of the observed rates, suggest the involvement of two completely different reaction mechanisms for the hbox {SO}_2 addition to dimeric and monomeric flavanol substrates; (d) direct sulfonation of epicatechin is slightly faster than that of catechin.
Highlights
Flavanols are an important class of natural products occurring in almost all plants, fruits and vegetables; they have a great influence on wine ageing potential, astringency, colour stability and biological activities
The time-dependent conversion of product 5 was followed for several days both in solution buffered at pH 3 (Fig. 4a) and at pH 4 (Fig. 4b) by electrospray ionisation (ESI)/MS quantitative measurements
Our kinetic investigation of the sulfonation of monomeric and dimeric flavanols in a wine-model reacting system, has allowed us to obtain a reliable estimate of pH- and temperature-dependence of the rate constants, of this process, and of several competitive processes occurring in wines
Summary
Flavanols are an important class of natural products occurring in almost all plants, fruits and vegetables; they have a great influence on wine ageing potential, astringency, colour stability and biological activities. To check if the sulfonation process in wine starts with the acid-catalysed cleavage of the polymeric proanthocyanidins, producing the intermediate electrophilic carbocation species at C(4) (9 starting from 3 or 10 starting from 4 in Fig. 2), it is necessary to know what the true products of the reaction and the main kinetic parameters (reaction order, rate constant, activation energy) are; these information are largely lacking in the current literature. To check if the sulfonation process in wine starts with the acid-catalysed cleavage of the polymeric proanthocyanidins, producing the intermediate electrophilic carbocation species at C(4) (9 starting from 3 or 10 starting from 4 in Fig. 2), it is necessary to know what the true products of the reaction and the main kinetic parameters (reaction order, rate constant, activation energy) are; these information are largely lacking in the current literature26,27 Whether this process occurs in a similar timescale as the sulfonation process or whether the ring opening occurs by acid induced processes, it is clear that sulfonation of epicatechin should lead to epicatechin- and ent-catechin-sulfonated derivatives and sulfonation of catechin should lead to both catechin- and ent-epicatechin-sulfonated derivatives. Regio- and stereochemistry must be carefully considered in any kinetic investigation of flavanols, because they allow to obtain relevant information
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