Kinetic investigations of sulfite addition to flavanols

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 ([Formula: see text]), 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 [Formula: see text]), of the reaction products obtained by [Formula: see text] 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 4[Formula: see text]-sulfonated derivatives passes through the acid-catalysed depolymerisation of proanthocyanidins; (b) monomeric flavanols lead to the same 4[Formula: see text]-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 [Formula: see text] addition to dimeric and monomeric flavanol substrates; (d) direct sulfonation of epicatechin is slightly faster than that of catechin.

Figure 1

Structure of monomeric flavanols 1-2, dimeric flavanols 3–4 and their corresponding 4-sulfo analogues 5–6 and 7–8.

Figure 2

(A) Proposed hypothetical mechanism for the production of 4$\beta$-sulfonate 5 from interflavanic bond cleavage of procyanidin B2 (3); (B) production of 4$\beta$-sulfonate 6 from interflavanic bond cleavage of procyanidin B3 (4). A different mechanism is required for the slower conversions 1 $\to$ 5 and 2 $\to$ 6.

Figure 3

Top: acid-induced C(2) epimerisation of (−) epicatechin (1) leading to ent-2 and/or, in presence of hydrogen sulfite, epicatechin ring-C opened sulfonates (12); Bottom: acid-induced C(2) epimerisation of ($+$) catechin (2) leading to ent-1 and/or, in presence of hydrogen sulfite, catechin ring-C opened sulfonates (14).

Figure 4

Comparison of the different temperature kinetics of epicatechin 4$\beta$-sulfonate (5) and catechin 4$\beta$-sulfonate (6) formation starting, respectively, from epicatechin (1) (a, b) and catechin (2) (c, d) at pH 3 and 4. R${}^2$ coefficients can be found in Supplementary Table S3 online.

Figure 5

3D geometry optimised structures of 5 and 6 as obtained by molecular mechanics (GMMX) calculations (energy minimisation).

Figure 6

Proposed mechanism for the sulfonation of epicatechin 1.

Figure 7

Comparison of the different temperature kinetics of epicatechin 4$\beta$-sulfonate (5) and catechin 4$\beta$-sulfonate (6) formation starting, respectively, from procyanidin B2 (3) (a, b) and procyanidin B3 (4) (c, d) at pH 3 and 4. R${}^2$ coefficients can be found in Supplementary Table S3 online.