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 (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 • C), of the reaction products obtained by 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 4 β-sulfonated derivatives passes through the acid-catalysed depolymerisation of proanthocyanidins; (b) monomeric flavanols lead to the same 4 β-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 SO 2 addition to dimeric and monomeric flavanol substrates; (d) direct sulfonation of epicatechin is slightly faster than that of catechin. Flavanols are among the most important groups of secondary metabolites, due to their ubiquity, biological activities, nutritional value and food quality impact. They include the monomeric epicatechin (1), catechin (2), gallocatechin, epigallocatechin gallate, catechin gallate, oligomeric procyanidins (e.g. procyanidin B2 (3) and procyanidin B3 (4)), prodelphinidins, and polymeric proanthocyanins (Fig. 1). Condensed tannins are defined as oligomeric/polymeric flavanols with mass higher than 500 Dalton 1. This group of secondary metabolites is found in most of the plants, fruits, vegetables and beverages, and so every day we intake huge amounts of them by consuming wine, tea, cocoa, coffee, chocolate, berries, apples, nuts, dry fruits, mint, basil, etc. 2-4. The first scientific works about tannins were focused on their utility in the leather industry 1,5,6 , but today they are known and worldwide studied for their role in (a) human health 3,7,8 , (b) plant physiology and defence 2,9 and (c) contribution to sensorial character of food, due to their astringent and bitter taste, or their ability to stabilise wine red colour 10-14. Their properties depend on their chemical structure, such as degree of polymerisation, Bring hydroxylation and C-ring configuration 15-20. Centuries ago, the addition of SO 2 to a tannin extract was a key process in order to obtain better quality material for leather manufacturing 21. Today, SO 2 (E220) addition is permitted and regularly used in a wide range of food including wine, dried fruits and meat products, because of its preservative effects 22. However, SO 2 and sulfites are among the food allergens and the added doses are subject to legal limits 23,24. ...

Improving the Phloroglucinolysis Protocol and Characterization of Sagrantino Wines Proanthocyanidins

“…Knowing the length and the structural characteristics of the wine oligomeric and polymeric proanthocyanidins, also provides important information from the enological point of view. Condensed tannins participate in several reactions occurring during winemaking and wine aging, such as interflavan linkage formation and acidic cleavage, aldehyde polycondensation, sulfonation, oxidation, rearrangement and reaction with anthocyanin (direct and ethyl linked) [ 6 , 12 , 14 , 42 , 43 , 44 ]. Since the oligomeric and polymeric forms are the main source of this group of metabolites and technologically we are not yet able to measure the single molecules of the large forms with precision, a degradative protocol is still the best tool for obtaining information about these high molecular weight compounds.…”

“…Due to their quasi-ubiquitary presence in our plant food, the human average intake per day is very high in comparison with that of other classes of polyphenols [ 4 , 5 ]. In wine, proanthocyanidins play important roles, affecting color, astringency, bitterness, stability, aroma and aging potential [ 6 , 7 , 8 , 9 , 10 ]; and can be found in various forms, such as the monomeric catechin (C), epicatechin (EC), gallocatechin (GC), epigallocatechin (EGC) and epicatechin 3-gallate, their linear and cyclic oligomers/polymers, and their sulfonated adducts [ 6 , 7 , 11 , 12 , 13 , 14 ] ( Figure 1 ). In wine science, there is a great demand for the structural elucidation and the absolute or relative quantification of the proanthocyanidins profile for various purposes like taxonomical aims, sensorial characterization, structure-function studies, biotic and abiotic stresses responses research and other technological issues [ 6 , 13 , 15 , 16 , 17 ].…”

H/D Exchange Processes in Flavonoids: Kinetics and Mechanistic Investigations

“…Moreover, the two active sites (6 and 8) on ring A have been proven to have different behaviours towards H/D exchange. Other reports have found different rates of exchange for the two sites [12,19,20], which can reflect differences in their electron density, triggered by the We recently described, within the class of flavan-3-ols, an interesting case of ring C reactivity towards sulfite addition leading to the corresponding 4-sulfonated analogues [8]. During this investigation, we also observed the slow disappearance of the NMR signals of H-C( 6) and H-C(8) in ring A when D 2 O was used as a solvent for the flavan-3-ol substrates, evidently due to solvent-mediated H/D exchange at these positions.…”

“…Two o-hydroxyl groups and an ether-oxygen atom are attached to ring A, in an alternated fashion, and two or more hydroxyl groups are attached to ring B generating a very unique reactivity on the carbon sites of the aromatic rings. Based on polyhydroxyl configuration on the benzene rings of flavonoids, the B ring is considered to be prone to redox processes (catechol to o-quinone conversion) [6,7], whilst ring A would be much more reactive to electrophilic substitutions at the strongly activated nucleophilic sites C (6) and C (8). Among the flavonoids, structural changes in the B ring are also considered to influence the antioxidant activity.…”

“…We recently described, within the class of flavan-3-ols, an interesting case of ring C reactivity towards sulfite addition leading to the corresponding 4-sulfonated analogues [8]. During this investigation, we also observed the slow disappearance of the NMR signals of H-C(6) and H-C(8) in ring A when D2O was used as a solvent for the flavan-3-ol substrates, evidently due to solvent-mediated H/D exchange at these positions.…”