446 M. Monagas and B. Bartolom ́e
(+)-catechin and ethanol (Remy-Tanneau et al. 2003). Based on their resistance to
thiolysis, it was postulated that the adducts were of bicyclic nature containing both
C-C and ether type bonds (structure similar to A-type procyanidins), confirming
the mechanism first proposed by Bishop and Nagel (1984). The major dimer was
identified by NMR as malvidin-3-glucoside-(C2-O-C7, C4-C8)-epicatechin (Remy-
Tanneau et al. 2003) and recently detected again in wine fractions obtained by high
speed countercurrent chromatography (HSCC) (Salas et al. 2005a). According to
Remy et al. (2000) both types of products, A-F and F-A, seem to be more associated
with oligomeric procyanidins than with the polymeric ones. However, Hayasaka and
Kennedy (2003) have detected molecular masses of A-F and F-A polymers in wine
up to the level of octamers. Evidence of (epi)catn-anthocyanins and anthocyanin-
O-(epi)catn(n≥2) has also been obtained by thiolysis of HSCC wine fractions
(Salas et al. 2005a).
Anthocyanin-anthocyanin dimers have also been detected in red wine fractions
(Vidal et al. 2004; Salas et al. 2005a; Alcalde-Eon et al. 2007). Both, the flavene
(malvidin-3-glucoside[flavene]-(C4-C8)-malvidin-3-glucoside[A+]) and the bicyclic
structure (malvidin-3-glucoside[flavane]-(C2-O-C7, C4-C8)-malvidin-3-glucoside
[A+]) have been proposed for these compounds. Other dimers containing malvidin-
3-glucoside and either delphinidin, cyaniding, peonidin and petunidin -3-glucosides
have also been detected in wine fractions (Salas et al. 2005a; Alcalde-Eon et al. 2007).
Recently, F-A-A+oligomers consisting of (epi)catechin or (epi)gallocatechin and
a dimeric anthocyanin have been detected in red wine fractions (Alcalde-Eon
et al. 2007).
Factors affecting the reaction.Salas et al. (2003) have studied the influence of
pH (2.0 and 3.8) on the progress of the reaction between the procyanidin dimer
B2-3′-O-gallate (Ec-EcG) and malvidin-3-glucoside. Recently, similar experiments
have been performed with a flavanol monomer ((–)-epicatechin) instead of B2-3′-
O-gallate in the pH range 2.0-6.0 (Due ̃nas et al. 2006). The nucleophilic addition
of epicatechin onto the flavylium cation of malvidin-3-glucoside occurred at all pH
values leading to the malvidin-3-glucoside-epicatechin adduct in the flavene form.
However, the conversion of this flavene into further products differed according to
the pH. At pH 2 it proceeded to a colorlessA-type dimer whereas at higher pH val-
ues (3.2–6.0), it was converted into new xanthylium pigments (Due ̃nas et al. 2006).
Nevertheless, the precursor of these compounds, the intermediate flavylium form
of the A-F product, could not be detected in this case as in previous experiments
carried out with procyanidin dimer B2-3′-O-gallate at pH 3.8 (Salas et al. 2003)
or with a flavanol trimer at pH 3.0 at high temperature (50◦C) (Malien-Aubert
et al. 2002), indicating the role of the flavanol mDP in the stabilization of this specie.
However, there is one case where a stable A-F dimer in the flavylium form was syn-
thesized from a flavanol monomer (Escribano-Bail ́on et al. 1996 ), but a synthetic
flavylium with no OH group at C-5 position was used instead of a common antho-
cyanin. Finally, when the experiments were performed at pH≥4.0, (–)-epicatechin
was oxidized too-quinones. The nucleophilic addition of the anthocyanin in its
hemiketal form onto this specie was promoted, leading to a new series of F-A
adducts.