Procyanidins oxidation reactions

Procyanidins are quite reactive and are therefore considered as some of the most unstable natural phenolic compounds [19-20]. They are subject to enzymatic oxidation by polyphenol oxidases as well as to spontaneous oxidation [21], Coupled oxidation reactions involving o-quinones of phenolic acids have been reported [22-24], Procyanidins are thermally labile [25] and can easily undergo molecular rearrangements in acidic or basic media [26]. In model solutions interflavanoid bonds of procyanidins were found to be unstable, but also new carbon-carbon bonds were formed... 

The analysis of procyanidins starts at the point of sample collection. Care should be taken not to damage the plant material when collecting it. Any disruption of membrane integrity increases the risk of oxidation reactions and the formation of complexes with biomolecules such as proteins. Consequently an alteration in extraction efficiency and chemical composition of the samples is inevitable [45-46],... 

During the barrel and bottle aging of wine, many oxidative reactions modify the structures of the original procyanidins. Phenolic compounds in wine may be fractionated into four classes (Section 6.4.6) that have characteristic reactivity to gelatin (Table 6.9). According to the percentage of each of these classes in a given wine, it is... 

Other reactions involving this aldehyde include the following (1) combination with sulphite ion, which substantially increases the proportion of bound sulphite in wine (2) formation of addition compounds with some polyphenols such as tannins and procyanidins, where it acts as a bridging molecule (Haslam and Lilley 1998) and (3) chemical oxidation to acetic acid, which only occurs to a small extent and has little influence on wine composition and quality. 

A major drawback of all functional group assays is that a satisfactory standard does not exist. For a given sample the most appropriate standard is a purified procyanidin fraction prepared from the same matrix. The isolation and characterization of such purified fractions are laborious. Added to that procyanidins undergo oxidation, complexation and self-polymerization very easily, rendering such purified fractions only reproducible to a limited degree. At least in the proanthocyanidin assay the color reaction depends not only on the polyphenols themselves, but also on the matrix. The use of specified proanthocyanidins as a standard in a suitable blank matrix is an attempt to correct for such effects [67],... 

The binding mechanism is based on hydrogen bonding and hydrophobic interaction [157-158]. An involvement of electrostatic attraction is unlikely in the case of procyanidins because their phenolic hydroxyls ionize only at a very alkaline pH. Associated with oxidation covalent binding may occur [43,86]. The precipitation reaction is only possible, if a minimum number of hydroxyl groups per molecule are present to enable sufficient cross linkages. The affinity of procyanidins to proteins was found to be proportional to the number of ortho-diphenol groups and... 

The procyanidins polymerized during the various reactions oxidize other components in the medium, especially ethanol into ethanal. 

In conclusion, flavanols, procyanidins and, consequently, condensed tannins react more-or-less easily with free radicals, according to their configuration. These chain reactions produce brown polymers with varying structures that precipitate. In wine, these phenomena depend on the phenol content. The oxidation kinetics are apparently slower than in a model medium, probably due to the presence of even more easily oxidizable compounds, also involved in the oxidation of procyanidins. 

In wine, this type of reaction occurs at the same time as the heterogeneous polymerization of the procyanidins (Section 6.3.7), as a result of the controlled oxidation during barrel aging, when traces of ethanal are produced by the oxidation of ethanol. The color of the wine becomes more intense and changes tone, becoming darker after a few months in the barrel. 

Reactions between tannins (-I-) and anthocyanins (—) (T+ A ). The formation of car-bocations (-I-) from procyanidins is promoted by higher temperatures and requires an acid medium (wine). Anthocyanins (—) correspond to the carbinol base. The molecules formed are theoretically colorless, but are rapidly dehydrated into a stable, reddish-orange form. This reaction is completely independent of the oxidation conditions in the medium. 

The first phenomenon that should be mentioned is the direct reaction of red anthocyanins, in the form of positive flavylium cations, with flavanol molecules (catechins, procyanidins, etc.). This results in the formation of a colorless complex (flavene) that produces a red pigment when oxidized (Section 6.3.10). This reaction is stimulated by an acid pH (<3.5) and aT/A molar ratio <5. This mechanism causes wine to deepen in color following running-off. 

The quantification of the procyanidins with a degree of polymerization higher than 2 also varied according to the cocoa source analyzed. For instance, cocoa powder showed the lowest content compared with the other cocoa sources studied, which could be related to the manufacturing process comprising oxidation steps and condensation reactions. Lacueva et al. [58] suggested a 60% loss of the flavonoid content during the process of alkalinization to obtain the cocoa powder. 

Several studies indicate that catechins and procyanidins are powerful scavengers of ROS. Some findings regarding the antioxidant activity of proanthocyanidins are listed in Ref. [100]. Other antioxidant mechanisms are the chelation of transition metals, as well as the mediation and inhibition of enzymes. The metal-chelating activity of proanthocyanidins is thought to be due to their capacity to reduce the concentration, and thus the oxidative activity, of hydroxyl radicals formed by Fenton reaction catalyzed by iron or copper. Flavanols also influence oxidative stress via enzyme modification and modulation of cell signaling pathways the extent of the effect relies greatly on flavanol structure-related protein reactivity [101]. 

Covalent bond formation between the oxidized polyphenolic compound and the protein, or extensive oxidation of the protein-procyanidin complex during reaction with the radical, are the most likely mechanisms for this reaction. We are pursuing characterization of the oxidized complex at the molecular level. 

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