# Polypphenols: Natural Antioxidants to Be Used as a Quality Tool in Wine Authenticity
## 2 Structural Information
### Flavonoid Compounds  
Flavonoids are secondary metabolites occurring in a wide variety of natural products such as vegetables, fruits, stems, cocoa, tea, grapes, and wine. Nowadays flavonoids are considered essential ingredients for various medicinal, nutraceuticaltial, pharmaceutical, and cosmetic applications. This is due to their potent anti-oxidative, anti-carcinogenic, anti-inflammatory, and anti-mutagenic properties combined with the ability to modulate significant enzyme functions [41, 42, 43]. In plants they are involved in a series of processes related to defense against pathogens and pests, protection from ultraviolet (UV) radiation, allelopathy, pollen fertilization, auxin transport regulation, and pigmentation [44, 45, 46, 47]. In wine, flavonoids have a fundamental role in the determination of its sensory attributes specifically wine color, flavor, astringency, and bitterness [27,48]. Total phenolic content in red wine ranges from 1200 to 1800 mg gallic acid equivalents/L, which is six to nine times more than the corresponding content in white wines [49].  
Chemically, they belong to a class displaying a diphenylpropane skeleton structure, essentially, two aromatic rings (rings A and B) linked by a three-carbon chain forming a closed pyran ring (heterocyclic ring containing oxygen, the C ring) with the A aromatic ring (Figure 3). In this C\({}_{6}\)-C\({}_{3}\)-C\({}_{6}\) structure, usually the B aromatic ring is attached to position 2 of C ring and on fewer occasions, it is found attached to positions 3 or 4 [7,17]. The above possible combinations, the patterns of glycosylation and/or hydroxylation of the aromatic rings, the structural features of the rings, along with the degree of unsaturation and oxidation of the C ring account for the large diversity of the flavonoid compounds.  
Figure 2: Scheme of formation of resveratrol oligomers and viniferins: (**a**) trans-resveratrol; (**b**) (E and Z) \(\epsilon\)-viniferin/ \(\omega\)-viniferin; (**c**) pallidol; (**d**) caraphenol B; (**e**) \(\delta\)-viniferin (E and Z); (**f**) \(\alpha\)-viniferin; (**g**) isohopeaphenol; (**h**) E-miyabenol C; (**i**) Z-miyabenol C; (**j**) vaticanol C isomer; and (**k**) ampelopsin H (from [PERSON] et al. (2016) [31]).  
With the exception of chalcones, flavonoids can be subdivided into various types of compounds according to the degree of oxidation of the central ring. Flavonoid compounds where the B ring is attached to position 3 or 4 of the central ring are called isoflav and neoflavonoids, respectively. Numerous compounds arise from the attachment of ring B in position 2 of the central ring. The corresponding subgroups are called: flavonols, flavones, flavonols, flavanones, flavan-3-ols, and anthocyanins. Flavonoids where the central ring is open are chalcones.  
Two pathways are involved in the biosynthesis of flavonoids; the shikimate and the acetate (or polyketide) pathway. Ring A and the linking chain (central ring) are synthesized from four \(p\)-coumaroyl-CoA molecules provided by phenylalanine through the shikimate pathway while the acetate pathway provides three malonyl-CoA molecules derived from glucose transformation and form ring B [45; 46]. The two rings are condensated forming chalcone, which followingly is transformed to flavanone. This is the starting unit for the biosynthesis of more than 5000 compounds.  
An overview of the main flavonoid groups in grapes and wines is presented in Figure 4.