Leucoanthocyanidines, oxidation

Finally, Weinges et al. (31, 33) postulated the formation of dimers such as 27 or 28, 29 or 30 directly from catechins without involving 3,4-flavandiols (leucoanthocyanidins). This process has never been demonstrated in fruits directly and specifically not in grapes. Only the proanthocyanin dimers have been positively identified through the formation of catechins and anthocyanidins during acid hydrolysis. Dimer formation proceeds by enzymatic oxidation of two molecules of catechin... 

Proanthocyanidins are so named because under oxidative and acidic conditions they are converted to anthocyanidins, a subclass of flavonoid (for general structure, see Fig. 11.3.4). Historically, they have also been referred to as leucoanthocyanidins, condensed tannins, or simply tannins because of their ability to fix or tan leather hides. 

This enzyme catalyzes the conversion of flavan 3,4-diols (leucoanthocyanidins) to their corresponding anthocyanidins.60 A cDNA encoding ANS was recently isolated from Perilla frutescens,61 and its recombinant protein catalyzed the oxidation of both leucocyanidin and leucopelargonidin to their corresponding anthocyanidins, most likely via their 2-flaven-3,4-diols upon subsequent acidification. The enzyme exhibited a 3-fold higher affinity for leucocyanidin over leucopelargonidin.61 Leucodelphinidin was not tested as a substrate. 

Condensed tannins, on the other hand, occur in the bark of all conifers and hardwoods examined to date, and they are frequently present in the wood. They are primarily responsible for the tan to brown color of wood after it is exposed to air. In their purest form, condensed tannins are colorless, but they become colored very readily once isolated because of their propensity to oxidize to quinones. The primary characteristic of the water-soluble condensed tannins (4) is dehydration/oxidation to intensely colored anthocyanidin pigments (5) when refluxed in butanol and hydrochloric acid (Figure 2). For this reason, there has been a tendency to refer to these compounds as proanthocyanidins in the last few years. Prior to that, they were referred to as leucoanthocyanidins (i.e., the colorless chemical form of anthocyanidins). All references earlier than the late 1950 s, when the structure of these substances was just beginning to be understood, used the term condensed tannin. 

Leucoanthocyanidin reductase (LAR) converted leucoanthocyanidins (flavan-3,4-diols, 37) to flavan-3-ols (40). The flavan-3-ols (40) were converted to condensed tannins (proanthocyanidins, PA, 44) by condensing enzyme (CE). Following this condensed tannins (proanthocyanidins, PA, 44) finally yielded their oxidized tannins (oxidized proanthocyanins, 45) by proanthocyanidine oxidase (PRO) (Fig. 9) [23,24]. 

Fig. 9 Biosynthesis of flavan-3-ols (40), condensed tannins (proanthocyanidins, 44) and oxidized tannins (oxidized proanthocyanidines, 45) from leucoanthocyanidins (flavan-3,4-diols, 37)...

The tea bush and in particular its young leaves contain a high concentration of polyphenols and oxidative enzymes, thus the young leaves are better for tea manufacture. Tea polyphenols, previously called tea tannins, are also known as tea flavonoids. Among the polyphenols in fresh tea leaves, catechins are the predominant form of polyphenols, which account for 12-24% of the dry weight. Besides catechins, flavonol, and their glycosides, anthocyanidin and leucoanthocyanidin, phenolic acids and depsides are also present. Their typical concentrations are shown in table 8.1. These phenolic compounds are directly or indirectly associated with the characteristics of tea, including its color, taste, and aroma. 

The o-quinones formed from phenolics further enhance the intensity of browning by oxidation of other substrates, complexing with amino acids and protein, and polymerization. Non-enzymatic discoloration is believed to involve metal-polyphenol complexing as reported in the processed potato (Bate-Smith et al., 1958), cauliflower (Donath, 1962), and asparagus (DeEds and Couch, 1948), conversion of leucoanthocyanidins to pink anthocyanidins in the processed broad bean (Dikinson et al., 1957), green bean puree (Roseman et al., 1957), and canned Bartlett pear (Luh et al., 1960), and protein-polyphenol complexing in chilled or stored beer (Schuster and Raab, 1961). 

Anthocyanidin synthase (ANS), the key enzyme in the biosynthesis of anthocyanins, catalyzes oxidation of leucoanthocyanidin (flavan-3,4-diol) to a 2-flaven-3,4-diol that spontaneously isomerizes to 3-flaven-2,3-diol (anthocyanidin) (Fig. 5). This is subsequently glycosylated at C-3, transported to the vacuole, and finally converted to the colored flavilium cation at the acidic... 

The name proanthocyanidins, previously called leucoanthocyanidins, implies that these are colorless precursors of anthocyanidins. On heating in acidic solution, the C—C bond made during formation is cleaved and terminal flavan units are released from the oligomers as carbocations, which are then oxidized to colored anthocyanidins (cf. 18.1.2.5.3) by atmospheric oxygen (Formula 18.21). Base-catalyzed cleavage via the quinone methode is also possible. 

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