Leucoanthocyanidin, structure

From extensive analysis of recombinant proteins, and the crystal structure of A. thaliana protein, detailed reaction mechanisms have been proposed. The ANS reaction likely proceeds via stereospecific hydroxylation of the leucoanthocyanidin (flavan-3,4-cA-diol) at the C-3 to give a flavan-3,3,4-triol, which spontaneously 2,3-dehydrates and isomerizes to 2-flaven-3,4-diol, which then spontaneously isomerizes to a thermodynamically more stable anthocyanidin pseudobase, 3-flaven-2,3-diol (Figure 3.2). The formation of 3-flaven-2,3-diol via the 2-flaven-3,4-diol was previously hypothesized by Heller and Forkmann. The reaction sequence, and the subsequent formation of the anthocyanidin 3-D-glycoside, does not require activity of a separate dehydratase, which was once postulated. Recombinant ANS and uridine diphosphate (UDP)-glucose flavonoid 3-D-glucosyltransferase (F3GT, sometimes... 

Among the hypotheses formulated, the most likely mechanism calls for the formation of a covalent bond between carbon 4 of the 3,4-flavan-diol (leucoanthocyanidin) and carbons 6 or 8 of another flavan molecule. Benzylic alcohol is a reactive electrophile (loss of OH"), and it donates readily in acid media leucoanthocyanin (25) functions similarly at position 4. The phenolic group is a mesomeric structure which displays negatively charged nucleophilic centers in the ortho and para positions analogous centers may be found at positions 6 and 8 of flavan molecules. This would allow the possibility of covalent bond formation between carbon 4 of 25 and carbons 6 or 8 of 26a or 26b. This bond is attributable to the elimination of a water molecule. 

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. 

Paolocci F, Robbins MP, Madeo L, Arcioni S, Martens S, Damiani F. 2007. Ectopic expression of a basic helix-loop-helix gene transactivates parallel pathways of proanthocyanidin biosynthesis. Structure, expression, analysis, and genetic control of leucoanthocyanidin 4-reductase and anthocyanidin reductase genes in Lotus corniculatus. Plant Physiol 143 504-516. 

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. 

Anthocyanins are water soluble pigments responsible for the red to purple colour in plants. The common basic structure of all these compounds is the cation flavylium, which was proposed for the first time by Wilstaer in 1913 and later confirmed by Robinson in 1922. A special kind of flavonoids related to anthocyanins are the proanthocyanidins, also referred to as leucoanthocyanidins, in the case of monomeric proanthocyanidins, and condensed proanthocyanidins for the polymers of flavan-3-ols [32],... 

In many types of plant tissue, colorless materials are present which, upon treatment with mineral acids, are converted to red substances exhibiting many of the properties of anthocyanidins. The colorless compounds were given the names of leucoanthocyanins and leucoanthocyanidins by Rosenheim (6), These compounds probably represent a heterogeneous class. They may have an important part in bleaching and color-stabilization problems, particularly in the western areas of the United States, where many woods have a reddish rather than the more usual yellowish cast. Although work has been done, particularly by the Robinsons, the structures of the leucoanthocyanins have not been established definitely. 

FIGURE 10 Basic structures of leucoanthocyanidins (1), anthocyanidins (2), and dimeric leucoanthocyanidins (3). 

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