Flavan-3-ols and Proanthocyanidins

Flavan-3-ols are widely distributed in the Filicopsida and are almost invariably accompanied by condensed tannins. Several flavan-3-ol glycosides (389-394) have been isolated from ferns (202, 207, 240, 241). Dryopterin (280) found in Dryopterisfilix-mas is a new type of flavan-3-ol which contains an additional a-pyrone ring (181). Trimeric proanthocyanidins, arachnitannins 1 (282), 2 (283) and 3 (284), which contain dryopterin as the terminating unit were isolated from Arachniodes species (183). Proanthocyanidin polymers from Cyathea dealbata are composed of procyanidin and prodelphinidin in the ratio 6 4 (251). 

Leucoanthocyanidins, especially those producing delphinidin on acids treatment, are present in almost all members of the Filicopsida 14, 252). A flavan-3,4-diol having C-methyl groups (281) was first isolated from Bolbitis subcordata 182). 

---

Feucht, W. and Treutter, D. (1999). The role of flavan-3-ols and proanthocyanidins in plant defense , in Inderjit, K. and Dakshini M.M F.C.L., Principles and Practices of Plant Ecology, CRC Press, London, 307-338. 

Dihydroflavonol 4-reductase (DFR EC 1.1.1.219) is a member of the short-chain dehydrogenase/reductase family and catalyzes the stereospecific conversion of (+)-(2R,3R)-dihydroflavonols to the corresponding (2R,3S,4S) flavan-3,4-cw-diols (leucoanthocyanidins), with NADPH as a required cofactor. The enzyme activity was first identified in cell suspension cultures of Douglas fir (Pseudotsuga menziesii) and was shown to be related to the accumulation of flavan-3-ols and proanthocyanidins [96]. Leucoanthocyanidins and DFR were later shown to be required for anthocyanidin formation by complementation of Matthiola incana mutants blocked between dihydroflavonol and anthocyanidin biosynthesis [97, 98], DFR has been purified to apparent homogeneity and biochemically analyzed from flower buds of Dahlia variabilis [99]. DFR was shown to accept different substrates depending on the plant species from which it was isolated (reviewed in 100). 

Petereit, F., Kolodziej, H., and Nahrstedt, A., Flavan-3-ols and proanthocyanidins from Cistus incanus. Phytochemistry, 30, 981, 1991. 

Figure 1.13 Monomeric flavan-3-ols and proanthocyanidin B2, B5 dimers, and C, trimer found in fresh cocoa beans.

Table 3.5 Methodological Overview of Analysis of Flavan-3-ols and Proanthocyanidins from Foods and Biological Samples...

Hashimoto, R, Nonaka, G., and Nishioka, I. 1989. Tannins and related compounds. LXXVII. Novel chalcan-flavan dimers, assamicains A, B and C, and a new flavan-3-ol and proanthocyanidins from the fresh leaves of Camellia sinensis L. var. assamica Kitamura. Chem. Pharm. Bull. 37 77-85. 

New combined absorbents are now proposed to brewers, such as PVPP mixed with silica xerogel, PVP bound onto silica, and tannin linked to silica [23, 39]. Another innovative way is the use of flavan-3-ol and proanthocyanidin-free malt which allows affording an excellent colloidal stability [40]. 

Table 4 - IC q values (iiM) for flavan-3-ols and proanthocyanidins (20-33) on the classical and the alternative pathway of complement [20]. ...

Stafford (338-341) has proposed an alternative scheme in which these compounds are formed by reductions of dihydroflavonols, but a C-3 epimerase is postulated to account for production of a 2,3-cis (2i ,3S)-dihydroflavonol. This could be held by an enzyme where it is reduced stereospecifically through two steps to give a 2,3-cis flavan-3-ol such as epicatechin. This scheme proposed by Stafford and that proposed by Jacques et al. (175, 177) both require two metabolic pools, one for the flavan-3,4-diols and one for the flavan-3-ols, to account for differences in the stereochemistry of the flavan-3-ols and proanthocyanidins. 

There are now a number of well authenticated examples of the occurrence of gallate esters of monomeric flavan-3-ols and proanthocyanidins. Recently Haslam suggested that, in view of the apparently similar function and properties of hydrolyzable and condensed tannins, there is some form of metabolic link between these two complex polyphenols (58). Further, he observed that where gallate esters of flavan-3-ols occur, they are accompanied by only low concentrations of proanthocyanidins. 

Cashew Anacardium occidentale) kernels contain flavan-3-ols and proanthocyanidins and pecans Carya illinoensis) a range of phenohc acids and flavan-3-ols (Shahidi and Naczk 1995). Isoflavones, such as 5,7-dimethoxyisoflavone (Figure 7.53), occur in peanuts Arachis hypogaea) but at a much lower concentration than found in soya (Turner et al. 1975). 

---