Augustin Scalbert

Antimicrobial properties of tannins

Abstract Tannin toxicity for fungi, bacteria and yeasts is reviewed and compared to toxicity of related lower molecular weight phenols. The dependence of toxicity on tannin structure is examined. The different mechanisms proposed so far to explain tannin antimicrobial activity include inhibition of extracellular microbial enzymes, deprivation of the substrates required for microbial growth or direct action on microbial metabolism through inhibition of oxidative phosphorylation. A further mechanism involving iron deprivation is proposed. Many microorganisms can overcome plant defences based on tannins. They may detoxify tannins through synthesis of tannin- complexing polymers, oxidation, tannin biodegradation or synthesis of siderophores.

Ether linkage between phenolic acids and lignin fractions from wheat straw

Abstract The bonding of the bound-phenolic acids present in three lignin preparations isolated from wheat straw where determined. p -Coumaric acid was mainly ester-linked whereas 35–75% of the recovered ferulic acid was ether-linked to milled straw lignin or enzyme lignin. Ferulic acid ethers accounted for 1.1% dry wt of alkali extracted lignin and might explain the high solubility of Gramineae lignins in soda. Isolated lignins were associated to hemicelluloses, principally arabinoglucuronoxylans. The possible existence of ferulic acid cross-links between lignin and arabinoglucuronoxylans is discussed.

Insoluble ellagitannins in Castanea sativa and Quercus petraea woods

Sapwood and heartwood samples of sweet chestnut (Castanea sativa) and sessile oak (Quercus petraea) were extracted with aqueous acetone. Ellagitannins were estimated in both extracts and extraction residues, by the Folin-Ciocalteu method and by determination of ellagic acid formed after acid degradation. Ellagitannins were found to accumulate in comparable concentrations (ca 10% DM) in the heartwoods of both species. The concentrations of soluble ellagitannins slowly decreased from the periphery to the centre of the heartwood as it ages. This decrease was balanced by an equivalent increase of insoluble ellagitannins, resulting probably from the slow oxidative polymerization or copolymerization with cell-wall components of soluble ellagitannins.

Polyphenols of Quercus robur: Adult tree and in vitro grown calli and shoots

Abstract Bark, wood, leaves and in vitro grown calli and shoots from pedunculate oak ( Quercus robur ) were extracted by aqueous methanol. The polyphenols, analysed by paper chromatography and HPLC, were principally hexadroxydiphenoylesters-vescalagin, castalagin and pedunculagin-proanthocyanidins and pentagalloylglucose. In the adult tree, inner bark, heartwood and leaves were found to contain high amounts of hexahydroxydiphenoylesters (3.5 to 8.7% DM), mainly castalagin and vescalagin. Leaves were also rich in pedunculagin. Proanthocyanidins were characteristic of bark and leaves. Calli synthesized both hexahydroxydiphenoylesters and proanthocyanidins; the former were largely oxidized and poorly resolved by chromatography. In in vitro grown shoots phenolic compounds in leaves were similar to those of adult tree leaves, whilst stems did not contain pedunculagin but were rich in pentagalloylglucose.

Iron withholding by plant polyphenols and resistance to pathogens and rots

Abstract Plant polyphenols ( syn. , tannins) inhibit growth of several mutants of the bacterium Erwinia chrysanthemi , altered in their siderophore-mediated iron transport pathway. Growth of the mutants is restored by addition of iron(III) to the medium. Polyfunctional polyphenols, with their several chelating o -dihydroxyphenyl groups per molecule, also remove iron(III) from other iron/ligand complexes more efficiently than monofunctional low molecular weight phenols. Growth inhibition of the mutants is thus explained by their inability to assimilate iron in the presence of polyphenols. Polyphenols mimic animal iron-binding proteins such as transferrin and protect plants by withholding iron away from pathogens and rots.

Ellagitannins in woods of sessile oak and sweet chestnut dimerization and hydrolysis during wood ageing

Abstract Monomeric and dimeric C -glucosidic ellagitannins and phenolic acids were studied by reverse-phase HPLC and gel permeation chromatography in various sapwood and heartwood samples of sessile oak ( Quercus petraea ) and sweet chestnut ( Castanea sativa ). Sessile oak differs from sweet chestnut by the presence in heartwood of pentose-substituted ellagitannins. Analysis of ellagitannin concentrations and a model experiment show that vescalagin is a common precursor of all dimers which are likely formed by a non-enzymatic reaction. Ellagic acid and gallic acid in oak and chestnut woods result, respectively, from the hydrolysis of ellagitannins and some unknown galloyl esters. Both dimerization and hydrolysis occur at the sapwood-heartwood transition and in the dead heartwood.

Polyphenols and chemical defence of the leaves of Quercus robur

Abstract Changes in the phenolic content and profile of leaves of pedunculate oak ( Quercus robur ) were determined through two growing seasons. The results are used to formulate an alternative explanation for the apparent relationship between changes in phenolic content and insect predation.

ELLAGITANNINS IN EUROPEAN OAK WOOD : POLYMERIZATION DURING WOOD AGEING

Abstract Ellagitannins in acetone-water extracts of outer and inner heartwood of European oak were assayed and their M r distribution studied by gel permeation chromatography. Ageing of heartwood in the living tree induces darkening of the extract, a decrease in its solubility in cold water and polymerization of ellagitannins.

Douglas-fir polyphenols and heartwood formation

Abstract Phenolic extractives have been isolated from Douglas-fir sapwood and heartwood and identified by NMR and mass spectroscopy. Pinocembrin, [5,5′]-bisdihydroquercetin and two lignans, pinoresinol and 2,3-dihydro-2-(4′- O -β-glucopyranosyl-3′-methoxyphenyl)-3-hydroxymethyl-5-(3-hydroxypropyl)-7-methoxy-benzofuran are reported for the first time in these tissues. The chemical structures of these phenolic extractives show that heartwood formation is characterized by both primary (biosynthetic) and secondary reactions. Secondary reactions include hydrolysis of phenol glucosides and oxidative polymerization.

Polyphenols isolated from the bark of Castanea Sativa. Mill. Chemical structures and auto-association

Abstract Eight phenolic compounds (castalin, castalagin, vescalagin, kurigalin, 5-O-galloylhamamelose, (3′, 5′-dimethoxy-4′-hydroxyphenol)-1-O-β- d -(6-O-galloyl)glucose, chestanin, and acutissimin A) were isolated from chestnut bark and their structures elucidated on the basis of spectroscopic analyses. Evidence for strong intermolecular associations of 5-O-galloylhamamelose is presented on the basis of NMR and mass spectroscopic data.