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Abstract

The bark of trees has a big potential as a source of green chemicals. The aim of the present work was to valorise the potential of deciduous tree species with this regard. Three widely spread trees in Europe (grey alder, ash tree, aspen) were in focus as a source of polyphenols, and the yields of polyphenolic compound in the extracts were considered as evaluation criteria. The highest yields of hydrophilic extractives were found in barks of grey alder and aspen (36.8 and 22.9%, respectively). In the former, the highest antioxidant activity was found towards free radicals (DPPH and ABTS•+) and superoxide anion radical. Open chain diarylheptanoids, mainly oregonin, were identified as the major constituents of the grey alder hydrophilic extract. In addition to oregonin, the presence of 2 linear diarylheptanoids [platyphylloside and 1,7-bis-(3,4-dihydroxyphenyl)-heptane-5-O-β-D-glucopyranoside] was confirmed. For the first time, the compounds 1,7-bis-(3,4-dihydroxyphenyl)-3-hydroxyheptane-5-O-β-D-xylopyranoside and 1,7-bis-(3,4-dihydroxyphenyl)-heptane-3-one-5-O-β-D-glucopyranoside were detected in grey alder bark. The results of experiments in vitro and in vivo have shown the high potential for diarylheptanoids-containing extracts in prophylaxis and/or treatment of diseases due to the metabolic disorders and ageing. The biological activity of grey alder extract was confirmed in in vitro experiments by incubation of human blood samples. In vivo experiments with rats also showed positive results. The conclusion is that grey alder extracts have a high potential for prevention of ageing related pathologies. Besides diarylheptanoids, the bark contains condensed tannins in commercially available quantity (12.5%). Eco-friendly wood adhesives were obtained on a tannins basis. The bark left after polyphenols isolation can be used in soil melioration and as a sorbent for the removal of oil products from water surface. The investigation of the phenolic pool of grey alder could contribute to cluster technologies within the biorefinery-based bark processing.

Abstract

The optimum steaming conditions for extraction of condensed tannins from Acacia mearnsii bark were explored, after which characterization of proanthocyanidins from bark steamed under optimum conditions was carried out. The optimum conditions were as follows: steaming time, 30 min; steaming temperature, 140°C; and water volume, 75 ml. The total phenolics content obtained under these conditions amounted to 25.31%, which was much higher than the 21.70% obtained by conventional hot water extraction. Six proanthocyanidin dimers were isolated from the steamed bark. Analysis revealed them to be fisetinidol-(4β-8)-catechin, fisetinidol-(4α-8)-catechin, robinetinidol-(4β-8)-catechin, robinetinidol-(4α-8)-catechin, robinetinidol-(4β-8)-gallocatechin, and robinetinidol-(4α-8)-gallocatechin. Proanthocyanidin polymers were found to consist mainly of prorobinetinidin extender units, similar to those obtained from the same bark by conventional extraction with 70% acetone aqueous solution. However, nuclear magnetic resonance and gel permeation chromatography analyses suggested that during steaming the polymers undergo condensation reactions, as well as inversion of the configuration of interflavanoid bonds.

interflavanoid bonds, first demonstrated by us,23 constitutes a problem in 1H and 13C n.m.r. spectroscopy of the derivatives of some biflavanoid, and most tn— and tetraflavanoid derivatives in terms of spectral complexity. Thus at ambient temperatures the 'H n.m.r. spectra of triflavanoid methyl ether triacetates exhibit both duplication of resonances and line-broadening, rendering analysis impossible.7''22 Exceptionally high temperatures (150°-200°C) are required for merging and sharpening of ab-sorption, in order to overcome the effects of dynamic rotational isomerism. As in

mixture of ortho- and/or purahydroxybenzyl alcohols with resorcinol, phloroglucinol and/or (H-)-catechin as model of reaction of polyphenols with condensed tan- nins. These reactions, in comparison to those of rcsor- Holzforschung /Vol. 47 / 1993 / No. 4 ©Copyright 1993 by Walter de Gruytcr · Berlin · New York 350 M. Fechtal and B. Riedl: Eucalyptus and Acacia Bark Adhesives in Particleboard cinol or (+)-catechin with formaldehyde, proceed very slowly. Recently, other studies (Hemingway and Me Graw 1983; Laks and Hemingway 1987) have shown that the interflavanoid bonds

quantities in tree barks, seed coats and other plant protective tissues. A generalized structure for the 3,3',4',5,7-pentahydroxy (procyanidin-type) Con- densed tannin found in loblolly pine bark is shown äs structure l (Hemingway et al. 1982; Hemingway et al. )H 1983). Two different types of interflavanoid bonds are present, (4ß->8) and 4ß->6), in relative propor- tions of about 3 to l, respectively. In an acetone/water extract these tannins have a number average molecu- lar weight of 2,500, to 3,000 (about 9 flavanoid units) and dispersivity ranging from 2 to 4 (Karchesy

,gh molecular weight pro- anthocyanidins and increasing content of poly- saccharides present in the NaOH extract may result in the higher zero shear viscosity and broad plateau with the low degree of shear sensitivity shown in Figure 1. Figure 1 shows that as the shear stress is increased the Na2SO3 extract shows an immediate shear thinning, which does not occur in the presence of NH3. This relatively low viscosity of Na2So3 extract is consistent with the cleavage of the interflavanoid bonds and the generation of lower molecular weight procyanidin-4- sulphonate derivates as