Heartwoods of 22 African tropical wood species were extracted with methanol and the contents of total phenolic compounds in these extracts were measured. Three bioassays were conducted to evaluate the antioxidant activity, tyrosinase inhibitory activity, and antifungal activity of the methanol extracts. The results indicated that the extracts from 13 species exhibited high antioxidant potential, and their inhibitory concentrations that caused 50% scavenging of the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical (IC 50 ) were less than 10 μg/ml. The crude extract from Cylicodiscus gabunensis showed the highest antioxidant activity, and was even higher than that of (+)-catechin, which is known as a potent antioxidant. There was a good correlation between the antioxidant activity and the content of total phenolic compounds, indicating that phenolic compounds played a predominant role in the antioxidant property of the wood extracts. Among all 22 species, only 1 species, Milicia excelsa, contained extractives that showed very high tyrosinase inhibitory activity. The extracts from 9 species showed high antifungal activity. No consistent relationship was observed between the tyrosinase inhibitory activity or antifungal activity and the content of total phenolics in the extracts.
The reaction behavior of condensed tannins and related polyphenols with ammonia (NH3) water was examined. NH3 water treatment of pyrogallol, gallic acid, and epigallocatechin, which have a pyrogallol nucleus, caused regioselective amination in the presence of molecular oxygen (O2) to form 2-aminoresorcinol, 4-amino-3,5-dihydroxybenzoic acid and 4′-C-amino-epigallocatechin, respectively. In contrast, other polyphenols, such as catechol, protocatechuic acid, and epicatechin, which have a catechol nucleus, did not undergo amination even in the presence of O2. Under nitrogen atmosphere (in the absence of O2), epimerization at the C-2 position took place during NH3 treatment of epigallocatechin, in addition to amination. The same epimerization took place during NH3 treatment of epicatechin regardless of the presence or absence of O2. By NH3 treatment, pyrogallol-type B-rings of condensed tannin from Acacia mearnsii underwent regioselective amination to yield 4′-amino-3′,5′-dihydroxybenzene-type B-rings in the presence of O2, whereas catechol-type B-rings of condensed tannin from quebracho showed little amination regardless of the presence or absence of O2. From these results, it was concluded that amination of pyrogallol nucleus by NH3 treatment should proceed via oxidation and reduction, and therefore this reaction would be attributed to the oxidation property of condensed tannins and related polyphenols.
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.
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