A comparative study of the catechin components in the barks of wattle species related to Acacia Mernsii

The distribution of flavonoid constituents in the barks of Acacia mearnsii De Wild.(black wattle) , A· decurens Willd. (green wattle), A. dealbata Link.(silver wattle) and A. pycnantha Benth.(golden wattle) has been studied. Bark extracts of the four wattle species have been fractionated into low molecular weight fractions containing mainly catechins and other low molecular weight constituents, and high molecular weight fractions containing the bulk of the polymerized tannins. The low molecular weight fractions have been further fractionated by "preparative paper chromatography". (-)-Robinetinidol, (-)-7:3': 4': 5'-tetrahydroxy flavan- 3-ol, a new naturally occurring catechin, (+)-catechin and (+)-gallocatechin have been isolated from the barks of A. mearnsii, A. dealbata and A. pycnantha. (-)-Epicatechin and (-)-epigallocatechin have been identified in the bark extracts of A. dealbata and A. pycnantha, but appeared to be absent in the barks of A. mearnsii and A. decurrens. (-)-Epicatechin has been isolated fron A. dealbata, and both (-}-epicatechin and (-)-epigallocatechin were isolated from A. pycnantha. (-)-Epicatechin gallate, (-)-epigallocatechin gallate and gallic acid were isolated from A. pycnantha only. These three constituents appeared to be absent in the barks of the three other wattle species. (-)-Epigallocatechin, (-)-epicatechin gallate and (-)-epigallocatechin gallate which were not available for direct comparison, were subsequently isolated from green tea where they are present as major phenolic constituents. A method for the quantitative estimation of polyphenolic substances on two dimensional paper chromatograms has been developed, and a photoelectric densitometer constructed. Two spray reagents, ammoniacal silver nitrate and bisdiazotised benzidine, were found to give straight line relationships of instrument deflection against log concentration for flavonoid substances. This estimation method for the first time supplied means for a detailed study of the concentration of catechin constituents in the bark extracts of A. mearnsii, A. decurrens, A. dealbata, A. pycnantha and of A. mearnsii x A. decurrens hybrids. The concentration of catechin constituents has been shown to vary considerably between species whereas variation within species was small. In the latter respect silver wattle is an exception. Taxonomic significance may possibly be attached to the distribution of catechin constituents in the bark of the four Acacia species. The concentration of (-)-robinetinidol, which appears to be the characteristic compound of these Acacias, progressively decreases in the sequence black-, black x green hybrid, green-, silver- and golden wattle, while the number of catechin constituents of the "phloroglucinol series" increases in the same sequence. It thus appears, that by the examination of their bark components, a differentiation between species of a subgenera may be possible. Two tannins, constituents D and B, which are related to the leuco-anthocyanidins (flavan-3:4-diols) have been found in the barks of the four wattle species. One of the two, constituent D, was isolated in a pure form from the barks of A. mearnsii and A. pycnantha. Constituent D was found to generate robinetinidin and an orange pigment, the structure of which has not yet been fully identified. Compound D and its acetyl- and methoxyl derivatives did not crystallize. From the results of alkaline-, acidic- and enzymatic degradations, colour reactions and light-absorption studies, combustion analysis of the compound and its derivatives and molecular weight estimations, constituent D is surmised to be a dimer of 7:3': 4': 5'-tetrahydroxyflavan-3:4-diol (leuco-robinetinidin), The isolation of this complex leuco-anthocyanidin tannin represents the first isolation of a flavonoid tannin from commercial vegetable tannin sources. The second tannin obtained from the bark of A.mearnsii, "constituent B" appears to consist of two overlapping substances, which have not yet been separated. The tannin (B) was found to have an average molecular weight of 676 and it is considered likely that both substances may be dimolecular. On heating with mineral acid robinetinidin, fisetinidin and an orange pigment are generated, the pigment being identical with the pigment generated from constituent D. It may therefore be assumed that "Constituent B" consists of a mixture of complex leuco-robinetinidins and leucofisetinidins. The distribution of complex leuco-anthocyanidins in the bark extracts of Acacia mearnsii, A. decurrens, A. dealbata and A. pycnantha has been examined. A correlation between the distribution of leuco-anthocyanidins in the bark of the four wattle species, and accepted systematics, does not, apparently, exist