Minor constituents of Quebracho tannin extract

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Abstract
Previous work on the general fractionation of Quebracho extract and the method of characterizing the components of the extract is summarized. The minor components of the extract were examined in some detail. The characteristic yellow fluorescence of the extract under UV light is due to the presence of at least 16 substances, of which 12 appear to be glycosyl and galloyl derivatives of the remaining 4. The 4 primary fluorescent substances are 3,7,4[image]-trihydroxyflavone; 3,7,3[image],4[image]-tetrahydroxyflavone (fisetin); 3,7,3[image],5[image]-pentahydroxyflavone (robinetin), and 3,7,3[image]-trihydroxy-4[image]-methoxyflavone (4[image]-methoxyfisetin), recalling the corresponding mono-, di- and tri-hydroxyphenol relationship of the aldehydic oxidation products of lignin or the corresponding sequence in anthocyanins. The possible significance of this in relation to the structure of tannins is commented on. Gallic acid, glucose, xylose, arabinose, fucose, rhamnose, ribose, galactose and 5-hydroxymethyl-furfural are present in the extract in the free condition. Gallic acid and glucose increase in amount on hydrolysis of the extract and its fractions. Ethyl gallate was isolated as an artifact arising during hydrolysis. Anthraquinone was isolated from 1 batch of extract and other samples (and other extracts) show evidence of its presence. The fact that these findings add to the complexity of the problem of determining the composition and the natural function of tannin extracts is emphasized.
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