Oak-bark tannins

Abstract

(+)-Catechin and (+)-gallocatechin, and leucodelphinidin, were respectively isolated from the ether and ethyl acetate-soluble fractions of oak-bark phlobatannins. Leucodelphinidin, C15H14O8.H2O, has been characterized by a hepta-acetate derivative. (+)-Catechin accounts for 1% of the dry weight of young bark, (+)-gallocatechin for up to 1% and leucodelphinidin for 0.5%. Oak-bark phlobatannin, C15 H10O7.2H2O, was isolated in considerable yield by cellulose-column chromatography, and exhibits an absorption band at 285 m[mu] and diminishing general absorption in the region 300-600 m[mu]. Oak cambium exhibits polyphenoloxidase activity which is inhibited by 4-nitrocatechol. (+)-Gallocatechin and leucodelphinidin oxidation polymers exhibit the same absorption spectrum, have the same analytical properties and similar elementary analyses as those of oak-bark phlobatannin''. Aerobic oxidation of mixed synthetic substrates, such as 5-methylpyrogallol and 5-methoxy-4-methylresorcinol, or 5-methylpyrogallol and phloroglucinol, by polyphenoloxidase gave polymers which showed the characteristic absorption of (+)-gallocatechin-oxidation polymer, oak-bark phlobatannin, and 5-methylpyrogallol-oxidation polymer. The conclusion is drawn that phlobatannin is formed by the aerobic oxidation in the cambium of principally (+)-gallocatechin through a tail-to-tail quinone-polymerization mechanism. The increase in tannin concentration of oak stembark from crown to butt suggests a downward movement of the phenolic metabolites from the leaves through the phloem.