Activity of polyphenolic constituents of leaves of Eucalyptus and other species in complexing and dissolving iron oxide

Abstract

The polyphenolic constituents of aqueous extracts from the leaves of 11 Eucalyptus species and 7 other Western Australian tree and shrub species are examined in relation to their capacity to complex iron and dissolve iron oxides. The leaves of the Eucalyptus species contain mainly gallic acid, ellagic acid, hydrolysable tannins, leucoanthocyanins, and flavonol glycosides. Gallic acid forms soluble complexes with ferrous and ferric iron which are relatively less stable than ferrous aa'-dipyridyl. Tannic acid forms a soluble ferrous complex which is more stable than ferrous aa'-dipyridyl. The complexes formed on addition of ferric iron to the hydrolysable tannins, leucoanthocyanins, and flavonol glycosides are apparently insoluble, but can be solubilized by reduction to ferrous iron in the presence of sufficiently strong reducing agents. Ferrous complexes with flavonol glycosides and leucoanthocyanins are less stable than ferrous aa'-dipyridyl. No simple correlation could be established between the total quantity of polyphenols and iron oxide solubilized under aerobic conditions. This may be due to the contrasting effects of solution and precipitation of iron-polyphenol complexes which are shown by different classes of polyphenols and to the appreciable capacity shown by substances other than polyphenols to dissolve iron oxides.