Study On Tannin–Metal Iinteractions in Aqueous Solution Using Spectrophotometric Titration and Micelle-Mediated Separation/Atomic Absorption Spectrometry.

Tannins, including hydrolysable and condensed tannins, are important secondary metabolites of vascular plants and are a major plant-derived carbon source in the environment. Due to the many phenolic hydroxyl groups characteristic of tannins, these compounds have been long been thought  to play significant roles in regulating the bioavailability and environmental cycling of metals in the natural environment. Until now, most studies have used poorly characterized mixtures of phenolics and tannins, such as tannic acid, quebracho, or wattle tannins to demonstrate that tannins chelate metals including iron, aluminum, copper and zinc. Redox active metals such as iron and copper are reduced by tannins, leading to complex reaction pathways involving both chelation and reduction.  Given the complexity of the possible reactions, and the structural diversity of the tannins, we systematically examined chemically defined pure polyphenolic compounds and their reactivity with metals in controlled conditions.  These compounds included representatives of the major groups of tannins and their subunits: pentagalloyl glucose (a simple hydrolysable tannin), procyanidin (a condensed tannin), oenothein (an ellagitannin), epigallocatechin gallate ( a mixed flavonoid ester), methyl gallate, and catechin (subunits of tannins). We employed two complementary techniques, spectrophotometric titration and micelle mediated separation/atomic absorption spectrometry, to evaluate stoichiometry of reaction, formation constants, and tendency to undergo redox reactions.  Results from both methods show metals interact with tannins via the ortho-dihydroxy phenolic groups.  Tannins from different structural families have different numbers and positions of reactive groups, leading to distinct stoichiometries and formation constants.