Chemistry of Tannins and Related Phenolics.

Up to 40% of the dry weight of plant tissues (leaves, bark, seeds, wood) is comprised of  polyphenolics, or tannins.  These natural products enter soil and water systems by various routes including rain throughfall, litter decomposition, and root exudation.  The functional definitions of tannins as secondary products that form protein precipitates, serve as metal ligands, and reduce redox active species have perpetuated a perspective that overlooks chemical detail.  The broad term “tannin” encompasses a range of compounds from the relatively low molecular weight flavonoid esters such as epigallocatechin gallate (Camellia spp.) to the high molecular weight tetrameric ellagitannins such as lambertianin D (Rubus spp ).  Even the familiar chemical subclasses of tannins (“condensed” and “hydrolysable”) are inadequate to delineate the chemical complexity of this suite of natural products.  The fundamental structural chemistry of tannins and the constituent monomeric phenolics will be reviewed.  Tannin reactivity will be discussed in the context of phenolic chemistry, and methods for analysis of phenolics based on both chemical reactivity and biological activity will be compared.  Examples will be drawn from the ecological and soil science literature to illustrate how studies of individual polyphenolic compounds reveal ecological activities and significance in ways that are impossible with studies of poorly defined mixtures of “tannins”.