Mechanisms of protein precipitation for two tannins, pentagalloyl glucose and epicatechin16 (4→8) catechin (procyanidin)

The precipitates that form when purified pentagalloylglucose or a purified procyanidin [epicatechin(16) (4-->8) catechin; EC16-C] are mixed with bovine serum albumin were quantitatively analyzed. EC16-C isa more efficient protein precipitating agent than pentagalloylglucose on a molar or a mass basis. EC16-C precipitates protein independently of temperature and presence of organic solvent. Precipitation by pentagalloylglucose increases as temperature is increased and decreases when alcohols are present. When tannin is in excess, up to 40 mol of pentagalloylglucose is bound per male of protein precipitated, but only 20 mol of EC16-C is bound per mole of protein precipitated. The data support different models of precipitation for the two types of tannin: pentagalloylglucose, which is very nonpolar, precipitates by forming a hydrophobic coat around the protein, whereas the much more polar EC16-C forms hydrogen-bonded cross-limbs between protein molecules.