Relationship between hydrophobicity and structure of hydrolyzable tannins, and association of tannins with crude drug constituents in aqueous solution.

The hydrophobicity values of hydrolyzable tannins were evaluated by measuring the distribution of the compounds between n-octanol and water. Of 8 gallotannins and 13 ellagitannins examined, pentagalloylglucose (7), the major polyphenol of Paeoniae Radix, showed the largest partition coefficient value. In aqueous solution, pentagalloylglucose associated with various crude drug constituents, such as paeoniflorin, glycyrrhizin potassium salt, aconitine trifluoroacetate, liquiritin apioside and amygdalin. The 1H-NMR spectroscopic examination suggested that the association occurred preferentially at the most hydrophobic sites of the molecules. The association with these compounds inhibited the distribution of pentagalloylglucose into the n-octanol phase and adsorption on hide powder. In addition, the water solubility of the biologically active polymeric proanthocyanidins of rhubarb was increased by association with rhein 8-O-glucoside potassium salt, the major anthraquinone glycoside of rhubarb.