The specificity of proanthocyanidin-protein interactions

The proanthocyanidins or condensed tannins, phenolic polymers which are synthesized by many plants, characteristically bind and precipitate proteins. The specificity of the interaction was investigated using a competitive binding assay to compare directly the affinities of various proteins and synthetic polymers for the tannin obtained from Sorghum bicolor (Lin.) Moench. At pH 4.9, the relative affinities range over more than 4 orders of magnitude, indicating that this proanthocyanidin interacts quite selectively with protein and protein-like polymers. The affinity for tannins is an inverse function of the size of the polymer, and peptides with less than six residues interact very weakly with tannin. Proteins are precipitated by proanthocyanidins most efficiently at pH values near their isoelectric points. Proline-rich proteins and polymers have very high affinities for tannin. Tightly coiled globular proteins have much lower affinities for tannin than conformationally loose proteins.