Polyphenol/peptide binding and precipitation.

Polyphenols are largely responsible for the astringency and "mouthfeel" of tea and wine by their interactions with basic salivary proline-rich proteins. Astringency arises from precipitation of polyphenol/peptide complexes, which is an important protective mechanism in animals that consume polyphenols. This paper presents biophysical studies of the interactions between chemically defined polyphenols and peptides. It is shown that intermolecular binding is dominated by stacking of polyphenolic rings onto planar hydrophobic surfaces and is strengthened by multiple cooperative binding of polyphenolic rings. Affinities weaken at higher temperatures and are unaffected by pH between pH 3.8 and 6.0. Measurements of self-diffusion rates for peptides with increasing concentrations of polyphenol demonstrate that peptides become increasingly coated with polyphenol. When the coating is sufficiently extensive to provide cooperative polyphenol bridges, the peptide dimerizes and precipitates. Light scattering measurements and electron microscopy indicate that the insoluble particles fall into two discrete size classes of ca. 80 and 500 nm diameter. The larger particles are favored at higher temperature and pH, suggesting that the particles are in a colloidal state, with the smaller particles being stabilized by charge repulsion between particles, and that precipitation of the complexes may be a phase separation process.