The unusually high summer temperatures in France have raised winemakers' hopes for a truly memorable vintage. The polyphenolic tannins, a component of red wines, come primarily in two varieties: the flavan-3-ol polymers (made of catechin and epicatechin monomers) from the grape skins and seeds, and the hydrolyzable tannins (made of ellagic and gallic acids) from the oaken barrels. These compounds are responsible for the astringent taste of red wine and are thought to bind to proline-rich proteins (PRPs) in saliva. Previous attempts to define this interaction have been hampered by the limited solubility of the complexes.
Working in the remarkably true-to-life solvent of water:ethanol (88:12, v/v; pH 3.5), Simon et al. describe the association of three procyanidin B3 (catechin-4a,8-catechin) molecules with a 14-residue fragment of salivary protein IB7. Using circular dichroism, mass spectrometry, NMR, and molecular dynamics calculations, they find that the peptide adopts a type II helical structure typical of proline-rich sequences. Furthermore, the B3 tannins bind via hydrogen bonds with only millimolar affinity to the hydrophilic side of the peptide, reducing its conformational flexibility. The potential range of binding stoichiometries and dynamics in a mixture of tannins and PRPs reminds us how infinitely varied wines can be. -- GJC
Biochemistry 10.1021/bi034354p (2003).
