Tannin interactions with a full‐length human salivary proline‐rich protein display a stronger affinity than with single proline‐rich repeats

The protein IB5 has been purified from human parotid saliva. This protein contains several repeats of a short proline-rich sequence. Dissociation constants have been measured at several discrete binding sites using ¹H-NMR for the hydrolysable tannins (polyphenols) ( , and and the condensed proanthocyanidin (−)-epicatechin. The dissociation constants for trigalloyl glucose and pentagalloyl glucose were 15 × 10⁻⁵ and 1.7 × 10⁻⁵ M, respectively, which are 115 and 1660 times stronger than those previously measured under the same conditions for a single repeat of a mouse salivary proline-rich protein. The increase in affinity is ascribed to intramolecular secondary interactions, which are strengthened by the rigidity of the interacting molecules.