Polyphenols (tannins) are known for their high propensity to precipitate proteins. They bind most strongly to proteins with a high proline content. Understanding the mechanism of this association is of prime interest because this interaction might induce protein conformational changes that may modify their biological activity. To investigate the interaction, an NMR study was carried out on the binding of a representative polyphenol, penta-O-galloyl-D-glucopyranose, to a nonapeptide hormone, bradykinin (BDK), where proline accounts for 30% of residues. Series of 1D and 2D-NMR experiments were performed. For the first time, a three-dimensional structure of complexes was determined using 2D-NMR experiments and molecular modeling. These structure calculations are a potent tool to understand how the association arises. They clearly show that the interaction is a complex phenomenon where several parameters are involved. The PGG/BDK complexes are formed by multiple weak interactions between peptide side chains and galloyl rings. Proline and arginine are good anchoring points and the glycine gives a certain flexibility in the peptide backbone that allows the polyphenol to approach and interact. Therefore, it is not only the hydrophobic stackings between galloyl rings and proline and hydrogen bonding involving arginine and aromatic rings which are important. The residue sequence and the side chain steric bulk also intervene.