| The condensed tannins or proanthocyanidins are phenolic polymers which are synthesized by the plant kingdom. They can characteristically bind with proteins, especially proline-rich proteins like salivary proteins, and also even with some synthetic polymers like poly(vinyl pyrrolidone). Understanding the complex formation of the condensed tannins with proteins and synthetic polymers is of interest because the biological functions and many of the commercial applications are due to the ability of the condensed tannins to form complexes with other polymers. Many studies have evaluated the photophysical and conformational properties of procyanidin and profisetinidin, two classes of the condensed tannins.;Proline, one of amino acids commonly found in protein, has an aliphatic side chain, but it differs from most other amino acids in that its side chain is bonded to both the nitrogen and $alpha$-carbon atoms. Therefore, poly(L-proline) has two different solid state structures, Form I and II, depending on the solvent used for preparation of the sample. Since Form I and Form II are helices of opposite handedness, the transition between the two different helical conformations can be measured by CD (circular dichroism) spectroscopy.;The specificity of the interaction between the condensed tannins and poly(L-proline) was investigated using the UV absorption spectroscopy, steady-state fluorescence spectroscopy, CD spectroscopy and computer modeling. The complex formation and collision between the condensed tannins and poly(L-proline) Form II reduced the fluorescence intensity of condensed tannins. But the fluorescence intensity was enhanced upon complexation with poly(vinyl pyrrolidone) and poly(L-proline) Form I. Viscosity data also showed that poly(vinyl pyrrolidone) was collapsed by the condensed tannins but poly(L-proline) Form II was not. This strange behavior was rationalized by using RIS and Monte Carlo simulation.;With these experimental results and topological modelings, the new binding mechanism was proposed, which is that the hydrogen bondings between the hydroxyl groups of the condensed tannins and carbonyl oxygens of poly(L-proline) or poly(vinyl pyrrolidone) induce additional hydrophobic interactions. Also, the difference in binding behavior between poly(L-proline) and poly($gamma$-hydroxy-L-proline), which is that poly(L-proline) is preferred to poly($gamma$-hydroxy-L-proline) for binding with the condensed tannins, was explained based on the new binding mechanism. |
