UV and NMR studies of tannin complexation

Vegetable tannins or plant polyphenols constitute a distinctive and large family of higher plants. Tannins have affinity for many biological molecules, of which proteins and peptides are probably the most important ones. Due to their physiological and industrial importance, tannins have attracted increasing scientific attention. The principal goal of the work described in this thesis was to have a better understanding of tannin complexation. The secondary aim was to investigate medicinal implications of a few tannins. A number of tannins were isolated, purified and characterised by mainly spectroscopic means. A model condensed tannin (procyanidin B-2) was targetted for multi-dimensional 1 H & 13C NMR studies coupled with molecular mechanics calculations. The complexation of various tannins with proteins and caffeine was studied by UV and 1H NMR spectroscopy. Effect of solvent composition on caffeine-tannin complexation was examined. Caffeine was used as a model for proline as well as being a physiologically significant compound. A complementary caffeine-calixarene complexation was also studied. Association constants were obtained from chemical shift changes for the caffeine-tannin/calixarene interactions . The complexation of tannins with the physiologically significant hormone peptide bradykinin was investigated by 1H NMR spectroscopy. Tannins were found to effectively complex with bradykinin and ultimately precipitate from aqueous media. The peptide was found to remain extended or unstructured during the complexation with tannins. Association constants for the interactions were obtained from chemical shift changes. Hydrophobic interactions have been confirmed to be the principal binding force in complexation and precipitation of substrates by tannin; hydrogen bonding probably acts as secondary stabilisation factor.