Ellagitannins as Al(III) chelators: Determined binding sites with a predictive model

Plant-toxic Al(III) ions released in acidic soils pose a global problem, affecting a substantial portion of agricultural land. One plant defense mechanism against Al(III) ions, which is poorly understood, is the release of organic acids or polyphenols with chelating properties. These compounds include ellagitannins (vescalin, castalin, vescalagin, castalagin, roburin A, and roburin D), whose chelating ability against Al(III) ions was investigated in this study. The formation of coordination compounds was monitored by Job's method at pH 3.5-5.5. NMR and CD spectroscopy, combined with DFT calculations, were used to determine the binding positions of Al(III) ions on these high-molecular-weight polyphenols. Using information about binding sites and the microscopic protolytic equilibrium of ellagitannins, a model was developed to describe and predict the chelating ability of Al(III) ions by the nonahydroxytriphenoyl and hexahydroxydiphenoyl groups of ellagitannins. This model was then fitted to the experimental Job plots. The obtained stoichiometries and apparent formation constants can be used to predict the Al(III) chelating ability of other ellagitannins with identical structural elements. The developed model can therefore serve as a predictive tool for the removal of Al(III) ions by ellagitannins at various pH values.