It has recently been proposed that high molecular weight polyphenols (PP) may plan an important role in aluminum detoxification in certain aluminum resistant plants such as Eucalyptus. Although interactions between small phenolics and metals are well characterized, less is known about high molecular weight (MW) polyphenol-metal interactions. In this paper, we examined the interaction between purified polyphenols pentagalloyl glucose (PGG), oenothein B (OeB) and methyl gallate (MeG) and Al(III) by using spectrophotometric titration and Job’s method to determine the apparent affinity constants. We did the study at two pH values, pH 4 and pH 6, representing the pH range of acidic, Al-toxic soils. We used molecular modeling (Gaussian) to evaluate structures and spectral features of methyl gallate-Al complexes. We found that PGG (MW 940) forms an Al2PP complex with LogK 11.94±0.09 at pH 6 and an AlPP complex with LogK 2.61±0.70 at pH 4. OeB formed AlPP complexes at both pH values with lower apparent binding constants than those of PGG. The low molecular weight monomer of these hydrolyzable tannins, methyl gallate, formed AlPP complexes at both pH values with lower apparent affinity constants of LogK 5.87±0.02 at pH 6 and 3.69±0.08 at pH 4. We examined the solubility of complexes formed at physiological concentrations of polyphenol and metal, and found that at intermediate metal:polyphenol ratios both OeB and PGG formed insoluble complexes, but when either metal or polyphenol was in excess the complexes remained soluble. MeG did not form insoluble complexes at any metal:polyphenol ratio tested. All three of these gallate-derived polyphenols are effective metal chelating agents. Either OeB or PGG could detoxify Al by forming insoluble complexes that either prevent Al uptake (exclusion) or trap Al in the vacuole (tolerance). We propose that OeB is a more effective Al-detoxifying agent because of its water solubility, compared to the very nonpolar and water insoluble PGG. Potential roles of polyphenols in metal uptake and utilization by plants need to be explored in living systems as well as by chemical analysis.

Polyphenol-aluminum complex formation: Implications for aluminum tolerance in plants

251st National Meeting (2016)

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