|ZHANG, LIANGLIANG - Institute Of Chemical Industry Of Forest Products (ICIFP)|
|LIU, RUIQIANG - The Ohio State University|
|GUNG, BENJAMIN - Miami University - Ohio|
|TINDALL, STEVEN - Miami University - Ohio|
|HAGERMAN, ANN - Miami University - Ohio|
Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/29/2016
Publication Date: 3/29/2016
Publication URL: https://handle.nal.usda.gov/10113/5231774
Citation: Zhang, L., Liu, R., Gung, B.W., Tindall, S., Gonzalez, J.M., Halvorson, J.J., Hagerman, A. 2016. Polyphenol-aluminum complex formation: Implications for aluminum tolerance in plants. Journal of Agricultural and Food Chemistry. 64:3025-3003. doi:10.1021/acs.jafc.6b00331.
Interpretive Summary: Although interactions between metals and small phenolics, produced by plants, are well characterized, less is known about interactions between metals and more complex polyphenols. However, large polyphenols may play an important role in aluminum detoxification in some aluminum resistant plants by forming insoluble complexes that either prevent its uptake (exclusion) or by trapping it in the cell wall or in the vacuole (tolerance). We used several techniques to examine some of the characteristics and solubility of complexes formed between aluminum (Al3+) and two comparatively large polyphenols (PGG and OeB), and the related small phenolic (MeG). We did the study at pH 4 and pH 6, representing the pH range of acidic, aluminum-rich soils and the plant cell vacuole, respectively. In the presence of excess Al3+, PGG and OeB formed similar but soluble complexes at both pH values but MeG did not complex aluminum at the lower pH. At intermediate metal-to-polyphenol ratios, more characteristic of physiological concentrations, the complexes formed with PGG and OeB were insoluble complexes but MeG did not form insoluble complexes at any metal:polyphenol ratio tested. Because each of these polyphenols can bind effectively to aluminum, they may contribute to future efforts to engineer aluminum-resistant plants or to produce effective soil amendments to neutralize aluminum toxicity.
Technical Abstract: Natural polyphenols may play an important role in aluminum detoxification in some plants. We examined the interaction between Al3+ and the purified high molecular weight polyphenols pentagalloyl glucose (940 Da) and oenothein B (1568 Da), and the related compound methyl gallate (184 Da) at pH 4 and 6. We used spectrophotometric titration and chemometric modeling to determine stability constants and stoichiometries for the aluminum-phenol (Al-L) complexes. The structures and spectral features of aluminum methyl gallate complexes were evaluated with quantum chemical calculations. The high molecular weight polyphenols formed Al3L2 complexes with conditional stability constants (ß) ~ 1 x 10^23 at pH 6 and AlL complexes with ß ~ 1 x 10^5 at pH 4. Methyl gallate formed AlL complexes with ß = 1 x 10^6 at pH 6 but did not complex aluminum at pH 4. At intermediate metal-to-polyphenol ratios, high molecular weight polyphenols formed insoluble Al-complexes but methyl gallate complexes were soluble. The high molecular weight polyphenols have high affinities and solubility features that are favorable for a role in aluminum detoxification in the environment.