Polyphenol–Aluminum Complex Formation: Implications for Aluminum Tolerance in Plants

Abstract

Natural polyphenols may play an important role in aluminum detoxification in some plants. We examined the interaction between Al(3+) 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 (AlL) 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 (β) ∼ 1 × 10(23) at pH 6 and AlL complexes with β ∼ 1 × 10(5) at pH 4. Methyl gallate formed AlL complexes with β = 1 × 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.

Deprotonation Mechanism of Methyl Gallate: UV Spectroscopic and Computational Studies

“…The spectrum of pH titration of MeG is showed in Figure 2 A. As was observed by our previous study [ 12 ], pure MeG solution has a maximum absorbance at 275 nm. Following the NaOH addition, the spectra showed a distinct bathometic shift of the absorbance maximum towards 320 nm, indicating that there was a deprotonation reaction of the phenolic hydroxyl in MeG.…”

“…Solvent effects were modeled with water as the solvent using the polarizable continuum model (PCM) [ 20 ]. The TD-DFT method has gained increasing popularity among chemistry scientists because of its ability to reproduce absorption spectra of both small compounds and its complexes [ 9 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ].…”

Macronutrients and metals released from soils by solutions of naturally occurring phenols

“…Phenolic compounds are also chelating agents, with the phenolic hydroxyl group acting a hard base that preferentially serves as a good ligand for hard acids such as Fe and Al, but a poor ligand for soft or borderline-hard metals such as Mn and Zn (Slabbert, 1992). Some Fe-and Al-phenol complexes have stability constants similar to metal-EDTA complexes (Perron and Brumaghim, 2009;Zhang et al, 2016), consistent to our finding that all the phenols extract Fe and Al more efficiently than water. However, extractability of these metals by the phenols was less than the Mehlich 3 standard, possibly because of the solubility of the chelated metals.…”

“…In our experiment, only gallic acid and methyl gallate extracted more P than the water control. We suggest that the effects of polyphenols on P are mediated by metals such as Al, which is able to form strong complexes with either polyphenols or phosphate ( McBride , ; Zhang et al, ). In the absence of completing phenolic ligands, soil phosphate can be at least partially immobilized in Al complexes, but stronger ligands can exchange with phosphate, increasing P extractability ( Hue et al, ; Ma , ).…”

Tannins and Their Complex Interaction with Different Organic Nitrogen Compounds and Enzymes: Old Paradigms versus Recent Advances

“…According to the definition by Bate‐Smith and Swain, tannins form precipitates with proteins, but also with alkaloids. However, tannins also create complexes with metals, and other compounds, i.e., tannic acid (TA), a common hydrolysable tannin, forms complexes with choline, an amine precursor of acetylcholine; TA also adsorbs to chitosan . It is widely assumed that tannins from the entire pool of organic N compounds precipitate only proteins/peptides .…”