Insights into the complexation behavior of polyphenol functional groups with Al3+: NMR and DFT analysis of the Al–protocatechuic acid system

To understand the complexation behavior of aluminum ion (Al(H 2 O) 6 3+ , Al 3+ ) with polyphenols in acidic environments, complexation between Al 3+ and protocatechuic acid (3,4-dihydroxybenzoic acid, C 7 H 6 O 4 , PCA) at pH 3 was investigated using nuclear magnetic resonance (NMR) and density functional theory (DFT) techniques. PCA has a carboxyl group in addition to the catechol moiety and can form a chelate structure with Al 3+ at the catechol moiety (bidentate complex) or a monodentate complex structure with the carboxyl group. Our experimental results ( 27 Al and 1 H NMR measurements) showed that both complexes coexisted in solution and that the main chemical species was a complex with a chelate structure. DFT calculations accurately represented multiple pieces of information, including the complex structure and the Al 3+ electron density changes. Based on the results of 27 Al NMR measurement, the conditional formation constant ( K ) of the bidentate complex was estimated as 0.02. The formation constant of the bidentate complex was larger than that of the gallic acid (3,4,5-trihydroxybenzoic acid, C 7 H 6 O 5 ) system used in our previous study, suggesting that the number and binding positions of functional groups other than the catechol moiety play important roles in complex formation with Al 3+ .