Complexation of metal ions by propyl gallate and procyanidins: A spectroscopic and quantum chemical calculation method

The deprotonation mechanism for the phenolic hydroxyl and the complexing of metal ions with a commonly used food additive, propyl gallate (PG) were studied theoretically and experimentally. The interaction of procyanidins [PC, epicatechin 16 (4→8) catechin], and its basic monomeric unit catechin (CA) with metal ions was studied by the fluorescence quenching spectra. The results showed that the 9-OH quinoid PG was formed at higher pH (10.9) by the oxidization of phenolic hydroxyl. The binding affinities ( K a ) and stoichiometry of these metal ions with PG were determined. The Al 3+ in PG-Al complex [Al(PG)(H 2 O) 2 Cl 2 ] − was coordinated at the 8,9-OH doubly deprotonated catechol site with double chloride ions (Cl − ) and double water molecules (H 2 O). The fluorescence quenching titration with Sn 2+ , Zn 2+ , Cu 2+ , Al 3+ and Fe 3+ revealed that the stoichiometries of metal-bound PC were 1:1, 2:3, 2:3, 2:3 and 1:1, respectively. The presence of bovine serum albumin (BSA) could enhance the complexing strength of PC with metal ions.