A spectroscopic and quantum chemical calculation method for the characterisation of metal ions complexed with propyl gallate and procyanidins

Abstract 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, epicatechin16 (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 Al3+ in PG-Al complex [Al(PG)(H2O)2Cl2]- was coordinated at the 8,9-OH doubly deprotonated catechol site with double chloride ions (Cl-) and double water molecules (H2O). The fluorescence quenching titration with Sn2+, Zn2+, Cu2+, Al3+ and Fe3+ 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.