CHARACTERIZATION OF ALUMINUM COMPLEXES IN TEA EXTRACT .1. AL-27 NMR-STUDIES

The high-field Al-27 NMR spectrum of aqueous tea extract contained two narrow resonances at 9.85 and 16.15 ppm. The spectra of aluminium chelates with a number of model ligands (catechol, pyrogallol, catechin, kojic acid, protocatechuic acid, ascorbic acid and salicylic acid) containing chelating moieties present in the constituents of tea showed broad resonances in the range 8-36 ppm. Potentiometric and NMR studies indicate the formation of a monohydroxo 1:1 Al-catechin chelate (delta 8.3 ppm) with highly distorted symmetry. From a comparison of the NMR spectra of tea infusions at different pH values with those of aluminium chelates with malic acid and oxalic acid (minor constituents in tea), the resonance at 16.15 ppm was assigned to the highly symmetric octahedral anionic chelate trisoxalatoaluniinium, [Al(C2O4)3]3-. This was confirmed by anion-exchange separation of the chelate and its identification by Al-27 NMR spectroscopy. The signal at 9.8 ppm could be due to a mixed ligand chelate containing oxalate. In the case of the aluminium-malate system, the resonance at 20 ppm was assigned to a 1:2 chelate in which malate functions as a tridentate resulting in five- and six-membered chelate rings.