Spectroscopic and theoretical investigation of the solvent effects on Al(III)-hydroxyflavone complexes

In methanol/water medium at pH 6, the chelation of Al(III) by three mono-site ligands: 3-hydroxyflavone, 5-hydroxyflavone and 3'4'-dihydroxyflavone has been studied by electronic absorption spectroscopy. A comparison of the results obtained for the three chelating sites shows that the alpha-hydroxy-carbonyl group presents the greatest affinity for Al(III). When the three sites are in competition within a single compound: the quercetin (Q) molecule, this site remains the preferential site for fixing the metal cation. Indeed, the combined use of electronic spectroscopy and TD-DFT calculations has allowed highlighting the formation of the species [Al(H2O)(OH)Q(2)](0) involving chelation with the alpha-hydroxy-carbonyl site. Comparisons with an Al(III) complexation experiment carried out in methanol solution show that whatever the ligand, the presence of water molecules in the medium decreases the amount of complex formed. (C) 2008 Elsevier Ltd. All rights reserved.