Complexes of aluminium(III) with isoquercitrin: spectroscopic characterization and quantum chemical calculations

The composition and conformation of complexes of aluminium(III) with isoquercitrin (Iso) have been studied in methanol solution. This molecule presents two potential chelating sites in competition. UV-Vis spectroscopy provides evidence of three different species in neutral solution: Al(Iso)(2+) Al(Iso)(2)(+) and Al-2(Iso)(3+). This last one is formed only if a great quantity of Al(III) is presented in the medium. The first site involved in complexation is the 5-hydroxy-4-keto group. FT-Raman spectroscopy has allowed to confirm this mechanism. The stability constants of this complexes have been determined using the SPECTFIT program. In acidic condition, only the first complex is obtained. In alkaline medium, Al(Iso)(+) and Al(ISO)(2)(-) complexes are formed, the catechol group is then the chelating site involved in these species To propose a molecular conformation for the free isoquercitrin and its complexes in neutral medium, both semiempirical molecular orbital and DFT calculations have been performed. It has been showed that the participation of cynnamoyl and benzoyle mesomeric forms stabilise the structure of Al(Iso)(2+). The structural models have been validated by the good agreement between theoretical and experimental electronic spectra. (C) 2002 Elsevier Science Ltd. All rights reserved.