Comparison of the chelating power of hydroxyflavones J.P. Cornard * , J.C. Merlin Lab. de Spect. Infra. & Raman, CNRS UMR 8516, Universite´ des Sciences et Technologies de Lille, Baˆt C5-59655, Villeneuve d’Ascq cedex, France Received 2 September 2002; accepted 23 September 2002 Abstract A series of hydroxyflavones, widely occurring in nature, have been used as model compounds for the study of the behaviours of humic substances towards aluminium ion complexation. The chelating power of 3-hydroxy-4-keto, 5-hydroxy-4-keto and catechol functional groups have been compared in methanol solution, first when these binding sites are isolated, and then when they are in competition on the a same ligand. Electronic and vibrational spectroscopy investigations were performed to characterize the Al(III) complexes: stoichiometry, stability constants and involved groups in multi-site ligands. Quantum chemical calculations have been carried out in order to know the structural modifications generated to the ligand by complexation. The influence of the medium, acidic, neutral and alkaline, has been studied. The 3-hydroxy-4-keto group has the strongest chelating power in acidic and neutral media whereas catechol presents the greatest ability to chelate Al(III) in alkaline solution. This investigation has shown that capacity of different studied functions to chelate Al(III) is the same when they are in competition and when they are individually taken. q 2003 Elsevier Science B.V. All rights reserved. Keywords: Hydroxyflavones; Al(III) complexation; Stability constants; UV – Visible spectroscopy; FT-Raman; Quantum chemical calculations 1. Introduction Natural organic matter (NOM) is a heterogeneous mixture of compounds that does not have a uniform structural formula and that presents various potential metal binding sites. Humic substances that represent the main part of NOM are polyfunctional. These macromolecules may contain a large number of different complexing sites in competition, e.g. car- boxylic, carbonyl, polyphenolic or amine functional groups [1]. The study of soil organic matter–metal ions interactions is of considerable interest in soil science chemistry, and two general approaches could be envisaged to the understanding of these inter- actions. One approach consists to study the inter- actions in term of general chemical and physical properties of the complexes formed with global NOM, but this kind of studies reveals little about the structure of metal-binding sites, because of the polyfunctionality of the macromolecule complicates the determination of its chemical reactivity. An other approach lies in the study of complexing properties of model molecules representative of small identifiable fractions of NOM. Previous studies have shown that polyphenols and notably flavonoids are nonnegligible components of NOM [2–5]. 0022-2860/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved. PII: S0022-2860(02)00655-5 Journal of Molecular Structure 651–653 (2003) 381–387 www.elsevier.com/locate/molstruc * Corresponding author. Tel.: þ33-3-20-43-69-26; fax: þ 33-3- 20-43-67-55. E-mail address: cornard@univ-lille1.fr (J.P. Cornard).