Spectral analysis of Al (III) - Quercetin complexes

Complex formation between aluminum and quercetin(Q) in methanol was studied by the combined use of ultriviolet-visible, infrared and electrospray ionization tandem mass spectroscopy. Quercetin presents in its structure three possible chelating sites in competition. In acidic and neutral medium, the complexs formation and stoichiometry and the preferentially occupied chelating sites have been investigated. Quercetin molecule forms a complex of stoichiometry Al(III) : Q = 1:1 in acidic medium and the chelating site involved in the complex formation is the 3-hydroxy-4-carbonyl, the aluminum central atom formed a five-membered chelate ring and was four-coordinated. Whereas in the neutral medium,the stoichiometry ratio of successive formation of two complexes (Al (III) : Q) was 1:2 and 2:1, respectively. The first site involved the complex fori-nation process is the 3-hydroxy4-carbonyl and the second one is the ortho-dihydroxyl group. The structure of the first complex is aluminum central atom chelated with two quercetin molecules and formed between the two five-membered chelate rings, its coordination number is four. However, the structure of the second complex is a quercetin molecule chelated with two aluminum atoms and formed, two different five-membered chelated rings in the above-mentioned two cheating sites, the coordination number of the two complexs is also four.