ARTICLES Study of the Complexation of Fisetin with Cyclodextrins Mariana R. Guzzo, ² Miriam Uemi, ‡ Paulo M. Donate, ² Sofia Nikolaou, § Antonio Eduardo H. Machado, | and Laura T. Okano* ,² Departamento de Quı ´mica, Faculdade de Filosofia, Cie ˆ ncias e Letras de Ribeira ˜ o Preto, UniVersidade de Sa ˜ o Paulo, 14040-901, Ribeira ˜ o Preto, SP, Brazil, Instituto de Quı ´mica, UniVersidade de Sa ˜ o Paulo, C. P. 26077, 05513-970, Sa ˜ o Paulo, SP, Brazil, Faculdade de Cie ˆ ncias Farmace ˆ uticas de Ribeira ˜ o Preto, UniVersidade de Sa ˜ o Paulo, 14040-903, Ribeira ˜ o Preto, SP, Brazil, and Instituto de Quı ´mica, UniVersidade Federal de Uberla ˆ ndia, C. P.593, 38408-100, Uberla ˆ ndia, MG, Brazil ReceiVed: March 2, 2006; In Final Form: July 12, 2006 In this work, the interaction between fisetin (3,3′,4′,7-tetrahydroxyflavone) (Fis) and cyclodextrins (CDs) (R and ) was studied through UV-vis absorption, steady-state fluorescence, induced circular dichroism, and 1 H NMR experiments with dependence on temperature and pH. Some experimental data were compared with quantum-mechanics studies based on the SAM1 (AMPAC) semiempirical model, as well as with the B3LYP and MPW1PW91 functional models from density functional theory using the 6-311G* and 3-21G* basis sets. The spectroscopic measurements show that Fis does not form stable complexes with R-CD. On the other hand, at pH 4.0 and 6.5, the complex Fis--CD is formed in a Fis:-CD 1:1 stoichiometry and an equilibrium constant (K) of 900 ( 100 M -1 . In basic medium (pH 11.5), K decreases to 240 ( 90 M -1 because Fis deprotonation leads to its better solubilization in water. Molecular modeling points out that Fis is not totally inserted into the inner cavity of -CD. The formation of the inclusion complex renders an environment that enhances intramolecular excited state proton transfer. The inclusion complex is formed preferentially via entry of the Fis phenyl group into -CD. 1. Introduction Fisetin (3,3′,4′,7-tetrahydroxyflavone) (Fis) has very interest- ing spectroscopic and pharmacological properties. For instance, it exhibits a dual fluorescence behavior that is very dependent on solvent polarity, 1,2 which enables its use as a laser dye. 3 Besides that, like other flavones and related compounds, Fis has antioxidant properties and a great biological potential for therapeutic applications. Although some published papers have reported on the interaction between Fis and some drug delivery systems, 2,4,5 there are no papers describing its complexation with cyclodextrins (CDs). The fluorescence behavior of Fis in media of different polarities has been compared with that of 3-hydroxyflavone (3- HF). 1 Both molecules present a higher dipole moment in the singlet excited state than in their ground state. In addition, in the singlet excited state, 3-HF and Fis can undergo an intramo- lecular excited state proton transfer (IESPT) from the hydroxyl group in position 3 to the carbonyl oxygen, 6 thus forming a tautomer (Scheme 1). The presence of the latter justifies the dual fluorescence emission spectra of these compounds. The structures of cyclodextrins (CDs), cyclic oligosaccharides produced from starch and cellulose, consist of an external hydrophilic surface and a hydrophobic cavity. The widely known R-, -, and γ-CDs are crystalline, homogeneous, and nonhy- groscopic, and they contain macrocycles of six, seven, and eight glucopyranose units, respectively. 7 They easily form inclusion complexes with a wide variety of nonpolar molecules, which greatly enhance their solubility in water, thus favoring their industrial application. When the guest is a drug, the formation of inclusion complexes improves drug bioavailability and stability. In pharmaceutical formulations, the toxic effects of the drugs are decreased and they are more easily dispensed from the body. 8 In the present work, the Fis complexation with CD was evaluated by UV-vis absorption, steady-state fluorescence, induced circular dichroism, and 1 H NMR measurements. The experimental data show that Fis is able to complex with -CD, but not with R-CD. The nature of the Fis--CD complexes is dependent on pH and on the diameter of the CD inner cavity. The equilibrium constant (K) and the thermodynamic parameters * Corresponding author. Phone: 55-16-3602-3757. Fax: 55-16-3602- 4838. E-mail: LTOkano@ffclrp.usp.br. ² Departamento de Quı ´mica, FFCLRP, USP. ‡ Instituto de Quı ´mica, USP. § Faculdade de Cie ˆncias Farmace ˆuticas de Ribeira ˜o Preto, USP. | Instituto de Quı ´mica, Universidade Federal de Uberla ˆndia. SCHEME 1: Intramolecular Excited State Proton Transfer Producing Two Fluorescent Species 6 10545 J. Phys. Chem. A 2006, 110, 10545-10551 10.1021/jp0613337 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/22/2006