In this work, the interaction between fisetin (3,3',4',7-tetrahydroxyflavone) (Fis) and cyclodextrins (CDs) (alpha and beta) 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 alpha-CD. On the other hand, at pH 4.0 and 6.5, the complex Fis-beta-CD is formed in a Fis:beta-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 beta-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 beta-CD.

Aggregation of the following cationic surfactants has been studied in aqueous solution: cetyltrimethylammonium chloride, CMe3ACl; cetyldimethylphenylammonium chloride, CMe2PhACl; cetyldimethylbenzylammonium chloride, CMe2BzACl; cetyldimethyl-2-phenylethylammonium chloride, CMe2PhEtACl; and cetyldimethyl-3-phenylpropylammonium chloride, CMe2PhPrACl. Critical micelle concentrations, cmc's, were obtained from surface tension and conductance measurements; data of the latter were also employed to determine the degree of the surfactant counterion dissociation, R. Static and quasi-elastic light-scattering measurements were employed to obtain micellar weight-average molecular weights, aggregation numbers, Nagg, micellar hydrodynamic radii, Rh, and interfacial area/surfactant headgroup. The latter area was also obtained from surface tension measurements. Finally, time-resolved fluorescence decay measurements with pyrene as probe were employed to obtain Nagg. The structure of micelles of CMe2PhACl is different from that of the other phenyl-group-bearing surfactants because its aromatic ring cannot fold back on the micellar interface. Increasing the number of the methylene segments in the headgroup results in an increase in R and interfacial surface area/headgroup and a decrease in the cmc, Nagg, and Rh. There is good agreement between micellar properties obtained by the different techniques employed.

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