Investigation of solvent effect and cyclodextrins on fluorescence properties of ochratoxin A

Fluorescence properties of ochratoxin A (OTA) solutions depend on the pH, solvent polarity and can be influenced by the presence of cyclodextrins (CDs). In this work, the effect of b-cyclodextrin (b-CD) and heptakis-2,6-dimethyl-o-b-cyclodextrin (ome-CD), on fluorescence properties of OTA in aqueous solutions has been investigated by means of steady-state fluorescence at different pHs (range 2–10). Binding constants of OTA/CDs inclusion complexes have been determined by applying by non-linear regression analysis. A 1:1 stoichiometry of OTA/CDs complexes has been observed at all tested pHs. The use of ome-CD generally resulted in the greatest fluorescence intensity. The effects of solvent and pH on the positions of λmax (excitation) and λmax (emission) of OTA was determined. Correlations between the excitation and emission wavelength of OTA (monoanion and dianionic forms) and the solvent parameters were analysed with Lippert–Mataga plots. Results show that the peak position is affected mainly by specific and non-specific types of interactions between the solvent and solute. The fluorescence quenching of OTA by chloroform (aprotic) and water (protic) were studied in methanol as solvent at room temperature. The quenching was found to be appreciable and a non-linear curve with downward curvature was obtained in the Stern–Volmer (SV) plot for the water in the concentration range studied. The quenching efficiency is related to hydrogen bond-donating capacity of the quencher molecule. It was inferred that non-linearity can be attributed to fractional accessibility of fluorophore to quencher. The quenching constant was calculated from the modified SV equation.