Fluorescence enhancement of the aflatoxin B1 by forming inclusion complexes with some cyclodextrins and molecular modeling study

The interaction between the aflatoxin B₁ (AFB₁) and three cyclodextrins, α-cyclodextrin (α-CD), β-cyclodextrin (β-CD) and heptakis-2,6-dimethyl-o-β-cyclodextrin (ome-CD), was studied by spectrofluorescence technique. It was found that the inclusion association behavior occurs for the complexes of cyclodextrins with AFB₁. The fluorescence of AFB₁ is generally enhanced in the complexes with cyclodextrins in aqueous solutions. The inclusion complex constants of the three types of cyclodextrins at different temperatures were evaluated from Benesi-Hildebrand plot and also by non-linear regression analysis. These cyclodextrins can only form the 1:1 (host:guest) inclusion complex in the studied temperature range of 20-50 °C. The enthalpy (ΔH°) and entropy (ΔS°) changes of complexation were extracted from the temperature dependency of complex formation constants (K). Temperature-dependent measurements showed that the association step is controlled by enthalpy-entropy compensation effect. The use of ome-CD generally resulted in the greatest fluorescence intensity. On the other hand, the discrepancy between the exhibited enhanced fluorescence and thermodynamic parameters (ΔG°) is proposed to be different only by the orientation of the AFB₁ within the cyclodextrin cavity. To find the most favorable structure, the geometry of complex was investigated by molecular modeling approach employing the semiemperical HF-SCF calculations.