Fluorimetric studies of the formation of riboflavin-beta-cyclodextrin inclusion complex: determination of the stability constant by use of a non-linear least-squares model.

The non-linear least-squares model for calculation of the stability constant (Kst) of a drug-cyclodextrin complex has been used in fluorimetry studies. Complexation of riboflavin with beta-cyclodextrin (beta-CyD) was monitored fluorimetrically by measuring changes in the fluorescence intensity of the vitamin in the presence of various amounts of beta-CyD. Formation of an inclusion complex was confirmed in the solid state by differential scanning calorimetry (DSC) and in aqueous solution by proton nuclear magnetic resonance spectroscopy (1H NMR). The experimental Kst value (2112 M-1) derived from the fluorimetry studies appeared to fit well to a 1:1 drug-to-cyclodextrin molar ratio according to the non-linear mathematical model. The model is particularly suitable for fluorescent compounds of which fluorescence intensity is influenced by the presence of cyclodextrins.