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 (K-st) 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 (H-1 NMR). The experimental K-st 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.