Fluorimetric Studies of the Formation of Riboflavin-β-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 β-cyclodextrin (β-CyD) was monitored fluorimetrically by measuring changes in the fluorescence intensity of the vitamin in the presence of various amounts of β-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 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.