2ND-DERIVATIVE SPECTROPHOTOMETRIC DETERMINATION OF THE BINDING CONSTANT BETWEEN CHLORPROMAZINE AND BETA-CYCLODEXTRIN IN AQUEOUS-SOLUTIONS

Second-derivative spectrophotometry was applied to determine the binding constant (K) between chlorpromazine (CPZ) and beta-cyclodextrin (CD) in aqueous solutions. An iterative calculation method which included a calculation of the free-ligand concentration was examined; two kinds of data were used in the calculation. One was the derivative intensity difference of CPZ at lambda(max) of CPZ between, before and after the addition of CD; the other was an increase in the derivative intensity induced by the CD addition at the zero-crossing point of the second-derivative spectrum of CPZ. Although both data gave similar K values, the latter data were highly scattered. The reason was considered to be that the small uncertainty in the wavelength reproducibility of the spectrophotometer would severely affect the derivative intensities at the zero-crossing point. The derivative intensity changes of CPZ induced by binding to CD were well reproduced by a calculation using the obtained K value. The results are useful for applications of derivative spectrophotometry to determine the binding constants between drugs and large molecules.