Ground and excited-state ionization behavior of 9-aminocamptothecin: An absorption and fluorescence spectral study

The absorption and fluorescence spectra of 9-amino-20(S)-camptothecin (ACAM ) have been measured as a function of pH. The photophysical properties, for example, fluorescence quantum yields and lifetimes, and radiative and nonr adiative decay rates of different prototropic species of ACAM in aqueous so lution have been estimated. The neutral lactone form as well as the ring-op ened carboxylate form of the drug are found to be nonfluorescent in aqueous medium. The ground and excited-state acidities of the prototropic forms of the molecule have been determined. The semi-empirical AM I calculations ha ve been performed to calculate the ground and excited-state charge densitie s at the basic centers of the molecule. The results have been compared with that for the parent drug, 20(S)-camptothecin.