Abstract
The effect of pH on the absorption and fluorescence spectral characteristics of 20(S)-camptothecin was studied in aqueous solution. In alkaline solution, the lactone ring of camptothecin hydrolyzes to produce camptothecin carboxylate. The equilibrium constants of the lactone ring hydrolysis of camptothecin at various pH values were determined using steady state fluorescence measurements. The 1H nuclear magnetic resonance (NMR) and circular dichroism spectra were measured to characterize the structure of camptothecin in strongly alkaline medium. The fluorescence intensity of camptothecin is quenched by OH − ions. A mechanism of fluorescence quenching of the carboxylate form by OH − ions, which involves tautomerization on electronic excitation, is proposed. The ground and excited state p K a values of the quinolinium ion are reported.
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