EXCITED-STATE TAUTOMERIZATION OF CAMPTOTHECIN IN AQUEOUS-SOLUTION

The effect of pH on the absorption and fluorescence spectral character istics of 20(S)-camptothecin was studied in aqueous solution. In alkal ine solution, the lactone ring of camptothecin hydrolyzes to produce c amptothecin carboxylate. The equilibrium constants of the lactone ring hydrolysis of camptothecin at various pH values were determined using steady state fluorescence measurements. The H-1 nuclear magnetic reso nance (NMR) and circular dichroism spectra were measured to characteri ze the structure of camptothecin in strongly alkaline medium. The fluo rescence intensity of camptothecin is quenched by OH- ions. A mechanis m of fluorescence quenching of the carboxylate form by OH- ions, which involves tautomerization on electronic excitation, is proposed. The g round and excited state pK(a) values of the quinolinium ion are report ed.