Acidity constants of β-carbolines in the ground and excited singlet states

Abstract The ground state ionization constants p K a (S 0 ) of β-carboline and three of its derivatives (harmane, harmine and 2-methylharmine) were measured spectrophotometrically in aqueous solutions. The corresponding p K a (S 1 ) of the first excited singlet state was calculated by means of the Forster cycle from the absorption and emission spectra. It was also established that protonated β-carboline cations are responsible for photoabsorption and fluorescence in acidic solutions. However, in alkaline solutions excitation of the neutral species leads to formation of the protonated β-carboline cations by rapid proton transfer from the solvent. A zwitterion species is produced by diffusion-controlled quenching of the excited state cation by OH − ions.