Prototropic equilibria of some Harmala alkaloids in acid solutions: proton-induced fluorescence quenching of the monocations of harmaline and harmalol

Abstract

Absorption and fluorescence spectra of harman homologues in acid solutions have shown the presence of mono-, di-, and tri-cations. Fluorescence spectra for the various cations at 77 K and their quantum yields at 298 K are reported. The acidity constants indicate that these alkaloids are more basic in the excited singlet state than in the ground state. The proton-induced fluorescence quenching rate constant for the monovalent cations is observed to be of the order of 10⁹ dm³ mol^–1 s^–1 for harmaline and harmalol and considerably less for harman, harmine, and harmol.