Tautomerism and spectral properties of δ-carboline (5H-pyrido[3,2-b]indole) and its N,N-dimethyl derivative in aqueous media

The influence of acidity on the absorption and fluorescence spectra of 5H-pyrido[3,2-b]indole or delta-carboline (delta-C) and its N,N-dimethyl derivative has been studied in aqueous media. The changes in the absorption spectra of delta-C are apparently simple and can be related with the formation of the cationic and anionic species resulting from the protonation of the pyridinic nitrogen atom and the deprotonation of the pyrrolic group, respectively. However, the wavelength dependence of the excitation and emission fluorescence spectra in acid media reveals the existence of tautomeric cations in the ground state. The influence of the acidity on the spectra of the N,N-dimethyl derivative, a model of fixed delta-C cation, allowed the identification of two tautomeric cations: the normal one, CN, where the positive charge is on the pyridinic nitrogen, and its tautomer, CT, with the positive charge mainly localised on the pyrrolic nitrogen. Upon excitation to the first singlet excited state, the pyridinic nitrogen of delta-C becomes more basic and the pyrrole group more acid by 7 and 7.6 pK(a) units, respectively. (C) 2003 Elsevier B.V. All rights reserved.