Acid–base equilibria of methyl β-carboline-3-carboxylate in aqueous solution

The photophysical properties and the acid–base equilibrium of methyl β-carboline-3-carboxylate in aqueous solution have been investigated. The pKa values in the ground and the first excited state have been spectrophotometrically determined using Förster cycle for the excited state and acidifying the solutions in the pH range with sulphuric acid and acetic acid. Some differences have been found in the absorption spectra depending on the acid added and have been explained in terms of a complex formation between the methyl β-carboline-3-carboxylate and acetic acid. This β-carboline derivative is the most acidic derivative studied till now both in the ground and excited state. The higher acidity of the pyridinic nitrogen, the extension of the conjugation and the steric hindrance due to the inclusion of the methyl ester group as substituent in position 3 of β-carboline justify the unusual acidity of this compound.