An experimental and theoretical approach to the acid–base and photophysical properties of 3-substituted β-carbolines in aqueous solutions

Abstract

The photophysical properties and the acid–base equilibrium of β-carboline-3-carboxylic acid N-methylamide (βCMAM) in aqueous solution have been investigated. The p K a values in the ground and the first excited state have been spectrophotometrically determined using the Förster cycle for the excited state. This compound and methyl-β-carboline-3-carboxylate (βCCM) are the most acidic β-carboline derivatives studied till now both in the ground and the first excited state. The acidity of these compounds is explained in terms of the optimised structure of the solvated molecules using the density functional method’s (DFT) computational method and the polarized continuum model’s (PCM) solvatation model. In contrast with most of the gas phase calculation published on β-carbolines, in this work, we have included solvatation effects in order to reproduce the relative experimental pyridinic nitrogen proton affinities of these 3-subtituted β-carbolines and Norharmane. The gas phase calculations are insufficient and the inclusion of these solvatation effects in the calculations is vital. Potential energy barrier when dihedral angle between the Norharmane skeleton and the substituent varies is also calculated in order to elucidate the importance of steric hindrance in these 3-substituted β-carboline derivatives. The different photophysical properties of the neutral and cationic forms of βCCM and βCMAM are explained in terms of their optimised structures.