Ionization of β-carbolines in concentrated hydroxide solutions

Abstract

The ionization of several β-carbolines (norharman or β-carboline, harman, harmine, harmaline, and tetrahydroharmine) in concentrated potassium hydroxide solutions have been examined. The pK_a values, calculated using the H_– acidity function, indicate that these compounds are stronger acids than indole and their acidities increase with the degree of aromaticity of the compounds in the order tetrahydro < dihydro < fully aromatic β-carbolines. This sequence can be interpreted in terms of resonance stabilization of the indolate anion.