New insights into the photo-tautomerisation process in β-carboline derivatives revealed by NMR spectroscopy

Abstract

In this work, we present a spectroscopic study of the interactions of harmine (HI), 2-methylharmine (2MHI) and harmaline (HLINE) with acetic acid (AcOH) in low polarity media, such as 1,4-dioxane and chloroform. UV-Vis absorption, fluorescence spectroscopy and ¹H NMR measurements show that, in all cases, a complex between these β-carboline derivatives (BCD) and acetic acid is formed. In addition, this complex exists in two different tautomeric forms in solution, depending on the concentration of acetic acid. The NMR data also indicates that electronic charge transfer takes place differently, depending on which tautomer (T₁ or T₂) is the most stable. Thus, when T₁ is the most stable tautomer species, charge transfer induced by hydrogen bond interactions takes place from the pyrrole to the pyridine ring; however, for T₂, the charge transfer occurs in reverse. We have also shown that loss of aromaticity in the pyridine ring strongly increases the acidity of the NH group of the pyrrolic ring of these derivatives.