Effect of Bile Salt Aggregates on the Prototropic Equilibria of Harmine, a Fluorescent β‐Carboline Alkaloid

Abstract

Interconversion between different ground and excited state prototropic forms of harmine (cation, neutral and zwitterionic forms) in presence of bile salt aggregates has been monitored using steady state and time‐resolved fluorescence spectroscopic techniques. The existence of two binding pockets in bile salt assembly with distinctly different polarity preferences gives rise to significant modulation in the prototropic equilibrium in terms of conversion of cation→neutral and cation*→zwitterionic* in the ground and excited states, respectively. The excited state conversion of cation*→zwitterionic* has been confirmed by both TRANES and TRES profiles and a clear iso‐emissive point indicates an excited state equilibrium between cation and zwitterionic forms. We have also observed that small variations in the hydrophilic head groups of the bile salt monomers changes the spectral feature of harmine appreciably. In non‐conjugated bile salt (NaC), both cation→neutral and cation*→zwitterionic* conversions occur while, in conjugated bile salt (NaTC) conversion from cation*→zwitterionic* is not observed due to stronger electrostatic interactions of C with NaTC head group. The coexistence of cation, neutral and zwitterionic forms in the excited state gives rise to simultaneous emission from three prototropic forms in NaC, which has not been previously observed at neutral pH.