Synthesis of a Carbazole-Coumarin Schiff Base Dyad and Its Biophysical Interactions with HSA and Lysozyme

A novel carbazole-coumarin Schiff Base dyad, HE1, was synthesized and characterized. Its interactions with HSA and lysozyme (Lyz) were examined by UV-visible absorption and steady-state/ three dimensional (3D) fluorescence spectroscopies and also molecular docking method. Fluorescence measurements indicated that the Schiff base quenched the intrinsic fluorescence of the proteins via static mechanism with the formation of complexes. According to thermodynamic assays at different temperatures, the positive signs of both ΔS and ΔH parameters for the binding affinity between the HE1 and Lyz indicated the presence of hydrophobic interactions. The negative signs of ΔH and ΔS indicated that hydrogen bonds and van der Waals forces were active between HE1 and HSA. Based on Förster’s theory, the binding distance (r) between Lyz/HSA and HE1 was calculated to be 3.38/2.88 nm, which implies that non-radiative energy transfer occurs from protein to HE1. 3D fluorescence spectra showed that binding of HE1 to proteins caused conformational changes in the peptide structure of protein molecule. The obtained experimental data were supported and validated by computational analysis by molecular docking confirming the types of binding between HE1 and proteins. The current results shed important and useful insight into binding interactions and affinities between biomolecules and similar analogs of Schiff bases.