Exploring the binding mechanism of amine-functionalized tetraaryl imidazole fluorescent ligands with lipase: Insights from multi-spectroscopic, thermodynamic and docking approaches

Environment-sensitive fluorescence ligands based on twisted intramolecular charge transfer (TICT) are often applied to target relevant biomacromolecules. In this work, several imidazole-based "D-pi-A " fluorescent ligands with different amine side chains have been designed, which showed sensitivity toward solvent polarity and viscosity. The binding of obtained ligands with lipase led to enhancement of emission intensity. Results from binding parameters exhibited that the ligands bound with lipase in a 1:1 stoichiometry, and the flexible amine side chain appended ligand was more sensitive to lipase. The fluorescence quenching experiments of ligands with lipase suggested the involvement of a static quenching mechanism in the binding process. From the estimated thermodynamic parameters, complexation between the ligands and lipase was found to occur primarily via Van der Waals forces and hydrogen bonds formation. Observations from synchronous, 3D fluorescence and CD spectra pointed that the binding of ligands to lipase impacted the microenvironment around tyrosine and typ-tophan residues, but was not enough to change the secondary structure of lipase. Moreover, modeling calculation demonstrated that ligands were mainly located within the hydrophobic pockets of lipase. This study provides evidence for expanding the application of environment-sensitive fluorescent ligands for probes or drug candi-dates targeting lipase.