Exploring the non-covalent binding behaviours of 7-hydroxyflavone and 3-hydroxyflavone with hen egg white lysozyme: Multi-spectroscopic and molecular docking perspectives.

The interactions of bio-active flavonoids, 7-hydroxyflavone (7HF) and 3-hydroxyflavone (3HF) with hen egg white lysozyme (HEWL) have been established using differential spectroscopic techniques along with the help of molecular docking method. The characteristic dual fluorescence of 3HF due to the excited intramolecular state proton transfer (ESIPT) process is altered markedly upon binding with HEWL. Both the flavonoids quenched the intrinsic fluorescence of HEWL through static quenching mechanism while the binding affinity of 7HF was found to be greater than 3HF under experimental conditions. The binding constant (K) values were estimated to be in the order of 10 M and decreased with the rise in temperature. The contributions of the thermodynamic parameters (ΔH° and ΔS°) revealed that hydrophobic forces along with hydrogen bonding played a crucial role in the interaction of HEWL with 7HF and 3HF respectively and this finding was aptly supported by the molecular docking studies. The donor (HEWL) to acceptors (7HF and 3HF) binding distances were calculated using the Föster's theory. The phenomena of blue shifting of the emission maxima of the residues indicated the increase in hydrophobicity around the Trp micro-environment upon addition of the flavonoids was observed from synchronous and 3D fluorescence measurements whereas REES study indicated the decrease in mobility of the Trp residues upon addition of the ligands. The CD, FTIR and thermal melting studies indicated the alteration in the structural stability of HEWL on ligand binding and it was found that the % α-helical content decreased on complexation with 7HF and 3HF respectively as compared to native state. The flavonoids were found to inhibit the enzymatic activity of HEWL. The molecular docking results and accessible surface area (ASA) calculations revealed that the flavonoids bind within the active site of HEWL. The negative ΔG° values obtained from experimental and molecular docking studies indicate the spontaneity of the interaction processes.