A spectral deciphering of the binding interaction of an intramolecular charge transfer fluorescence probe with a cationic protein: thermodynamic analysis of the binding phenomenon combined with blind docking study.

The binding interaction of an antimicrobial protein lysozyme (chicken egg white lysozyme; abbreviated as Lz from now and onwards) with intramolecular charge transfer (ICT) probe N,N-dimethylaminonaphthyl-(acrylo)-nitrile (DMANAN) has been explored using fluorescence spectroscopic techniques. The thermodynamic parameters like ΔH, ΔS and ΔG for the binding phenomenon have been evaluated on the basis of van't Hoff equation to reveal that the binding is characterized by negative enthalpy and positive entropy changes substantiating the predominant role of hydrophobic forces in the binding process. Quenching of intrinsic tryptophanyl fluorescence of Lz with increasing DMANAN concentration is the actuating tool in the analysis. Particular focus has been rested on fluorescence resonance energy transfer (FRET) between excited tryptophan in the protein and protein-bound DMANAN. Synchronous fluorescence, three-dimensional excitation-emission matrix fluorescence and circular dichroism (CD) spectroscopic techniques have been exploited to unravel the conformational changes in Lz induced by DMANAN binding. Further, molecular docking has been employed to explore the binding site of the probe in Lz based on the AutoDock-based blind docking strategy.