Photoinduced electron transfer between hen egg white lysozyme and anticancer drug menadione

The protein, hen egg white lysozyme, on photoexcitation undergoes electron transfer with menadione (2-methyl-1,4-naphthoquinone), a widely known anticancer drug. With the addition of menadione the fluorescence of lysozyme is quenched with the simultaneous formation of an excited state charge-transfer complex in the longer wavelength and a ground state complex. The former is further evident from laser flash photolysis studies, which indicate a tryptophan to menadione electron transfer. From fluorescence quenching studies the binding constant is found to be ∼1.7×10⁴ M⁻¹ with the corresponding changes in enthalpy (ΔH°) and entropy (ΔS°) as 12.24 kJ mol⁻¹ and 124.12 J mol⁻¹ K⁻¹, respectively, indicative of an entropy-driven process. The circular dichroism studies also show some structural changes with increase in α-helical content in the protein on interaction with menadione. Finally, docking studies give some insight into the role of Trp 108 of lysozyme in the interaction.