Thermodynamics of cytochrome c adsorption on negatively charged gold nanoparticles

In this letter, we have probed the interaction of the heme protein cytochrome c from equine heart (ECc) with negatively charged gold nanoparticles (GNPs) by second harmonic light scattering (SHLS) in solution at pH 10.75 where the protein is also negatively charged. The giant SH response generated from the GNP surface gets quenched due to adsorption of ECc and the isotherm when modelled by a modified Langmuir adsorption model produces the apparent equilibrium binding constant (K), Gibbs free energy change (Delta G) and the maximum number of adsorbed ECc (n(sat)) on the GNP surface. The thickness of the protein layer on the surface obtained from dynamic light scattering (DLS) experiments show that only monolayer of ECc is formed on the surface and a simple calculation put the fraction of the surface coverage to 0.5 and 0.87 for 41 and 69 nm GNPs, respectively. The enthalpic (Delta H) and entropic (Delta S) contributions to the overall free energy change in adsorption obtained from a temperature dependent SHLS measurement show that the adsorption is primarily driven by enthalpy change indicating the insignificant role played by the electrostatic repulsion between the protein and the GNP surface in the process. Noncovalent interactions seem to dictate the adsorption process while displacement of the water molecules from the surface by the protein molecules do not affect the adsorption process in any major way as evidenced from the measured entropy change.