Effect of denaturants and stabilisers on protein adsorption at solid-liquid interfaces

The adsorption isotherms of different proteins from aqueous solution to the surface of different solids have been compared in the presence of additives such as urea, surfactants and high concentration of various neutral salts. The adsorption isotherms of lysozyme on alumina are not affected much in the presence of 8 M urea showing the rigid structure of lysozyme whereas isotherms of hemoglobin show surface coagulation even in presence of 2 M urea. In presence of 8 M urea, adsorption isotherms of BSA on alumina show two distinct steps. The extent of protein adsorption in the presence of surfactants depends on the nature of surfactants as well as of the underlying surface. The adsorption isotherms of BSA and lysozyme in presence of 2 M concentration of different neutral salts have also been compared with each other. In the presence of denaturants such as NaI and LiCl, the proteins are adsorbed in unfolded beta-conformation whereas in the presence of protein stabilizers such as NaCl, KCl and Na2SO4, amount of protein adsorbed at saturation is zero or extremely small showing that unfolding of proteins at the interface is necessary for initial stage of protein adsorption. The standard free energy change (delta G degrees) per square meter of the surface, signifying relative affinity of adsorption at the state of monolayer saturation, have been calculated. The magnitude of standard free energy of transfer (delta G degrees B) of one mole of protein to the surface in presence of all the additives was found close to 40 kJ/mole.