Modeling of the salt effects on hydrophobic adsorption equilibrium of protein.

A two-state protein model is proposed to describe the salt effects on protein adsorption equilibrium on hydrophobic media. This model assumes that protein molecules exist in two equilibrium states in a salt solution, that is, hydrated and dehydrated states, and only the dehydrated-state protein can bind to hydrophobic ligands. In terms of the two-state protein hypothesis and the steric mass-action theory, protein adsorption equilibrium on hydrophobic media is formulated by a five-parameter equation. The model is demonstrated with the adsorption of bovine serum albumin to Phenyl Sepharose gels as a model system. The effects of salt type (sodium chloride, sodium sulfate and ammonium sulfate) on the model parameters are discussed. Then, the model formulism is simplified in terms of the small magnitude of the protein dehydration equilibrium constant in the model. This simplification has returned the model derived on the basis of the two-state protein hypothesis to its original mechanism of salt effects on the hydrophobic adsorption of protein. This simplified model also creates satisfactory prediction of protein adsorption isotherms.