Distinct roles of salt cations and anions upon the salting-out of electro-positive albumin

Precipitation experiments of electro-positive albumin by the action of a wide number of salts, and at different concentrations, were performed at a constant temperature (25 °C). The pH range studied covered extreme acidic conditions up to hydronium concentrations where the dissociation of the protein carboxyl groups becomes noticeable. The time required for the clouding phenomenon to occur and the quantity of salted-out protein were also ascertained. The results here reported show that the salt anion is the main salting-out species for the positively charged protein, where their efficacy in salting-out albumin from aqueous solution increases in the order: F− < Cl− < Br− < NO3− < I− < SCN− ~ ClO4− < SO42−. Although at extreme pH conditions the salt cation has no significant influence on the protein salting-out, experiments performed at higher pH values, where the carboxyl groups starts to dissociate, revealed a non-monotonic effect of the salt upon protein precipitation. We interpret this observation as a result of the presence of different protein forms, with which the salt cation participates in chemical equilibrium. Overall, the proteins salting-out phenomenon induced by salt can be rationalized by a general mechanism driven by electrostatic interactions and chemical equilibrium concepts.