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

Abstract

Precipitation experiments of electro-positive albumin by the action of a wide number of salts, and at differentconcentrations, were performed at a constant temperature (25 °C). The pH range studied covered extreme acidicconditions up to hydronium concentrations where the dissociation of the protein carboxyl groups becomes no-ticeable. The time required for the clouding phenomenon to occur and the quantity of salted-out protein werealso ascertained. The results here reported show that the salt anion is the main salting-out species for the posi-tively charged protein, where their efficacy in salting-out albumin from aqueous solution increases in theorder: F−bCl−bBr−bNO3−bI−bSCN−~ ClO4−bSO42−. Although at extreme pH conditions the salt cationhas no significant influence on the protein salting-out, experiments performed at higher pH values, where thecarboxyl groups starts to dissociate, revealed a non-monotonic effect of the salt upon protein precipitation. Weinterpret this observation as a result of the presence of different protein forms, with which the salt cation partic-ipates in chemical equilibrium. Overall, the proteins salting-out phenomenon induced by salt can be rationalizedby a general mechanism driven by electrostatic interactions and chemical equilibrium concepts.