Hofmeister Effects on the Phase Stability of Aqueous BSA Solutions with Added PEG

Abstract

Research on the impact of co-solutes on protein aggregation is of crucial importance both for the life sciences and for industrial applications, particularly in the pharmaceutical sector. Measuring the cloud-point temperatures, we investigated the phase stability of bovine serum albumin (BSA) solutions mixed with polyethylene glycol (PEG) and salt in acetate buffer at a pH of 4.6, which is close to the isoionic point of BSA (pH ≈ pI), and at a pH of 4.0, at which the protein has a net positive charge (pH < pI). Our results show that: 1) PEG induces BSA aggregation both in the presence and absence of added salt, consistent with previous studies on protein solutions and explainable by depletion forces. 2) The addition of NaCl to BSA-PEG solutions has different effects depending on the pH: at pH ≈ pI, the salt stabilizes the solution against phase separation, while at pH < pI salt reduces the colloidal stability of the solution. 3) Ion-specific trends depend on the pH: at pH ≈ pI, the phase stability of the solution increases towards separation from NaF (least stable) to NaI (most stable) and from LiCl to CsCl. The opposite trend is observed at pH < pI.