PROTEIN PARTITIONING EQUILIBRIUM BETWEEN THE AQUEOUS POLY(ETHYLENE GLYCOL) AND SALT PHASES AND THE SOLID PROTEIN PHASE IN POLY(ETHYLENE GLYCOL) SALT 2-PHASE SYSTEMS

True partitioning behaviour, which is independent of the protein conce ntration in aqueous two-phase systems, only occurs at relatively low p rotein concentration. The actual concentration limit depends on the pr operties of the protein. When the concentration of a protein exceeds r elatively low values, precipitation at the interface can be observed. This protein precipitate is in equilibrium with the protein solubilize d in each of the phases. This paper discusses the effect of protein so lubility in view of the equilibrium of the protein concentration betwe en the aqueous poly(ethylene glycol) and salt phases and the solid pro tein phase using three proteins. It was found that only rarely will th e proteins be completely in solution as the concentration is increased until a solubility limit is reached and then the protein precipitates fully out of solution. A behaviour that came close to this was only s een in one case out of six. In virtually all cases, a third phase is f ormed which represents a solid aggregate phase which is in equilibrium with the other two, largely aqueous, phases. As the overall concentra tion of protein in the system is increased and the concentration in th e top and bottom aqueous phases increases, the pseudo concentration in the solid-phase, C'(s), also increases. This could have interesting i mplications in terms of the amount of water associated with this phase and it certainly means that in this particular case, the solid phase is not a crystal.