Preferential solvent interactions between proteins and polyethylene glycols.

Preferential solvent interactions between polyethylene glycols and five proteins were investigated by density measurements and analyzed by the multicomponent theory. These measurements were conducted as a function of concentration and molecular size of the synthetic polymer at different pH values. The results showed that proteins are preferentially hydrated under the experimental conditions employed, i.e. polyethylene glycol is excluded from the protein domain. The introduction of protein thermodynamically destabilizes the solvent system. The magnitude of instability increases with increasing concentration of the polymer. Furthermore, systems of polyethylene glycols of higher molecular weight are more destabilized. A linear relationship was observed between the magnitude of destabilization and average hydrophobicity of the proteins employed with the exception of tubulin. The system is more destabilized in the presence of proteins with higher content of hydrophilic residues indicating that the interaction between polyethylene glycol and ionized residues is thermodynamically unfavorable with a negative ion being more so than a positive one. After correcting for the contribution of ionic effect on the instability of the system it was found that at least for a protein of average hydrophobicity of 1000 cal/residue the mass of the protein contributes to the instability also. It may, therefore, be concluded that in a polyethylene glycol system the presence of protein leads to unfavorable thermodynamic interaction which in turn leads to phase separation. The causes of such unfavorable interaction include the charges residing on the protein.