Protein crystallization induced by polyethylene glycol: A model study using apoferritin

Abstract

The phase behavior of apoferritin solutions induced by the addition of polyethylene glycol (PEG) was studied. The interaction between apoferritin molecules was determined by dynamic light scattering. The comparison of the experiments with the theoretical calculations showed that the addition of NaCl to the protein solution only screened the electrostatic repulsion and did not induce attraction. By the addition of PEG, on the other hand, significant attraction was induced and three types of precipitation (crystals, liquid domains, and random aggregates) appeared depending on the concentration of PEG and on its molecular weight. The strength of the attraction could be explained by the depletion mechanism, although there was slight discrepancy between the simple theory and the experiments. Superiority of PEG is thus demonstrated since the depletion mechanism does not depend on specific nature of proteins. From the phase diagram, we suggest that the control of the concentration and molecular weight of PEG are both needed for protein crystallization.