Interactions and aggregation of apoferritin molecules in solution: Effectsof added electrolytes

We have studied the structure of the protein species and the protein-protei n interactions in solutions containing two apoferritin molecular forms, mon omers and dimers, in the presence of Na+ and Cd2+ ions. We used chromatogra phic, and static and dynamic light scattering techniques, and atomic force microscopy (AFM). Size-exclusion chromatography was used to isolate these t wo protein fractions. The sizes and shapes of the monomers and dimers were determined by dynamic light scattering and AFM. Although the monomer is an apparent sphere with a diameter corresponding to previous x-ray crystallogr aphy determinations, the dimer shape corresponds to two, bound monomer sphe res. Static light scattering was applied to characterize the interactions b etween solute molecules of monomers and dimers in terms of the second osmot ic virial coefficients. The results for the monomers indicate that Na+ ions cause strong intermolecular repulsion even at concentrations higher than 0 .15 M, contrary to the predictions of the commonly applied Derjaguin-Landau -Verwey-Overbeek theory. We argue that the reason for such behavior is hydr ation force due to the formation of a water shell around the protein molecu les with the help of the sodium ions. The addition of even small amounts of Cd2+ changes the repulsive interactions to attractive but does not lead to oligomer formation, at least at the protein concentrations used. Thus, the two ions provide examples of strong specificity of their interactions with the protein molecules, in solutions of the apoferritin dimer, the molecule s attract even in the presence of Na+ only, indicating a change in the surf ace of the apoferritin molecule. in view of the strong repulsion between th e monomers, this indicates that the dimers and higher oligomers form only a fter partial denaturation of some of the apoferritin monomers. These observ ations suggest that aggregation and self-assembly of protein molecules or m olecular subunits may be driven by forces other than those responsible for crystallization and other phase transitions in the protein solution.