Controlling Phase Separation of Lysozyme with Polyvalent Anions

The ability of polyvalent anions to influence protein–protein interactions and protein net charge was investigated through solubility and turbidity experiments, determination of osmotic second virial coefficients (B22), and ζ-potential values for lysozyme solutions. B22 values showed that all anions reduce protein–protein repulsion between positively charged lysozyme molecules, and those anions with higher net valencies are more effective. The polyvalent anions pyrophosphate and tripolyphosphate were observed to induce protein reentrant condensation, which has been previously observed with negatively charged proteins in the presence of trivalent cations. Reentrant condensation is a phenomenon in which low concentrations of polyvalent ions induce protein precipitation, but further increasing polyvalent ion concentration causes the protein precipitate to resolubilize. Interestingly, citrate does not induce lysozyme reentrant condensation despite having a similar charge, size, and shape to pyrophosphate. We observe qualitative differences in protein behavior when compared against negatively charged proteins in solutions of trivalent cations. The polyphosphate ions induce a much stronger protein–protein attraction, which correlates with the occurrence of a liquid–gel transition that replaces the liquid–liquid transition observed with trivalent cations. The results indicate that solutions of polyphosphate ions provide a model system for exploring the link between the protein-phase diagram and model interaction potentials and also highlight the importance that ion-specific effects can have on protein solubility.