Patterns of protein-protein interactions in salt solutions and implications for protein crystallization

The second osmotic virial coefficients of seven proteins - ovalbumin, ribonuclease A, bovine serum albumin, α-lactalbumin, myoglobin, cytochrome c, and catalase - were measured in salt solutions. Comparison of the interaction trends in terms of the dimensionless second virial coefficient b₂ shows that, at low salt concentrations, protein-protein interactions can be either attractive or repulsive, possibly due to the anisotropy of the protein charge distribution. At high salt concentrations, the behavior depends on the salt: In sodium chloride, protein interactions generally show little salt dependence up to very high salt concentrations, whereas in ammonium sulfate, proteins show a sharp drop in b₂ with increasing salt concentration beyond a particular threshold. The experimental phase behavior of the proteins corroborates these observations in that precipitation always follows the drop in b₂. When the proteins crystallize, they do so at slightly lower salt concentrations than seen for precipitation. The b₂ measurements were extended to other salts for ovalbumin and catalase. The trends follow the Hofmeister series, and the effect of the salt can be interpreted as a water-mediated effect between the protein and salt molecules. The b₂ trends quantify protein-protein interactions and provide some understanding of the corresponding phase behavior. The results explain both why ammonium sulfate is among the best crystallization agents, as well as some of the difficulties that can be encountered in protein crystallization.