Comparative effects of salt, organic, and polymer precipitants on protein phase behavior and implications for vapor diffusion

Salts, polymers, and organic precipitants commonly used to crystallize proteins share the ability to induce attractive protein-protein interactions, and eventually lead to the formation of crystals. Their effects on protein phase behavior are investigated here. First, the ovalbumin phase diagram at pH 7 was determined separately in ammonium sulfate, poly(ethylene glycol) (PEG) 8000, and 2-methyl-2,4-pentanediol (MPD) solutions. Increasing concentrations of each of these three additives lead to well-defined phase separations that have the same characteristic trends, but differ in their physical appearance. Second, the phase diagrams of ovalbumin in ammonium sulfate and ribonuclease A in MPD were determined at pH 6, conditions under which the two proteins crystallize. The general shape of the phase diagram and the formation kinetics of the different phases are similar for both proteins, suggesting that the main characteristics of the phase behavior are independent of the physical origin of the attraction between protein molecules. Finally, the phase diagrams are compared to the results of vapor diffusion experiments carried out under similar solution conditions. These results illustrate how vapor diffusion experiments can be interpreted in terms of the phase diagram, and how crystallization conditions can be designed based on knowledge of the experimental phase diagrams of proteins.