Protein precipitation

The solubility of proteins in aqueous buffers depends on the distribution of hydrophilic and hydrophobic amino acid residues on the protein's surface. Hydrophobic residues predominantly occur in the globular protein core, but some exist in patches on the surface. Proteins that have high hydrophobic amino acid content on the surface have low solubility in an aqueous solvent. Charged and polar surface residues interact with ionic groups in the solvent and increase the solubility of a protein. Knowledge of a protein's amino acid composition will aid in determining an ideal precipitation solvent and methods. Repulsive electrostatic forces form when proteins are dissolved in an electrolyte solution. These repulsive forces between proteins prevent aggregation and facilitate dissolution. Upon dissolution in an electrolyte solution, solvent counterions migrate towards charged surface residues on the protein, forming a rigid matrix of counterions on the protein's surface. Next to this layer is another solvation layer that is less rigid and, as one moves away from the protein surface, contains a decreasing concentration of counterions and an increasing concentration of co-ions. The presence of these solvation layers cause the protein to have fewer ionic interactions with other proteins and decreases the likelihood of aggregation.Repulsive electrostatic forces also form when proteins are dissolved in water. Water forms a solvation layer around the hydrophilic surface residues of a protein. Water establishes a concentration gradient around the protein, with the highest concentration at the protein surface. This water network has a damping effect on the attractive forces between proteins. Dispersive or attractive forces exist between proteins through permanent and induced dipoles. For example, basic residues on a protein can have electrostatic interactions with acidic residues on another protein. However, solvation by ions in an electrolytic solution or water will decrease protein–protein attractive forces. Therefore, to precipitate or induce accumulation of proteins, the hydration layer around the protein should be reduced. The purpose of the added reagents in protein precipitation is to reduce the hydration layer. Protein precipitate formation occurs in a stepwise process. First, a precipitating agent is added and the solution is steadily mixed. Mixing causes the precipitant and protein to collide. Enough mixing time is required for molecules to diffuse across the fluid eddies. Next, proteins undergo a nucleation phase, where submicroscopic sized protein aggregates, or particles, are generated. Growth of these particles is under Brownian diffusion control. Once the particles reach a critical size (0.1 µm to 10 µm for high and low shear fields, respectively), by diffusive addition of individual protein molecules to it, they continue to grow by colliding into each other and sticking or flocculating. This phase occurs at a slower rate. During the final step, called aging in a shear field, the precipitate particles repeatedly collide and stick, then break apart, until a stable mean particle size is reached, which is dependent upon individual proteins. The mechanical strength of the protein particles correlates with the product of the mean shear rate and the aging time, which is known as the Camp number. Aging helps particles withstand the fluid shear forces encountered in pumps and centrifuge feed zones without reducing in size. Salting out is the most common method used to precipitate a protein. Addition of a neutral salt, such as ammonium sulfate, compresses the solvation layer and increases protein–protein interactions. As the salt concentration of a solution is increased, the charges on the surface of the protein interact with the salt, not the water, thereby exposing hydrophobic patches on the protein surface and causing the protein to fall out of solution (aggregate and precipitate). Salting out is a spontaneous process when the right concentration of the salt is reached in solution. The hydrophobic patches on the protein surface generate highly ordered water shells. This results in a small decrease in enthalpy, ΔH, and a larger decrease in entropy, ΔS, of the ordered water molecules relative to the molecules in the bulk solution. The overall free energy change, ΔG, of the process is given by the Gibbs free energy equation: ΔG = Free energy change, ΔH = Enthalpy change upon precipitation, ΔS = Entropy change upon precipitation, T = Absolute temperature.When water molecules in the rigid solvation layer are brought back into the bulk phase through interactions with the added salt, their greater freedom of movement causes a significant increase in their entropy. Thus, ΔG becomes negative and precipitation occurs spontaneously.