[1] The underlying mechanism of precipitation is to alter the solvation potential of the solvent, more specifically, by lowering the solubility of the solute by addition of a reagent.
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.
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.
Addition of a neutral salt, such as ammonium sulfate, compresses the solvation layer and increases protein–protein interactions.
The hydrophobic patches on the protein surface generate highly ordered water shells.
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.
The ideal salt for protein precipitation is most effective for a particular amino acid composition, inexpensive, non-buffering, and non-polluting.
The isoelectric point (pI) is the pH of a solution at which the net primary charge of a protein becomes zero.
At a solution pH that is above the pI the surface of the protein is predominantly negatively charged and therefore like-charged molecules will exhibit repulsive forces.
The greatest disadvantage to isoelectric point precipitation is the irreversible denaturation caused by the mineral acids.
With smaller hydration layers, the proteins can aggregate by attractive electrostatic and dipole forces.
Important parameters to consider are temperature, which should be less than 0 °C to avoid denaturation, pH and protein concentration in solution.
Miscible organic solvents decrease the dielectric constant of water, which in effect allows two proteins to come close together.
For the specific case of polyethylene glycol, precipitation can be modeled by the equation: C is the polymer concentration, P is a protein–protein interaction coefficient, a is a protein–polymer interaction coefficient and μ is the chemical potential of component I, R is the universal gas constant and T is the absolute temperature.
It is important to note that an excess of polyelectrolytes will cause the precipitate to dissolve back into the solution.
An example of polyelectrolyte flocculation is the removal of protein cloud from beer wort using Irish moss.
Metal salts can be used at low concentrations to precipitate enzymes and nucleic acids from solutions.
There are numerous industrial scaled reactors than can be used to precipitate large amounts of proteins, such as recombinant DNA polymerases from a solution.
Since the particles are exposed to a wide range of shear stresses for a long period of time, they tend to be compact, dense and mechanically stable.
The fluid in volume elements approach plug flow as they move though the tubes of the reactor.
CSTR reactors run at steady state with a continuous flow of reactants and products in a well-mixed tank.