Precipitation (chemistry)

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Introduction
The process by which a dissolved chemical species comes out of solution as a solid is called precipitation. Precipitation is often employed in bioseparation applications for protein purification and concentration, although it may be used to extract other compounds, such as nucleic acids and other cell structures. Precipitation is achieved by altering the solubility of a target protein in a solution to cause it to become insoluble and thus precipitate, allowing for this concentrated solid to be more easily extracted from the liquid phase (supernatant).

Solubility of Macromolecules
Solubility is the property of a substance (solute) – in either gas, liquid, or solid phases – to dissolve in a liquid (called the solvent) and form a homogenous (single-phase) solution. The extent of which a solute dissolves in a solvent is its solubility, and is based on factors such as temperature, the similarity between the solute’s net charge and the solvent’s polarity, the formation of hydration layers, and the concentration of solutes already dissolved in the solvent, as well as the solvent’s ionic strength.

Macromolecules (such as proteins and nucleic acids) are typically present in aqueous solvents. They are able to dissolve as a result of assuming a stable conformation in the solvent and then being surrounded by hydration layer(s). When placed in an aqueous solution, the macromolecules adopt a structure in which most hydrophobic (nonpolar) portions of the molecules gather inwards, and most hydrophilic (charged/polar) portions surround the exterior, guided to a final conformation that has the lowest Gibbs free energy. The attraction between the hydrophilic surface of the macromolecule and the polar water molecules creates an interface in which similarly-oriented water molecules associate with, and surround, the solute’s surface to form a highly ordered layer, called the hydration layer (also called the interfacial double layer). The formation of hydration layers enhances the solubility of macromolecules by greatly reducing inter-macromolecular dipole-dipole attraction and thus preventing their association with other solutes.

Effects of Solute Net Charge
In an aqueous solvent, solutes with a higher net charge are more soluble, due to their enhanced ability to associate with water molecules and create a hydration layer, and due to lessened inter-solute interactions due to increased repulsion from like-charges.

Effects of Solvent Ionic Strength
Increasing a solvent’s ionic strength (the concentration of ions in solution) decreases the availability of unassociated water molecules, which are required for the formation of hydration layers. This increases intermolecular attraction between solutes and promotes coalescence, thus decreasing the solute’s solubility.

History
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Invention and early development
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Recent developments
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Design and Operation
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Applications
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Examples
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