Salting In & Salting Out: How to Manipulate Protein Solubility
Salting in and salting out are important techniques used to change the properties of proteins in solution for lab research purposes. As their name suggests, these techniques use salt (usually ammonium sulfate) to either solubilize (salting in) or isolate proteins (salting out).
To "salt a protein in" to a solution, add a low concentration of salt to the solution. The charged portions of the proteins will interact with salt ions. This stabilizes protein structure, since the charged particles serve as reinforcement for the maintained position of hydrophobic amino acid residues towards the interior of the protein; these residues do not want to make contact with water, but they definitely do not want to interact with charged particles. Thus, low salt concentrations reinforce the hydrophobic effect of proteins in solution. In addition, the neutralizing effect of the charged particles on the protein's surface evens out the distribution of water and protein, thereby solubilizing the protein. Charged areas tend to create areas of water molecule congregation due to hydrogen bonding, resulting in charged areas having a disproportionate amount of water nearby. However, neutralization allows for even distribution of water molecules throughout the solution. Thus, salting in enhances stability and solubility of proteins in solution.
To "salt a protein out" of a solution, add a high concentration of salt to the solution. The salts will saturate all charged portions of amino acids, and will need to move to forming interactions with water. Water is thereby "stolen" by the excess ions, so water can no longer easily interact with proteins. This results in separation of water from proteins, and eventual precipitation of the proteins from solution. Because proteins are typically denser than water, this insoluble protein precipitate sinks to the bottom of the solution, facilitating extraction from cell lysate.
Salting in and salting out are integral to protein biochemistry. Because of these techniques, manipulating the chemical properties of proteins to achieve a desired result is so simple, yet so effective.
Here are some resources to learn more:
https://youtu.be/q8zg6X7hc9U - this video clearly explains the molecular basis of salting in and salting out.
https://en.wikipedia.org/wiki/Salting_in - we love Wikipedia! Short and sweet encyclopedia entry. It's got some cool math, too.
