[1] Ammonium sulfate is especially useful as a precipitant because it is highly soluble, stabilizes protein structure, has a relatively low density, is readily available, and is relatively inexpensive.
At a sufficiently high ionic strength, the protein will precipitate out of the solution, an effect termed "salting out".
[3] When the ammonium (NH+4) and sulfate (SO2−4) ions are within the aqueous solution they are attracted to the opposite charges evident on the compound that is being purified.
This attraction of opposite charges prevents the water molecules from interacting with the compound being purified, leading to the precipitation or "salting out".
The folding and self-association of the protein pushes out free water, leading to an increase in entropy and making this process energetically favorable.
[5] The direct addition of solid ammonium sulfate does change the pH of the solution, which can lead to loss of enzyme activity.
The precipitated protein of interest can subsequently be recovered by centrifugation and dissolved in standard buffer to prepare the sample for the next stage of purification.
[4] It is also often employed during the later stages of purification to concentrate protein from dilute solution following procedures such as gel filtration.