Salts have different effects on proteins depending on ionic strength ● Protein solubility increases with neutral salts at low ionic strength – “Salting-in” ● Protein solubility decreases with neutral salts at high ionic strength – “Salting-out” ● Salting-in stabilizes charged groups of proteins ● Salting-out is competition between protein and salt for waters of hydration ● As salt concentration increases protein molecules aggregate and some fall out of solution
Ammonium Sulfate Precipitation ● Method that allows us to use the relative ionic strength of different proteins to purify individual proteins ● Different proteins precipitate at different levels of ionic strength due to different secondary and tertiary structure ● Ammonium sulfate used in protein purification and crystallography to help “salt-out” proteins ● Need to watch pH ● % Saturation is unit used to denote ionic strength ● “Salted-out” proteins are separated after salt addition by centrifugation