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Salting in refers to the effect where increasing the ionic strength of a solution increases the solubility of some solute (such as a protein). This effect tends to be observed at lower ionic strengths. Protein solubility is a complex function of the physicochemical nature of the protein, pH, temperature, and the concentration of the salt used. It also depends on whether the salt is kosmotropic (stabilizes water) the proteins usually increases slightly (salting in). But at high concentrations of salt, the solubility of the proteins drop sharply (salting out).
Initial salting in at low concentrations is explained by the Debye-Huckel theory. Proteins are surrounded by the salt counterions (ions of opposite net charge) and this screening results in decreasing electrostatic free energy of the protein and increasing the activity of the solvent, which in turn, leads to increasing solubility. This theory predicts in the logarithm of solubility to be proportional to the square root of the ionic strength.
The behavior of proteins in solutions at high salt concentrations was explained by John Gamble Kirkwood. The abundance of the salt ions decreases the solvating power of the salt ions, the solubility of the proteins decreases and precipitation results.
At high salt concentrations, the solubility is given by the following empirical expression.
log S = B - KI
where S is the solubility of the protein, B is a constant (function of protein, pH and temperature), K is the salting out constant (function of pH, mixing and salt), and I is the ionic strength of the salt. This expression is an approximation to that proposed by Long and McDevit.
Here you could plot the log of protein solubility versus the salt concentration, and it would look like this:
- F.A. Long, W.F. McDevit, Chem. Rev., 51, (119) 1952
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