Question

Why is sodium chloride not generally used for the fractional precipitation of proteins?

Answer 1

sodium chloride not generally used for the fractional precipitation of proteins. since it does not preserves protein activity and not promotes precipitation at lower concentrations than other salts. (sodium Phosphate should be better than sodium sulfate on theoretical grounds, but is ruled out because trivalent phosphate occurs only at extremely low pH. Sodium sulfate and many potassium salts are not sufficiently soluble; many multivalent cations are toxic; also potassium phosphate at even fairly low molar concentration is too dense to allow precipitate to settle - economy of the pure salt also plays a role).sodium chloride is not sufficiently soluble.

Salting out can be a powerful tool to separate classes of proteins that vary in size, charge, and surface area among other characteristics. One method of controlling the precipitation is the utilize the different effects of various salts and their respective concentrations. A salt's ability to induce selective precipitation is dependent on many interactions with the water and solutes. Research by Franz Hofmeister in the early 20th century organized various anions and cations by their ability to salt out.

The ordering of cations and anions is called the Hoffmeister Series (1). The cations are arranged as follows

NH4+> K+> Na+ >Li+ >Mg2+ >Ca2+

where ammonium has the highest ability to precipitate other proteinacious solutes. Likewise, the order for anions is

F- ≥ SO42-> H2PO4-> H3CCOO-> Cl-> NO3-> Br-> ClO3-> I->ClO

Between cations and anions in solution the concentration of the anion typically has the greatest effect on protein precipitation.

One of the most commonly used salts is ammonium sulfate, which is typically used because the ions produced in an aqueous solution are very high on the Hofmeister series, and their interaction with the protein itself is relatively low. Other ions such as iodide are very good at precipitating proteins, but are not used due to their propensity to denature or modify the protein.