What's the function of NaAc in DNA precipitation(using Eth)? - (Sep/26/2004 )

I have read some books but cannot find the answer. I think NaAc may increase the ionic strength to stabilize DNA duplexes. Is it right?

Thanks.

No, NaAc has nothing to do with stabilizing duplexes but provides moderate concentrations of monovalent cations which is required for DNA to precipitate.

I see. Cations can neutralize the negative charges carried by phosphate groups, which are important for solubility. Thank you.

Further more it increases the competition between salt and DNA for hydrate water.

The purpose of adding salts is to neutralize the charge on the sugar-phosphate backbone of the DNA. For a precipitation, you need to form ion pairs between the polyanion (DNA) and the cation (usually sodium or ammonium). The DNA and counterions are more or less in the free ion form rather than the ion pair form in dilute aqueous solution, and are surrounded by one or more layers of water molecules. Adding ethanol decreases the dielectric constant of the solution. The anions and cations form ion pairs and this results in precipitation of DNA.

Thank you very much.