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ucleic acid precipitation - why alchohol and low temperature (Jan/29/2002 )

Would anyone out there be able to tell as to why alchohols like isopropanol or ethanol are used to precipitate nucleic acids, and also, why is the precipitation done at low temperatures


Precipitation is probably easier at lower temperatures because of less Brownian motion of the molecules?


I think DNA is not soluble in alcohol because this kind of substance cannot shield the high charge of phosphate groups of the backbone. Water, a molecule that is more polar and has good capacity of shielding charged or polar groups, is more efficient to do that. When you increased the alcohol concentration in water, the DNA becomes progressively less shielded and eventually, precipitate. A good point to note here is that by adding salt, you  accentuate a lot this effect (salt ions sequester water molecules wich become less available to solubilize DNA).

For the temperature, it is simply because it reduce the solubility (in general) of dissolved substances. So, it helps to precipitate things (DNA, and unfortunately, salts...). Someone else has given a more in-depht point of view for this explanation.


Regarding temperature dependence of solubility it depends on whether it is endo- or egzotemic process. In the former case, solubility incrises with temperature.



ok i remember something about...

DNA is soluble in water because the water molecules intercalate into the phosphate backbone of the DNA and thus maintain it in a soluble state. When you add salt and alcohol you are essentially removing the water molecules and teh DNA then precipitates.