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EtOH ppt

Frauke Naumann fnaumann at scan.genetik.uni-koeln.de
Mon Jan 10 10:21:15 EST 2000

Silke Feineis schrieb in Nachricht <38721428 at uni-wuerzburg.de>...
>Hi everybody!
>I`m wodering what`s exactly happening while ethanol-precipitation of DNA?
>just curious - thank you

>>If I didn't recognize your name from prior postings, I would suspect that
>>this was a "troll". :-). I've heard more grad students and post-docs argue
>>about this over the years than I can remember. Most of the arguments
>>struck me as biologists trying to impress themselves with their knowledge
>>(or lack of it) of physical chemistry. I've never seen anything in print
>>on the issue, although I suppose there must be somewhere. Good luck in
>>your quest, although I suspect you may find yourself tilting at windmills.

Hi, the following explanation might be an "impressive try", but - as I'm
also ONLY a biologist ;-), but the combination of those explanations somehow
sounds reasonable:
The added salt ions compete successfully for the water molecules that keep
the DNA in solution, because they possess a higher field strength due to
their smaller diameter (like salting out proteins).
Adding ethanol or isopropanol reduces the dielectricity constant (the
epsilon in Coulomb's Law of the strength between two charges) of the aqueous
solution thus reinforcing the force of attraction between charged molecules
(epsilon is in the denominator in Coulomb's law). Thus, the salt-ions and
the charged DNA form a salt and precipitate. The remaining salt ions
continue to snatch away the water molecules from the DNA hydrat cover
because they exert a greater force on the dipolar water molecules and uuups,
the DNA falls down. I can't remember where I read it, but I'm sure it was in
some textbook.
About the best conditions for ethanol precipitation, there are a couple of
publications I think in Biotechniques, I can't remember. With little DNA,
you can't make a mistake leaving it over night in  the freezer.
Hope that helped,

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