Why use 75% ethanol to wash DNA during DNA extraction? - (Aug/26/2007 )

[size="4"][/size]Dear all:
We use 75% ethanol to wash DNA during DNA extraction. I don’t know what the actual mechanism is. Why not 50% or 95%? Is it related to the solubility of DNA in ethanol?
Thanks!

you're right.
DNA becomes soluble in 60-65% ethanol. As it's a pelleting procedure, you don't want this.
The aim of ethanol wazsh is desalting. Salts are vey bad soluble in 95% ethanol. At "30%water in ethanol", salts are soluble enough to be separated from DNA.

hi wly..
for a particular solute to be dissolved in a hydrophilic solvent, it has to interact with the solvent, making some H-bond...
Dna dissolves both in ethanol and water. but ethanol has a more affinity to water than dna.
so, when u add water to ethanol, the absolute interaction between dna and ethanol subsides, as a result, dna is to fall out as ppt.

additionally, 30% water dissolves some other impurities(salt, protein) from ur extract.
however, remaining to a 75% is not absolute. for eg, 70% is sometimes well-gone. lab tradition may work here.

lilliput

thank u very much！