biochemistry - Why do many DNA solutions contain additional compounds?

A previous question asked for a quantification of DNA solubility in water. It seemed like it would be easily answerable, however isn't quite that simple since no data seems to exist for DNA solubility in exclusively water.

Even the small amount of data about water was in the context of washing away organic compounds from the solution. This got me thinking. DNA is in an aqueous solution in biological situations, and is usually handled in aqueous solutions in labs. So why is solubility data so scarce for pure water? What is important about organic compounds for DNA solutions?

So my question is why is DNA dissolved in exclusively water seemingly "uncommon" in laboratories? Is there something about pure water that is bad for DNA storage? Is it that functionally DNA never needs to be in water because it is inaccessible to proteins? Or have I misinterpreted the lack of solubility data; water is common-place and no data exists because there is no need?

I imagine PCR is where most of the data on DNA solutions exists. What about the polar organic solvents/compounds makes them important for DNA solutions?

Answer

The only time that nucleic acids would encounter pure water would be in a laboratory setting--for example after an oligonucleotide is synthesized in vitro, the protecting groups are removed from the reactive atoms in the finished sequence and the final product is cleaved from the supporting matrix. At that point you can lyophilize (freeze dry) the ammonium hydroxide solution, and resuspend the single-stranded oligo in pure water.

However, as noted in the comments, nucleic acid solubility, particularly high molecular weight DNA, like genomic DNA, is enhanced in solutions with dilute monovalent cations. 10 mM TrisHCl, pH 8.0, 1 mM EDTA suffices for almost every application. The EDTA inhibits any errant DNAses, and also slightly inhibits microbial growth.

Tris is not the best biological buffer but has high solubility and is relatively cheap and stable. If the solution becomes too basic the DNA strands will melt, and if the solution becomes too acidic the purines will start to deaminate.

From the moment that DNA is synthesized in a cell until that cell dies and its DNA is ultimately degraded, it does not encounter a pure water environment.