91132 Study Guide - Final Guide: Hydrophile, Ethanol Precipitation, Relative Permittivity
Document Summary
Ethanol precipitation of dna with salts - theory. The purpose of adding salts is to neutralize the charge on the sugar-phosphate backbone of the dna. Ethanol"s task is a little more complex than removing the water. For a precipitation, you"re interested in forming ion pairs between the polyanion (dna) and the cation (na+, mg++, etc). Water is a high dielectric insulator, which means that the electrostatic force between two ions of opposite charge is very low in water: Adding organic solvent decreases the dielectric constant of the solution (in this case water no longer insulates individual ions; anion and cation form an ion pair and promptly pop out of solution. Nacl increase the stability of dna duplexes, although you might expect salts to interfere with hydrogen bonds, rather than strengthen them. Each strand of dna has an enormous charge density (charge per unit volume), so the two strands tend to push each other apart.
