Nonaqueous solutions of DNA: Factors determining the stability of the helical configuration in solution

Observations demonstrate the importance of hydrophobic forces and argue against the assignment of the stability of the aqueous configuration of DNA solely to hydrogen bonds.

Abstract

The disorganization of the aqueous helical configuration of deoxyribonucleic acid (DNA) by a number of structurally related organic solvents has been investigated. In all the solvents investigated, denaturation is accompanied by 35–50% increase in absorbance (at 259 mμ) and a decrease in optical rotation (at 436 mμ) of 200–350 °. The following observations have been made: (a) The effectiveness of the denaturant increases with both chain length and increasing hydrocarbon content. Thus ethyl and propyl alcohols were found to be more effective than methanol. (b) The alkyl-substituted solvents N,N′-dimethylformamide, dimethyl sulfoxide, and tetramethylurea are the most effective denaturants among the various solvents employed. The midpoints of the denaturation transition due to these solvents in the presence of 1–5 × 10−2M salt, range from 19 to 27 mole % (57–62 vol.%), and the changes produced in optical rotation (at 436 mμ) upon denaturation are of the order of −300 to −350 °. (c) N,N′-dimethylformamide is a more effective DNA denaturant than formamide. (d) Increasing the hydroxyl content of the solvent, on the other hand, had no significant effect; the denaturation midpoints in methanol-water and ethylene glycol-water mixtures (in the presence of 0.5–5 × 10−2M salt) occur at 80 ± 1 mole % (90 ± 2 vol.%) of the nonaqueous component. These observations demonstrate the importance of hydrophobic forces and argue against the assignment of the stability of the aqueous configuration of DNA solely to hydrogen bonds. Except in dimethyl sulfoxide, DNA denaturation by the organic solvents employed could be largely reversed by water in the presence of electrolyte.