Condensed states of nucleic acids. III. Ψ(+) and Ψ(−) Conformational transitions of DNA induced by ethanol and salt

The conformational response of calf thymus DNA to solvent conditions altered by varying amounts of ethanol and NaCl has been monitored by circular dichroism (CD). These measurements, which extend over a much greater range of conditions than previously examined, reveal (above critical concentrations of ethanol and salt) a condensed form of the macro‐molecule with unusually large positive ellipticity in the 250–300‐nm region [the Ψ(+) state]. Mere increase in NaCl concentration at constant 35% (v/v) concentration of ethanol suffices to convert such Ψ(+) samples—via a series of intermediate forms with CD spectra resembling those of A‐DNA, then B‐DNA—into Ψ(−) states having anomalously large negative ellipticity similar to the well‐known Ψ(−) forms produced by above‐critical concentrations of poly‐(ethylene oxide) and salt. These ethanol/salt‐induced transitions are all completely reversible and can occur without formation of any visible precipitates. We suggest that they represent predominantly tertiary structural changes of B‐form DNA molecules analogous to the changes which occur in several other systems where Ψ(+) ⇌ Ψ(−) interconversion has been reported. A “skein‐of‐yarn” model for the condensed tertiary (and quaternary, i.e., aggregated) state of the DNA affords one possible explanation for the inversions of ellipticity in all these cases. Such a model accords well with the accepted description of cholesteric liquid crystals.