A structural transition in duplex DNA induced by ethylene glycol.

The twist energy parameter (E T ) that governs the supercoiling free energy, and the linking difference (Δl) are measured for p306 DNA in solutions containing 0-40 w/v % ethylene glycol (EG). A plot of E T vs -In a w, where a w is the water activity, displays the full (reverse) sigmoidal profile of a discrete structural transition. A general theory for the effect of added osmolyte on a cooperative structural transition between two duplex states, 1 ⇄ 2, is formulated in terms of parameters applicable to individual base-pair subunits. The resulting fraction of base pairs in the 2-state (f 0 2 ) is incorporated into expressions for the effective torsion and bending elastic constants, the effective twist energy parameter (E eff T ), and the change in intrinsic twist (δl 0 ). Fitting the expression for E eff T to the measured E T values yields reasonably unambiguous estimates of E T1 and E T2, the midpoint value (In a w ) ½, and the midpoint slope (∂E T /∂ In a w ) ½ , but does not yield unambiguous estimates of the equilibrium constant (K 0 ), the difference in DNA-water preferential interaction coefficient (ΔΓ), or the inverse cooperativity parameter (J). Fitting a noncooperative model (assumed J = 1.0) to the data yields K 0 = 0.067 and ΔΓ = -30.0 per base pair (bp). Essentially equivalent fits are provided by models with a wide range of correlated J, ΔΓ, and K 0 values. Other results favor ΔΓ in the range -1.0 to 0, which then requires K 0 ≥ 0.914, and a cooperativity parameter, 1/J ≥ 30.0 bp. The measured δl 0 and circular dichroism (CD) at 272 nm are found to be compatible with curves predicted using the same f 0 2 values that best-fit the E T data. At least 7-10% of the base pairs are inferred to exist in the 2-state in 0.1 M NaCI in the complete absence of added osmolyte. Compared with the 1-state, the 2-state has a ∼2.0- to 2.1-fold greater torsion elastic constant, a ∼0.70-fold smaller bending elastic constant, a ∼0.91-fold smaller E T value, a ∼0.2% lower intrinsic twist, a somewhat lower CD near both 272 and 245 nm, and less water and/or more EG in its neighborhood. However, the relative change in preferential interaction coefficient associated with the transition is likely rather slight.