Statistical mechanics of the B----Z transition of DNA: contribution of diffuse ionic interactions.

A theoretical framework is constructed to treat the effects of salt ions on polyionic structural transitions in the absence of specific ion binding. As an application, the salt concentration-dependent part of the free energy difference governing the B----ZI transition of [d(C-G) . d(C-G)]6 in 1:1 electrolytes is calculated; quantitative agreement with the experimental findings is obtained. The effects of temperature and multivalent cations are also discussed. Preliminary calculations indicate that the ZII conformation in solution is thermodynamically less favorable than ZI in the high-salt regime (2.0-5.0 M) but more favorable than ZI below 2.0 M salt.