[The Z-form of DNA. Nonmonotonous change in stability with increase in ionic strength]

To specify the stability area of the Z-DNA conformation as a function of ambient conditions (ionic strength, temperature, water activity) a 3-D phase diagram for the (B,Z) equilibrium as exemplified by poly(dG-5ethyldC) has been constructed in experiment. Its main peculiarity proves to be the nonmonotonic change in stability of the Z form with ionic strength, with a minimum at 0.1 M NaCl, i.e. the increase or decrease in salt concentration from the above value leads to stabilization of the Z form relatively to the B form. The shape of the phase diagram section (temperature, ionic strength) is indicative of the existence of two B-type conformations, equilibrium between them being unaffected by ionic strength though being sensitive to temperature. The B1 form is stable at t < 30 degrees C while the B2 one--at t > 30 degrees C. Using the general 3-D phase diagram together with a polyelectrolyte theory of the B-Z transition, thermodynamic parameters were calculated for the B1-Z, B2-Z, and B1-B2 conformational shifts as well as the B/Z junction energy. The totality of the obtained results with poly(dG-5ethyldC) and those earlier obtained by us with poly(dG-5methyldC) is in full qualitative and quantitative agreement with the polyelectrolyte model, and at the same time it makes unlikely the explanation suggested by others for the low-ionic Z to B transition as the one totally stipulated by a metal ion contamination.