Z-FORM OF DNA - NONMONOTONIC CHANGE IN STABILITY WITH INCREASING IONIC-STRENGTH

To specify the stability area of the Z-DNA conformation as a function of ambient conditions (ionic strength, temperature, water activity), a 3D phase diagram for the (B,Z) equilibrium as exemplified with poly(d G-5ethyldC) was constructed experimentally. Its main peculiarity prove d to be a nonmonotonic change in Z-form stability with ionic strength, with a minimum at 0.1 M NaCl; i.e., an increase or decrease of salt c oncentration from the above value stabilized the Z form relative to th e B form. The shape of the phase diagram section (temperature, ionic s tength) was indicative of the existence of two B-type conformations, t heir equilibrium unaffected by ionic strength though sensitive to temp erature. The B-1 form was stable at t < 30 degrees C while the B-2 at t > 30 degrees C. Using the general 3D phase diagram together with the polyelectrolyte theory of the B-Z transition, thermodynamic parameter s were calculated for the B-1-Z, B-2-Z, and B-1-B-2 conformational shi fts. The aggregate of the results obtained with poly(dG-5ethyldC) and those obtained by us earlier with poly(dG-5methyldC) was in full quali tative and quantitative agreement with the polyelectrolyte model, and at the same time made unlikely the explanation of other researchers fo r the low-ionic Z B transition as being solely due to metal ion contam ination.