Abstract
Of the two known families of double‐stranded DNA conformations, A is stable in less‐polar, and B in more‐polar solutions. In some water‐nonelectrolyte solutions, B to A transition occurs when water activity in the system is near 0.8. In such systems, however, as water‐methanol, water‐ethylene glycol, etc., B conformation is stable at very low water activity. Hyperfine splitting constant of a spin label (A), a widely used criterion of the solvent polarity, was measured by EPR method in the solutions, in which B to A transition takes place at 25°C. A values in solutions of different compositions are close to each other. A method for quantitative estimation of Van der Waals and hydrogen‐bond contributions to A values is proposed. A values in water‐methanol solutions show that their polarity is too high for the A form to be stable. Decreasing temperature shifts the B‐A equilibrium to the B form, which correlates with increasing polarity of the water‐alcohol solutions, as measured by the EPR method. Thus B to A transition is mainly determined by the polarity of the environment, which, in turn, is determined by the ability of the solvent molecules to participate in Van der Waals interactions and hydrogen bonding.