No abstract

No abstract

An attempt to estimate the relative contributions of twisting and bending to the free energy of superhelix formation from the relaxed DNA is undertaken. The extent of teritiary ordering (number of DNA axis turns tau) and that of secondary ordering (duplex twist angle beta) have been taken as thermodynamical parameters, which characterize the state of the supercoild DNA at the fixed linking number (Lk) value. Such a thermodynamical approach implies the phenomenological parameters of rigidities of twisting and supercoiling (Gbeta, Gtau). Gtau/Gbeta ratio is estimated from the presented experimental data on the winding of the double helix upon increasing the ionic strength when twist alterations are followed by circular dichroism method. The adequacy of such interpretation of CD spectra changes are discussed. The values of Gtau and Gbeta are estimated to be of the same order of magnitude.

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 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 splittingconstant of a spin label ( A ) , a widely used criterion of the solvent polarity, was measured by EPR method in the solutions, in which fi 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.