EFFECT OF THE SUPERHELICITY ON THE DOUBLE HELIX TWIST ANGLE IN DNA

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 tertiary ordering (number of DNA axis turns τ ) and that of secondary ordering (duplex twist angle β) have been taken as thermodynamical parameters, which characterize the state of the supercoild DNA at the fixed linking number (Lk) value. Such a thermodynamical approaoh implies the phenomenological parameters of rigidities of twisting and supercoiling (Gβ, Gτ). Gτ/Gβ 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 valuee of Gτ and Gβ are estimated to be of the same order of magnitude. © 1979 Information Retrieval Limited.