[Toroidal globular state of circular DNA]

Abstract

The influence of torsional elasticity of the double helix on compactization and structure of circular DNA in a compact form is studied in the case when the compact (globular) particle has a torus shape. For closed circular DNA the topological invariant, linking number of two strains, yields strict connection between conformation of double helix, considered as a unifilar homopolymer, and elastic energy of torsional twisting. The contribution of torsional elasticity to free energy of the toruslike globule is calculated. This contribution is shown to be proportional to the square of superturn's density. Torsional elasticity decreases the equilibrium radius of the toruslike globule formed by circular DNA in comparison with the case of linear DNA. Closure of linear DNA into a ring widens the stability range of the relatively short DNA compact form and tightens it for long DNA.