Dependence of DNA steady-state electric birefringence on field strength

A major task in our work has been to calculate the dependence of DNA steady-state electric birefringence on field strength for a ‘hinged bent rod’ (HBR) composed of two rod-like arms of length L with a restoring force proportional to the angle of bending at the hinge. The system is considered to have a permanent dipole moment due to asymmetric charge distribution with respect to its centre of diffusion. The electric and optical polarizability tensors of HBR macroions are also calculated. To calculate the electric birefringence, the statistical averages of the differential optical polarizability per molecule in the directions parallel and perpendicular to the applied electric field are made over all conformations as well as over all spatial orientations and all the velocities in the presence of the electric field. Using this approach, numerical calculations are carried out to achieve a fitting of electric birefringence experimental curves versus the square of the electric field for the sonicated and fractioned DNA and restriction enzyme fragments of DNA. It is shown that the HBR macroion presents a reversal of the birefringence sign at low electric fields for certain chain lengths, electric polarizability and stiffness constant values, in agreement with the experimental results obtained for DNA. It is observed that the HBR model adequately represents DNA elastic properties.