Transient electric birefringence of T7 viral DNA

The electric birefringence properties of intact bacteriophage T7 DNA are investigated as a function of field strength and ionic environment. At low field strengths, E < 40 V/cm, Kerr behavior is observed. The birefringence decay kinetics are consistent with a Gaussian chain viscoelastic treatment for an induced dipole model of orientation. The observed relaxation time for the longest mode is 37 msec, in 0.5 m M Na + at 15°C, and is in agreement with flow experiments. In order to account for the observed differences in rise and decay kinetics, a field‐induced, anharmonic restoring force is postulated. The magnitude of the apparent induced dipole moment is found to be inversely proportional to the Debye‐Hückel shielding parameter κ for a series of Na + concentrations. The magnitude of the induced dipole moment is found to be very sensitive to the addition of Mg 2+ . When analyzed in terms of the counterion condensation theory of Manning, it appears that the induced dipole moment is proportional to the square of the reduced effective DNA charge density.