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synopsisA conductivity dispersion has been measured at very low frequencies (VLF) on several concentrated DNA solutions. By measuring simultaneously their electric birefringence decay, it is shown that the dielectric relaxation (which is related to the conductivity dispersion) is due to the molecular orientation. Different polarization mechanisms are discussed. It is concluded that the DNA polariaability measured in the VLF range can only be explained by the orientation of a permanent ionic dipole. It is suggested that such permanent dipoles could be caused by small differences in the ionic composition between the two molecular "ends;" the difference could either be stable (asymmetrical localization of protein impurities for instance) or transient (fluctuating dipoles explained by the Kirkwood-Schumaker theory).

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Birefringence studies on conducting solutions are not easily performed for several experimental reasons; these difficulties are discussed. An apparatus which allows Kerr effect experiments on high ionic strength solutions is described. It is built around a fast high-power switching device, which generates the electric pulse. Rectangular pulses of 30 kV can be applied to weakly conducting solutions (<10−2M NaCl). For a 0.1 M NaCl solution, the amplitude still reaches 15 kV. In any case, the 10%–90% rise and decay time is about 50 nsec. The pulse amplitude and duration are adjustable in large limits. This pulse generator can also be used to apply reversing pulses or sinusoïdal wave bursts. It is possible with this apparatus to measure the electric birefringence of urea, for instance, in high ionic strength solvents (NaCl 0.1 M).

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