Polarized fluorescence in an electric field: comparison with other electrooptical effects for rodlike fragments of DNA and the problem of the saturation of the induced moment in polyelectrolytes

Electrical birefringence, electrical dichroism and polarisation of fluorescence in an electric field experiments have been performed at high fields on sonicated fragments of DNA labelled with Acridine Orange. The latter electrooptical effect gives access to the field dependence of the fourth moment of the orientation function while the two former give access to the field dependence of the second moment. The origin of the large departure from an E2 dependence at rather low degrees of orientation is extensively discussed. Following a suggestion of Shirai on the calculation of orientational averages for a saturated induced moment, we can show that this model rationalizes the existence of a linear E dependence of the orientation factor at intermediate fields and explains very well our experimental results. When applied to previous dichroic data at higher fields it shows that the low value of the dichroism at saturation introduced to fit with other models, in contradiction with the absence of base tilting in the B form of DNA, is not required for a quantitative fit with this new orientation mechanism. The transition from an E2 dependence at low fields to an E dependence at intermediate fields gives an estimate of the field required for the saturation of the ionic polarisation E approximately 6 kV/cm.