Etude des propriétés électro‐optiques de i' adn par biréfringence électrique a faible champ, 2. Mécanismes d'orientation et de relaxation

Abstract

AbstractThe electric birefringence of calf thymus DNA is measured at different low electric field strengths with rectangular and bipolar pulses as a function of concentration, ionic strength and molecular weight, and these results are interpreted in terms of orientation and relaxation mechanisms. The orientation of DNA is mainly due to an induced dipole, but the results reveal a slow induced moment, which is saturated, when low field strengths are applied. A well‐defined slow relaxation mechanism is demonstrated, which is attributed to the orientation of DNA as a whole. The long relaxation time decreases when high electric fields or high DNA concentrations are used, and the fact that the decay of the birefringence also contains short relaxation times does not reflect the polydispersity of the samples. The comparison between the molecular lengths determined from long relaxation times and the theoretical lengths of rigid cylinders shows an important coiling of DNA, when the fragments are longer than 110 – 120 nm.