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

Abstract

The 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.

Transient electric birefringence measurements of the rotational and internal motions of a 1010 base pair DNA fragment - field strength and pulse length effects

“…The changes observed at decay times less than approximately 2 µß are not significant because data was collected only every 0.64 µß. The position of the rotational peak at 150-200 µß and the firet internal decay at [30][31][32][33][34][35][36][37][38][39][40] µß ie consistent with our previous measurements. When contin is used to analyze decays consisting of significant contributions from three or more decay times, the data are rarely accurate enough to precisely determine the time constants of each of the decays and we see an experiment to experiment variability in peak positions on the order of 30%.…”

Transient electric birefringence of N4 DNA (71 kb) in solution

“…However, multiexponential decays were found for longer DNAs. [11][12][13][14][15][16][17]19 The slowest relaxation time among the multidecays was mostly identified as the transverse rotational motion of the DNA molecular helix, while the faster ones might be associated with several mechansms, including segmental orienting, internal bending and twisting, and rotation of a semistiff chain.2,11 •13,16-19 The average relaxation time decreased with increasing field strength11,17,19 or with increasing frequency by means of an applied pulsed sine-wave electric field.4 Such relaxation time changes were attributed to the increase in the contribution of shorter segmental motions as the field strength was increased and the decrease in the contribution of longer segmental motions as the sine-wave frequency in the pulsed electric field was increased. The effect of pulse width on the relaxation time was also examined.…”