Magnetic birefringence study of the electrostatic and intrinsic persistence length of DNA

Magnetic birefringence experiments were performed on solutions of DNA of intermediate molecular weight at several concentrations ( c p ) over a wide range of ionic strengths (of NaCl and MgCl 2 ). The specific Cotton‐Mouton constant ( CM / c p ) is found to be independent of c p when contributions from c p to the ionic strength (μ eff ) are taken into account according to the concept of counterion condensation. For μ eff ≳ 10 −2 M , CM / c p is also independent of the ionic strength; the plateau value results in an acceptable value of the intrinsic persistence length, when a revised theoretical expression for the magnetic birefringence of wormlike chains is used, combined with new experimental data for the monomeric optical and magnetic anisotropy. For μ eff < 10 −2 M , CM / c p strongly, or wealky, increases with decreasing μ eff , depending on the valency of the counterion used (Na + or Mg 2+ , respectively). This increase agrees quantitatively with the variation of the electrostatic persistence length as predicted by Odijk [(1977) J. Polym. Sci. Polym. Phys. Ed. 15 , 477–483], Odijk and Houwaart [(1978) J. Polym. Sci. Polym. Phys. Ed. 16 , 627–639], and by Skolnick and Fixman [(1977) Macromolecules 10 , 944–948]. A comparison with other experimental data seems to reveal the importance of excluded‐volume effects, which are particularly pronounced in the low‐salt regime.