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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 (cp) over a wide range of ionic strengths (of NaCl and MgCl2). The specific Cotton‐Mouton constant (CM/cp) is found to be independent of cp when contributions from cp to the ionic strength (μeff) are taken into account according to the concept of counterion condensation. For μeff ≳ 10−2M, CM/cp 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 μeff < 10−2M, CM/cp strongly, or wealky, increases with decreasing μeff, depending on the valency of the counterion used (Na+ or Mg2+, 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.