The self-diffusion coefficient of a series of DNA fragments ranging from 280 to 5386 bases has been measured by fluorescence recovery after photobleaching after thermal denaturation in 8 M urea. The total persistence length p of single-stranded DNAs and its variation in ionic strength down to 10-3 M has been deduced. The importance of the value of p versus the pore size a and contour length L of the DNA in the optimization of sequencing by gel electrophoresis is emphasized.

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SynopsisMagnetic birefringence experiments were performed on solutions of DNA of intermediate molecular weight at several concentrations (c,) over a wide range of ionic strengths (of NaCl and MgC12). The specific Cotton-Mouton constant (CMIc,) is found to be independent of cp when contributions from cp to the ionic strength (peff) are taken into account according to the concept of counterion condensation. For p e f f t 10-2M, CMIc, 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 peff < 10-2M, CMIc, strongly, or weakly, increases with decreasing jieff, 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. 15,477-4831, Odijk and Houwaart [(1978) J. Polym. Sci. Polym. Phys. Ed. 16,627-6391, and by Skolnick and Fixman [(1977) Macromolecules 10,944-9481. A comparison with other experimental data seems to reveal the importance of excluded-volume effects, which are particularly pronounced in the low-salt regime.