FIGURE 2.

Pressure in a hexagonal lattice of DNA, according to experiment and Poisson-Boltzmann theory. Experimental data points are from the data of Rau et al. (1984) for 25 mM and 5 mM Mg²⁺ concentrations. Our theoretical calculations follow from a discrete one-dimensional Poisson-Boltzmann solver, assuming cylindrical symmetry. The free energy was calculated as a sum of the Shannon entropy of the ions and the electrostatic energy of ions and DNA, with the zero point for the potential set so that internal and external ionic concentrations were related by a Boltzmann factor. The theoretical predictions differ from the experimental points by a factor of 10, although the slopes are approximately correct and it is difficult to distinguish between the data for the two different concentrations. Also shown is a least-squares fit to the empirical datapoints, resulting in the parameters c = 0.30 nm and F₀ = 1.2 × 10⁴ pN/nm².