FIGURE 4.

Osmotic stress force curves are shown for DNA condensed by A₃R₆, R₃A₃R₃, and R₆A₃. Force curves for DNA condensed by R₃A₂R₃ and R₃A₄R₃ are also shown for comparison. The arrows indicate equilibrium spacings in the absence of applied osmotic pressure for A₃R₆, R₃A₃R₃, and R₆A₃ condensed DNA. The solid lines are the best fits to Equation 3. Placement of A₃ makes little difference to the observed forces. The data with R₆A₃ show a transition at low pressures. The asterisk indicates the best fitting equilibrium spacing after the transition. Osmotic pressure and free energy contributions from the repulsive and attractive force components calculated from the fitting parameters are given in Table 1. The DNA pellet bathing solutions contained various concentrations of PEG, 10 mm Tris-Cl (pH 7.5), and 100 μm peptide.