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Force curves for short fragment DNA at 0.2, 0.5, and 1.0 M NaCl obtained by x-ray scattering. Higher ionic strength reduces electrostatic repulsion, thus allowing DNA chains to come into closer proximity. For the range of DNA interaxial spacings observed in the cholesteric regime, the interaxial force decays exponentially with approximately four times the Debye screening length, indicating fluctuation-enhanced electrostatic interactions dominate (20,21).
