Abstract
Small-angle X-ray scattering was used to measure the distribution of Tl{sup +} and Ba{sup 2+} counterions near the surface of cylindrical, persistence length (500 {angstrom}) DNA fragments in aqueous solutions with or without the presence of salts. The absolute scattering intensity distributions were compared with numerical solutions of the nonlinear Poisson-Boltzmann equation in a cell model. Quantitative agreement between calculations and experiments was obtained when they include contributions to the intensity from both the DNA and the diffuse ion cloud. The agreement is slightly better if they assume about 10% of the counterions are within the cylindrical DNA core, namely, in the major and minor grooves of the double helices.
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