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Physics Department, Stanford University, Stanford, CA 94305
Counterion atmospheres condensed onto charged biopolymers strongly affect their
physical properties and biological functions, but have been difficult to
quantify experimentally. Here, monovalent and divalent counterion atmospheres
around DNA double helices in solution are probed using small-angle x-ray
scattering (SAXS) techniques. Modulation of the ion scattering factors by
anomalous (resonant) x-ray scattering and by interchanging ion identities
yields direct measurements of the scattering signal due to the spatial
correlation of surrounding ions to the DNA. The quality of the data permit, for
the first time, quantitative tests of extended counterion distributions
calculated from atomic-scale models of biologically relevant molecules.
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