30th Annual SSRL Users' Meeting — October 9-10, 2003

Probing the Counterion Atmosphere around DNA by Anomalous Small-Angle X-ray Scattering

R. Das

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.