Counting Ions around DNA with Anomalous Small-Angle X-ray Scattering

The majority of charge-compensating ions around nucleic acids form a diffuse counterion “cloud” that is not amenable to investigation by traditional methods that rely on rigid structural interactions. Although various techniques have been employed to characterize the ion atmosphere around nucleic acids, only anomalous small-angle X-ray scattering (ASAXS) provides information about the spatial distribution of ions. Here we present an experimentally straightforward extension of ASAXS that can be used to count the number of ions around nucleic acids.