Counterion Liquid Dynamics in F-Actin Bundles: A High Resolution Inelastic X-Ray Scattering Study

The behavior of counterions plays a pivotal role in the interaction between charged polyelectrolytes. Previously, we observed the counterion distribution in bundles of the polyelectrolyte F-actin using small angle x-ray scattering (SAXS) and found that the ions take on a periodic arrangement in the form of 1D counterion density waves (CDW), coupling to twist deformations of the F-actin helix [Angelini, et. al., PNAS 100 (2003)]. To probe the \textbf{\textit{dynamics}} of the CDW ions, we carried out a series of $\sim $meV resolution inelastic x-ray scattering (IXS) experiments on F-actin bundles condensed by different ion species. We find a new acoustic-like mode associated with the CDW, which has dispersive properties sensitive to ion species as well as ion density.