Structure and dynamics of condensed multivalent ions within polyelectrolyte bundles: a combined x-ray diffraction and solid-state NMR study

Like-charged polyelectrolytes can attract and condense into compact ordered states via counterion-mediated interactions (Gelbart et al 2000 Phys. Today 53 38-44). Recent examples include DNA toroids and F-actin bundles. We have investigated the structure and dynamics of condensed divalent ions within F-actin polyelectrolyte bundles. Using synchrotron x-ray diffraction, the structural organization of Ba2+ ions on F-actin has been directly observed. The Ba2+ ions organize into counterion charge density waves (CDWs) parallel to the actin filaments. Moreover, these 1 D counterion charge density waves couple to twist deformations of the oppositely charged actin filaments, and mediate attractions by effecting a 'zipper-like' charge alignment between the counterions and the polyelectrolyte charge distribution. We have also examined condensed divalent Mg-25 ions within F-actin bundles using solid-state NMR. Preliminary measurements indicate that the longitudinal relaxation time T-1 of Mg2+ ions decreases by approximately an order of magnitude as they organize into the CDW state within condensed F-actin bundles. The measured value of T-1 for Mg2+ ions in the CDW is intermediate between typical liquid-like and solid-like values.