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 1D 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 25Mg ions within F-actin bundles using solid-state NMR. Preliminary measurements indicate that the longitudinal relaxation time T1 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 T1 for Mg2+ ions in the CDW is intermediate between typical liquid-like and solid-like values. (Some figures in this article are in colour only in the electronic version)