Counterion Behavior on the Surfaces of Condensed Biopolymers

T. E. Angelini,¹ J. Butler,² H. Liang,² and G. C. L. Wong²

¹Department of Physics, University of Illinois, Urbana, IL 61801 USA
²Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801 USA

Electrostatics in aqueous media is commonly understood in terms of screened Coulomb interactions, where like-charged objects, such as polyelectrolytes, always repel. These intuitive expectations are based on mean field theories, such as the Poisson-Boltzmann formalism, which are routinely employed in colloid science and computational biology. Like-charge attractions, however, have been experimentally observed in a wide variety of systems. Intense theoretical scrutiny over the last 30 years has suggested that counterions play a central role, but no consensus exists for the precise mechanism. We have examined the organization of multivalent ions on actin filaments using synchrotron x-ray diffraction, and discovered a new collective mechanism in which the counterions self-organize into one-dimensional charge density waves parallel to the actin filaments and couple to their torsional distortions.