30th Annual SSRL Users' Meeting — October 9-10, 2003

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.