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 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.

As reported in the July 22, 2003, issue of PNAS, Physics graduate student Thomas Angelini and his colleagues have directly observed the organization of multivalent ions on cytoskeletal filamentous actin (a well-defined biological polyelectrolyte) using synchrotron x-ray diffraction and have discovered an unanticipated symmetry-breaking collective counterion mechanism for generating attractions.

Surprisingly, the counterions do not form a lattice that simply follows actin's helical symmetry; rather, they organize into one-dimensional (1D) charge density waves parallel to the actin filaments. Moreover, this 1D counterion charge density wave forms a coupled mode with twist distortions of the oppositely charged polyelectrolyte. This effect is analogous to superconductivity, in which a lattice distortion mediates an attraction between two like-charged particles; in the polyelectrolyte system, charged particles mediate an attraction between two like-charged distorted lattices.

Angelini is supervised by Professor of Materials Science and Engineering, Physics, and Bioengineering Gerard C.L. Wong. Related work of Professor Wong's is featured in this week's Physical Review Focus.

This work is based on work supported by National Science Foundation Grant NSF-DMR-0071761, the Beckman Young Investigator Program, Grant 00G0 from the Cystic Fibrosis Foundation, and the U.S. Department of Energy, Division of Materials Sciences under Award DEF G02-91ER45439, through the Frederick Seitz Materials Research Laboratory at the University of Illinois. The conclusions are those of the authors and not of the funding organizations.