Eur. Phys. J. E (2005) 16: 389-400
https://doi.org/10.1140/epje/i2004-10097-9

Direct observation of counterion organization in F-actin polyelectrolyte bundles

¹ Department of Materials Science & Engineering, Department of Physics, Department of Bioengineering, University of Illinois, IL 61801, Urbana-Champaign, USA
² School of Health Information Sciences, University of Texas Health Science Center at Houston, 7000 Fannin St., Suite 600, TX 77030, Houston, USA

^* e-mail: gclwong@uiuc.edu

Received: 8 September 2004
Accepted: 18 January 2005
Published online: 18 March 2005

Abstract

Attractions between like-charged polyelectrolytes have been observed in a variety of systems (W.M. Gelbart, R.F. Bruinsma, P.A. Pincus, V.A. Parsegian, Phys. Today 53, September issue, 38 (2000)). Recent biological examples include DNA, filamentous viruses, and F-actin. Theoretical investigations on idealized systems indicate that counterion correlations play a central role, but no experiments that specifically probe such correlations have been performed. Using synchrotron X-ray diffraction, we have directly observed the organization of multivalent ions on cytoskeletal filamentous actin (a well-defined biological polyelectrolyte) and found 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, the counterions organize into “frozen” ripples parallel to the actin filaments and form structures reminiscent of charge density waves. Moreover, these 1D counterion charge density waves form a coupled mode with twist deformations of the oppositely charged actin filaments. This counterion organization is not sensitive to thermal fluctuations in temperature range accessible to protein-based polyelectrolyte systems. Moreover, the counterion density waves are “pinned” to the spatial periodicity of charges on the actin filament even if the global filament charge density is varied, indicating the importance of charge periodicity on the polyelectrolyte substrate.