Inter-Ion Correlations In One- And Two-Dimensional Biopolymer Systems
Robert Coridan, Physics, UIUC
The aim of the program of research is to study the dynamics of counterions condensed on biological polyelectrolytes. We recently found evidence for large-scale counterion organization on polyelectroytes recently, by investigating F-actin, a highly charged rod-like biopolymer, using SAXS at SSRL. Surprisingly, the counterions do not follow actin’s helical symmetry; rather, they organize into 1-D charge density waves (CDW) parallel to the actin filaments. Moreover, this CDW forms a coupled mode with torsional distortions of the oppositely charged actin, in a way analogous to the formation of polarons in ionic solids, and mediates attractions by facilitating a 'zipperlike' charge alignment between the counterions and polyelectrolyte. The dynamic behavior of collective ion excitations on polyelectrolytes, which would allow detailed comparison with theory, however, is experimentally completely unknown.
Preliminary IXS data shows the existence of a new acoustic phonon mode in samples of condensed actin bundles, and a dispersion relation has been found. Moreover, the width of this Brillouin excitation at high wave vector Q reveals unexpected dynamics due to ions interacting with their ‘cages’ of nearest neighbors. We have extended this research to two-dimensional ion distributions, where divalent ions are organized between lipid bilayers in a multilamellar complex. Preliminary results from these experiments will also be presented.