CPM Seminar
Twisting, bundling and collapsing biopolymers:
The role of counterions in polyelectrolyte solutions
Erik Luijten
University of Illinois at Urbana-Champaign
Filamentous actin is an abundant biopolymer that can be condensed into
hexagonally-packed bundles by means of multivalent counterions. Recent x-ray
scattering experiments have probed the distribution of counterions inside
these self-assembled structures. Strikingly, the experiments revealed that
the counterions exhibit a periodical arrangement inside the bundles, whereas
the helical twist of the polymers changes upon condensation. By means of
molecular dynamics simulations of a coarse-grained model that explicitly
accounts for the charge distribution on the actin, we reproduce this periodic
arrangement and explain how the periodicity arises from an interplay between
the change in helical twist and the hexagonal packing of the bundle. Unlike
stiff polymers such as actin, flexible polyelectrolytes collapse under the
influence of multivalent counterions. The nature of this collapse and its
relation to `charge inversion' of the polyelectrolyte are the focus
of extensive theoretical debate. We use Monte Carlo and molecular dynamics
techniques to clarify the situation. In particular, we demonstrate how
various scenarios can arise depending on ionic size and valency.
Thursday, May 6th 2004, 15:30
Ernest Rutherford Physics Building, R.E. Bell Conference Room (room 103)
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