7,647 research outputs found
Effect of chain stiffness on ion distributions around a polyelectrolyte in multivalent salt solutions
Ion distributions in dilute polyelectrolyte solutions are studied by means of
Langevin dynamics simulations. We show that the distributions depend on the
conformation of a chain while the conformation is determined by the chain
stiffness and the salt concentration. We observe that the monovalent
counterions originally condensed on a chain can be replaced by the multivalent
ones dissociated from the added salt due to strong electrostatic interaction.
These newly condensed ions give an important impact on the chain structure. At
low and at high salt concentrations, the conformation of a semiflexible chain
is rodlike. The ion distributions show similarity to those for a rigid chain,
but difference to those for a flexible chain whose conformation is a coil. In
the mid-salt region, the flexible chain and the semiflexible chain collapse but
the collapsed chain structures are, respectively, disordered and ordered
structures. The ion distributions hence show different profiles for these three
chain stiffness with the curves for the semiflexible chain lying between those
for the flexible and the rigid chains. The number of the condensed multivalent
counterions, as well as the effective chain charge, also shows similar
behavior, demonstrating a direct connection with the chain morphology.
Moreover, we find that the condensed multivalent counterions form triplets with
two adjacent monomers and are localized on the chain axis at intermediate salt
concentration when the chain stiffness is semiflexible or rigid. The
microscopic information obtained here provides valuable insight to the
phenomena of DNA condensation and is very useful for researchers to develop new
models.Comment: 28 pages, 10 figures, accepted for publication in JC
Unfolding Polyelectrolytes in Trivalent Salt Solutions Using DC Electric Fields: A Study by Langevin Dynamics Simulations
We study the behavior of single linear polyelectrolytes condensed by
trivalent salt under the action of electric fields through computer
simulations. The chain is unfolded when the strength of the electric field is
stronger than a critical value. This critical electric field follows a scaling
law against chain length and the exponent of the scaling law is ,
smaller than the theoretical prediction, [Netz, Phys. Rev. Lett. 90
(2003) 128104], and the one obtained by simulations in tetravalent salt
solutions, [Hsiao and Wu, J. Phys. Chem. B 112 (2008) 13179]. It
demonstrates that the scaling exponent depends sensitively on the salt valence.
Hence, it is easier to unfold chains condensed by multivalent salt of smaller
valence. Moreover, the absolute value of chain electrophoretic mobility
increases drastically when the chain is unfolded in an electric field. The
dependence of the mobility on electric field and chain length provides a
plausible way to impart chain-length dependence in free-solution
electrophoresis via chain unfolding transition induced by electric fields.
Finally, we show that, in addition to an elongated structure, a condensed chain
can be unfolded into an U-shaped structure. The formation of this structure in
our study is purely a result of the electric polarization, but not of the
elasto-hydrodynamics dominated in sedimentation of polymers.Comment: 15 pages, 7 figures, accepted for publication in Biomicrofluidic
- …