Collective Behaviors in F-actin Polyelectrolytes

Polyelectrolytes (PE), polymers with ionizable groups, in polar solvents can dissociate into charged polymer chains (macroions) and small counterions [1]. Their importance in different fields like molecular biology and polymer industry leads to drastic need for understanding their rich physics. This significance will be increased by knowing that all nucleic acids, including DNA, RNA and also many protein assembled structures are PE. Due to complexity of the problem, and the size of the systems, we are far from complete solving the problem theoretically, while, computer simulations give us feasible tools to study the system.In a good solvent, in low concentration of the salt and low temperatures, PEs have a rod-like shape, while they go to random coil phase if the electrostatic repulsion is screened. The interaction between the rod-like PEs is the source of many interesting properties of their solution.F-actin, one of the fundamental components of eukaryotic cytoskeletons, is assembled by protein subgroups and is a rod-like PE in physiological environment. Recently, Wong et al. [2] have examined liquid crystalline phases of F-actin PE system in presence of multivalent salts, and they observed a new unexpected lamellar phase between isotropic and close-packed bundle phases when the density of ions changes [2].Because of computational cost, most of the simulation studies of PEs are limited to a few numbers of them in specific conditions [3, 4]. Effective interactions between two charged rod-like objects have been studied by Lee et al. [3]. Recently, Fazli et al. [4] have looked at their orientation ordering. They have shown two rods attract each other and have the tendency to be perpendicular in a specific salt concentration. This is a pseudo-stable state. Because of large entropic barrier it is very time consuming that a melt of PEs find their equilibrium configuration in molecular dynamics simulations. In the present study by introducing a soft core interaction between the PE monomers, we reduce the size of the barriers and let the melt relaxes in feasible simulation time. We simulate the collective behavior of F-actin PEs in presence of multivalent ions and observe their stable states, namely raft-like or closed packed bundle phases, as reported by Wong [