Potential of mean force and transient states in polyelectrolyte pair complexation

Abstract

The association between polyelectrolytes (PEs) of the same size but opposite charge is systematically studied in terms of the potential of mean force (PMF) along their center-of-mass reaction coordinate via coarse-grained, implicit-solvent, explicit-salt computer simulations. The focus is set on the onset and the intermediate, transient stages of complexation. At conditions above the counterion-condensation threshold, the PE association process exhibits a distinct sliding-rod-like behavior where the polymer chains approach each other by first stretching out at a critical distance close to their contour length, then 'shaking hand' and sliding along each other in a parallel fashion, before eventually folding into a neutral complex. The essential part of the PMF for highly charged PEs can be very well described by a simple theory based on sliding charged 'Debye-Hückel' rods with renormalized charges in addition to an explicit entropy contribution owing to the release of condensed counterions. Interestingly, at the onset of complex formation, the mean force between the PE chains is found to be discontinuous, reflecting a bimodal structural behavior that arises from the coexistence of interconnected-rod and isolated-coil states. These two microstates of the PE complex are balanced by subtle counterion release effects and separated by a free-energy barrier due to unfavorable stretching entropy.1 arXiv:1602.06547v1 [cond-mat.soft]

Recent progress in the science of complex coacervation

“…29,109 This effect is often invoked during the complexation of two individual polyelectrolytes, 109 and is directly apparent from molecular simulation. [131][132][133][134] Despite the widespread use of this concept to describe complexation of pair complexes, 29,109 only recently has charge localization been regularly invoked in understanding bulk coacervation. 31,97,135…”

The Mechanism for siRNA Transmembrane Assisted by PMAL

“…Molecular simulations have been applied to probe the interactions among polyelectrolytes [ 26 , 27 , 28 ], including siRNA and synthetic polymers [ 29 ]. Ouyang et al adopted an atomic molecular dynamics simulation to study the complexation of short strand duplex RNA with four cationic carrier systems of varying charge and surface topology at different ratios.…”

Probing the protein corona around charged macromolecules: interpretation of isothermal titration calorimetry by binding models and computer simulations

“…For proteins interacting with the highly charged dPGS macromolecule, it was found that the intrinsic part is dominated by a highly localized electrostatic effect, the counterion-release (CR) contribution: For the highly charged dPGS macromolecule, strong chargerenormalization was observed by a massive uptake of counterions [37]. A few of those counterions "condensed" on the dPGS surface layer are liberated when the protein binds, whereupon an oppositely charged protein patch becomes a multivalent counterion for the polyelectrolyte [58][59][60][61][62][63][64]. The resulting favorable (purely entropic) free energy in dependence of the salt concentration c s can be formulated as:…”