The conformational phase diagram of charged polymers in the presence of attractive bridging crowders

TL;DR

This study uses molecular dynamics simulations to map the phase diagram of charged polymers with crowders and counterions, revealing three distinct conformational phases influenced by electrostatic and attractive interactions.

Contribution

It introduces a detailed phase diagram for charged polymers considering crowders and counterions, highlighting crowders' role in inducing collapse and phase transitions.

Findings

Three conformational phases identified: extended, counterion-condensed collapsed, crowder-bridged collapsed.

Crowders can induce polymer collapse even with electrostatic repulsion.

Direct phase transition from extended or counterion-condensed to crowder-bridged phase observed.

Abstract

Using extensive molecular dynamics simulations, we obtain the conformational phase diagram of a charged polymer in the presence of oppositely charged counterions and neutral attractive crowders for monovalent, divalent and trivalent counterion valencies. We demonstrate that the charged polymer can exist in three phases: (1) an extended phase for low charge densities and weak polymer-crowder attractive interactions ($CE$), (2) a collapsed phase for high charge densities and weak polymer-crowder attractive interactions, primarily driven by counterion condensation ($CCI$), and (3) a collapsed phase for strong polymer-crowder attractive interactions, irrespective of the charge density, driven by crowders acting as bridges or crosslinks ($CCB$). Importantly, the simulations reveal that the interaction with crowders can induce collapse, despite the presence of strong repulsive electrostatic interactions, and can replace condensed counterions to facilitate a direct transition from the $CCI$ and $CE$ phases to the $CCB$ phase.