Accelerated Collapse Kinetics of Charged Polymers in Good Solvent: Role of Counterion Condensation

Abstract: We investigate the collapse kinetics of charged polymers (polyelectrolytes) induced by counterion condensation using coarse-grained molecular dynamics simulations. Under good solvent conditions, polyelectrolytes above the critical charge density ($A > A_c$) exhibit significantly faster collapse dynamics compared to neutral polymers, with dynamic scaling exponents ($\nu_c \approx 0.76-0.84$) distinctly smaller than those observed for neutral polymers ($\nu_c \approx 1.44$) . This accelerated collapse is driven primarily by three mechanisms: (1) local charge neutralization due to counterion condensation, which facilitates immediate local compaction, (2) screening of long-range electrostatic repulsions, reducing the conformational search space, and (3) bridging interactions mediated by multivalent counterions, enhancing efficient formation of intra-chain contacts. We systematically explore the effects of polymer length, charge density, and counterion valency (monovalent, divalent, and trivalent) on collapse dynamics, demonstrating that increased counterion valency significantly lowers the critical charge density required for collapse and accelerates the collapse process. Our findings highlight the limitations of modeling charged biopolymers using purely neutral coarse-grained models, underscoring the importance of electrostatic interactions and counterion dynamics in determining their kinetic pathways. These insights may aid in better understanding the folding, organization, and dynamics of inherently charged biomolecules, such as proteins and nucleic acids.