Randomly Charged Polymers: Excess Charge Dependence of Spatial Configurations

TL;DR

This study investigates how the excess charge Q influences the spatial configurations of polyampholytes, revealing a critical charge Q_c that determines whether the polymer is compact or stretched, supported by analytical and simulation methods.

Contribution

It introduces a detailed analysis of the charge-dependent conformational transition in polyampholytes, combining theoretical, simulation, and enumeration approaches.

Findings

For Q < Q_c, polymers become compact at low T.

For Q > Q_c, polymers stretch and resemble a necklace of globules.

Transition between states is similar to charged drop breakup.

Abstract

Spatial configurations of randomly charged polymers, known as polyampholytes (PAs), are very sensitive to the overall excess charge Q. Analytical arguments, supported by Monte Carlo simulations and exact enumeration studies, lead to the following picture: For Q < Q_c = q_0 N^2 (q_0 is the elementary charge, N is the number of monomers in the polymer), the radius of gyration R_g of the polymer decreases with decreasing temperature T and the polymer becomes compact, while for Q>Q_c the polymer stretches with decreasing T. At low T, the dense states are described by Debye-Huckel theory, while the expanded states resemble a necklace of globules connected by strings. At such temperatures, the transition between the dense and the expanded states with increasing Q, is reminiscent of the breakup of a charged drop.