A numerical study of polyampholyte configuration

Monte Carlo simulation and variational mean field calculations are used to study the structure of isolated polyampholyte chains at conditions roughly corresponding to dilute aqueous solutions. The simulations are performed by modeling the polymer as a necklace of charged hard spheres connected by rigid bonds with free rotations. A random distribution of cationic and anionic groups on the chain is assumed and average properties for samples with restricted or fluctuating net charge on individual chains are computed. The chains swell with increasing net charge while they are contracted when a balance of positive and negative charges is attained. The variational mean field theory successfully describes the swelling at high net charge while it underestimates the attractive effects characteristic of neutral or nearly neutral chains. This difference is interpreted as a result of spatial correlations among ionized polyampholyte beads in compact coils. The effect is studied by determining the structure of ionic atmospheres surrounding individual charges in the coil in analogy with the known behavior of simple ionic solutions. The mean field model neglects this effect but still captures the essential features of the temperature dependence of the average coil size for both the ensemble with enforced chain neutrality and the unrestricted ensemble with fluctuations of charge on individual chains.