Aggregation and counterion condensation in solution of charged proteinlike copolymers: A molecular-dynamics study

We present the results of molecular dynamics simulations of charged proteinlike hydrophobic–hydrophilic (ℋ𝒫) copolymers with a fixed charge distribution under pure solvent conditions. The processes of coil-to-globule transition, aggregation of polymer globules, and counterion condensation are studied in detail as a function of temperature. Various static structure factors and pair correlation functions, that occur in polyelectrolyte solutions, are also analyzed. Our simulations show that the chains pass through strong conformational changes while changing temperature. We find three different temperature regimes which are characterized by a different behavior of Coulomb energy, chain sizes, and pair correlation functions. In the high-temperature regime, at the reduced temperature T>3.0, the chains have an extended conformation with many hydrophobic blobs. As temperature is decreased, one observes a counterion condensation and sharp decrease in chain size. In this regime, we observe a solution of nonaggregating polymer globules which form a stable array resembling a charge-stabilized (micro)suspension or microgel phase. A further decrease in temperature (at T<0.5) leads to an aggregation of the isolated globules which form aggregates built up from individual globules connected by a layer of counterions condensed on the globular surface. It is found that the transitions between the temperature regimes are thermodynamically reversible.