Clumps of randomly charged polymers: Molecular dynamics simulation of condensation, crystallization, and swelling

The behavior of randomly charged polyampholytes against a wide range of the Coulomb coupling parameter Gamma (the ratio of the Coulomb energy to thermal energy) is studied with the use of molecular dynamics simulations. Neutral polyampholyte collapses for Gamma>1, where large volume changes are due to multichain effects. Charged chains reptate significantly in a globule. Polyampholyte with widely extensible bonds condenses to a cubic crystal for Gamma much greater than 1, while that with finitely extensible bonds remains in an imperfectly ordered glass structure. Non-neutral polyampholyte whose charge offset exceeds 1/2N(1/2) behaves as polyelectrolyte: it consists of nonoverlapped chains for Gamma>1, and shrinks to the noncharged polymer regime for Gamma<1 (N is the number of charged monomers). Condensed counterions on polyampholyte screen the electric field, making non-neutral polyampholyte close to the neutral one. Added salt of comparable charge density as that of the polyampholyte further compactifies it. However, the addition of more-salt results in the weakening of the polyampholyte nature and reentrant swelling of non-neutral polyampholyte.