Monte Carlo simulations of polyampholyte-polyelectrolyte complexes: effect of charge sequence and strength of electrostatic interactions.

We present the results of Monte Carlo simulations of complexation between polyampholyte and polyelectrolyte chains. Polymers are modeled as bead-spring chains of charged Lennard-Jones particles each consisting of 32 monomers. Formation of a polyampholyte-polyelectrolyte complex is driven by polarization-induced attractive interactions. The complex is usually formed at the end of the polyelectrolyte with the polyampholyte chain elongated and aligned along the polyelectrolyte backbone. This complex structure between the polarized polyampholyte chain and the polyelectrolyte leads to maximization of the attractive and minimization of the repulsive electrostatic interactions. The size of a polyampholyte in a complex is usually larger than that of an isolated polyampholyte chain. We also observed that initially collapsed polyampholytes undergo a coil-globule transition by forming a complex. The structure of a polyampholyte-polyelectrolyte complex was analyzed by tail and loop distribution functions. We have found that the number of loops increases while their sizes decrease with the strength of the electrostatic interactions. Polyampholytes with random charge sequence form stronger complexes with polyelectrolytes than those with alternating charge sequence. Polyampholytes with long blocky sequences form a double helix with a polyelectrolyte at sufficiently large values of the Bjerrum length.