Statistical mechanics of polyampholytes

Abstract

Polyampholytes are polymers with a random sequence of positive and negative charges along their backbone. We examine the question of whether such polymers swell or contract due to Coulomb interactions, by Monte Carlo simulations and analytical arguments. We find that the answer depends crucially on whether the polyampholyte is neutral or charged, in turn reflecting its preparation conditions. If the monomers comprising the chain are assembled together in an organic solvent, the strong Coulomb forces ensure the neutrality of the resulting polymer. The neutral polymer in any environment then behaves as a ``microelectrolyte'' and compactifies to screen the charges. There is no such bias towards neutrality if the polymer is prepared in a solution where the Coulomb forces are screened. The typical polyampholyte will then have a net charge due to the random imbalance of positive and negative components. Such a net charge is actually sufficient to stretch the chain in an environment with no Coulomb screening. Even with a net charge, the polyampholyte initially contracts at high temperatures before stretching at lower temperatures. A perturbative analysis, valid at high temperatures, describes this initial contraction.