Molecular-dynamics simulations of polyampholytes: Instabilities due to excess charges and external fields

In this paper we study the conformational properties of polyampholytes (PAs; polymers with positively and negatively charged monomers) by molecular-dynamics (MD) simulations and by scaling arguments. As is well known, in the absence of external electrical fields PAs with a small total charge $Q_{\mathrm{tot}}$ collapse into spherical globules, whereas PAs whose $Q_{\mathrm{tot}}$ exceeds a critical value $Q_c$ are expanded; our MD simulations confirm this fact. For $Q_{\mathrm{tot}}=0\mathrm{}$ we study the influence of external electric fields on single PAs. We find that the PA globule is unstable above a critical field strength $E_{c1\mathrm{}};$ for $E>\mathrm{}{E}_{c1\mathrm{}}$ the PA becomes highly stretched. Lowering the external field again, one observes hysteresis; the PA collapses back into a globule only for $E<\mathrm{}{E}_{c2\mathrm{}}$ where $E_{c2\mathrm{}}<E_{c1\mathrm{}}.$ In the weak coupling limit (when the intrachain electrostatic interactions can be neglected) the PA attains in an external field a trumpetlike shape, whose end-to-end distance obeys Pincus scaling.