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 mo lecular-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(tot) collapse into spherical globules, whereas PAs whos e Q(tot) exceeds a critical value Q(c) are expanded; our MD simulation s confirm this fact. For Q(tot)=0 we study the influence of external e lectric fields on single PAs. We find that the PA globule is unstable above a critical field strength E-c1; for E>E-c1 the PA becomes highly stretched. Lowering the external field again, one observes hysteresis ; the PA collapses back into a globule only for E<E-c2 where E-c2<E-c1 . In the weak coupling limit (when the intrachain electrostatic intera ctions can be neglected) the PA attains in an external field a trumpet like shape, whose end-to-end distance obeys Pincus scaling.