Excess Charge in Polyampholytes

Polyampholytes (PAs) are polymers composed of (quenched) random sequences of positive and negatively charged monomeric groups. We show that the radius of gyration, Rg, of a PA strongly depends on its overall excess chargeQ, and is very weakly influenced by other aspects of the sequence. For Q < Qc ≈ q0 √ N , where N is the number of monomers of charge ±q0, the PA is compact, while for Q > Qc it is stretched. Some aspects of this behavior can be understood by analogy with the shape instability of a charged drop where the Rayleigh charge plays the role of Qc. PACS. 36.20.–r Macromolecules and polymer molecules PACS. 35.20.Bm General molecular conformation and symmetry; stereochemistry 64.60.–i General studies of phase transitions PACS. 41.20.Cv Electrostatics; Poisson and Laplace equations, boundary–value problems