EXCESS CHARGE IN POLYAMPHOLYTES

Polyampholytes (PAs) are composed of (quenched) random sequences of positive and negatively charged monomers. We show that the radius of gyration of a PA strongly depends on its charge Q, and is very weakly influenced by other aspects of the sequence. For Q < Q(c) almost-equal-to almost-equal-to q0 square-root N, where N is the number of monomers of charge +/- q0, the PA is compact, while for Q > Q(c) it is stretched. We explain this transition by analogy with the shape instability charged drops (where the Rayleigh charge plays the role of Q(c)), and suggest a ''necklace'' model for the shapes of highly charged PAs.