Theta-point universality of polyampholytes with screened interactions

By an efficient algorithm, we evaluate exactly the disorder-averaged statistics of globally-neutral self-avoiding chains with quenched random charge q(i)= +/-1 in monomer i and nearest neighbor interactions proportional to q(i)q(j) on square (22 monomers) and cubic (16 monomers) lattices. At the Theta transition in two dimensions, the radius of gyration and entropic and crossover exponents are very compatible with the universality class of the corresponding transition of homopolymers. A further strong indication of such a class comes from direct comparison with the corresponding annealed problem. In three dimensions, classical exponents are recovered. The percentage of charge sequences leading to folding in a unique ground state approaches zero exponentially with the chain length.