Theta-point universality of random polyampholytes with screened interactions

By an efficient algorithm, we evaluate exactly the disorder-averaged statis tics 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 The ta transition in two dimensions, the radius of gyration and entropic and cr ossover exponents are very compatible with the universality class of the co rresponding transition of homopolymers. A further strong indication of such a class comes from direct comparison with the corresponding annealed probl em. 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.