Necklace model of randomly charged polymers

Spatial conformations of randomly charged polymers (polyampholytes (PAs)) strongly depend on their total (excess) charge Q. For Q larger (smaller) that some critical value the PA is expanded (collapsed). The transition between the collapsed and the expanded states is reminiscent of the Rayley shape instability of a charged drop. The expanded states can be approximately described using the necklace model, as a chain of interconnected compact globules. Randomness of the charge sequence along the chain modifies the simple model creating wide distribution of sizes of the "beads" of the necklace. A simple mathematical model was proposed to describe the shape of the necklace.