Partially Globular Conformations from Random Charge Sequences

Overall charged polymers with quenched charge sequences often adopt partially globular structures which result from the interplay between the disorder in charge sequences and thermal fluctuations. Simple energetic considerations show that structures consisting of alike (equal-size-equal-charge) globules are not favorable: the structures are intrinsically heterogeneous. We predict the globule distributions with the lowest energies in the size-charge space. The favorable structures comprise large (undercharged) and a majority of small (overcharged) globules. These distributions build a well characterized compact subset, which suggests some order. We also perform large scale molecular dynamics simulations on random quenched +/- sequences. Simulation results show that, despite disorder, the random charge sequences preferentially visit the predicted low energy structures and the predicted order emerges in the pearl-size distribution. This good agreement validates a posteriori the simple expression used for the energy. Implications for polyampholytes, polyelectrolytes, and intrinsically disordered proteins are discussed.