Polyampholyte sequence controls the type of electrostatic coil-globule transition in good solvent

Coarse-grained molecular dynamics simulations are employed to explore the conformational behavior of globally neutral polyampholytes under good solvent conditions. The interplay between non-Coulomb repulsions and sequence-dependent Coulomb attractions of monomers results in qualitatively different types of electrostatically driven chain contraction for diblock, random, and alternating statistics. At increasing the solution Bjerrum length lb, diblock and random polyampholytes exhibit a smooth coil-globule crossover, with the globule size continuously decreasing with lb according to the theoretical power laws. This confirms the scaling picture of the globule interior consisting of oppositely charged blobs attracting each other via long-range electrostatic forces and repelling via short-range two-body interactions. In contrast, alternating polyampholytes collapse completely analogously to neutral chains because Coulomb interactions in them are effectively dipole-dipole short-range. The transition region shrinks with increasing chain length, implying phase transition behavior in the limit of infinitely long chains. These collapse curves fall on the universal master curve, which is well-fitted by the theory of coil-globule transitions and demonstrates that Coulomb interactions in alternating polyampholytes renormalize (reduce) the dimensionless second virial coefficient by δB∝-lb2. This study highlights the profound impact of primary sequence on the conformational behavior of charge-balanced polyampholytes in good solvents, particularly the nature of the electrostatically driven coil-globule transition they undergo.