Scaling Theory of Neutral Sequence-Specific Polyampholytes

The primary sequence of charged monomers in a polyampholyte has a profound effect on its conformational properties. In this work, theory and simulations are used to predict how the sequence influences the single-chain behavior of globally neutral polyampholytes (PAs) under salt-free conditions. We consider PAs with Markov statistics of charges, where charge "blockiness"is defined by the correlation λ along the chain. These PAs cover a wide spectrum of primary sequences ranging from alternating (λ = -1) to ideally random (λ = 0) to diblock (λ → 1). In a theta solvent, sufficiently long PAs of any primary sequence form globules, but their internal structure, as well as the position and sharpness of the coil-globule transition, crucially depends on λ. Using scaling arguments, we demonstrate that the excess charge of the globule correlation volume (blob) increases with increasing λ, thereby inducing stronger Coulomb attractions between neighboring blobs of opposite charge and leading to higher density PA globules. Scaling predictions are supported by the results of the random phase approximation (RPA) and coarse-grained molecular dynamics simulations. A generalization to good and poor solvents is presented that enables the construction of the resulting scaling diagram of PA conformations in terms of solvent quality and charge blockiness, thereby providing a comprehensive molecular understanding of the role of the PA primary sequence in the chain structure.