(356l) Scaling Theory of Single-Chain Sequence-Specific Polyampholytes

The primary sequence of charged monomers in polyampholytes (PAs) crucially controls their physical properties. In this work, we use theory and simulations to reveal the role of sequences in single-chain conformational behaviors of PAs in salt-free solutions. PAs with the charge statistics given by the first-order Markov process are considered. In these PAs, charge “blockiness” is defined by the correlation λ along the chain, and they cover a wide spectrum of primary sequences ranging from alternating (λ = −1) to ideally random (λ = 0) to diblock PA/homopolyelectrolyte (λ → 1). First, globally neutral PAs with zero net charge are studied. By means of scaling and the random phase approximation, it is demonstrated that sufficiently long PAs of any statistics form globules under theta solvent conditions. However, the physical properties of these globules are sequence-dependent and defined by the charge blockiness, λ. The higher the clustering of the opposite charges, the denser and more compact the globule of the globally neutral PA. Three different scaling regimes are distinguished corresponding to the different ranges of the charge blockiness. These findings are supported by the results of coarse-grained molecular dynamics simulations. Then we pass to the case of non-neutral PAs where charge imbalance of the chains may result in their pearl-necklace or fully stretched (polyelectrolyte-like) conformations. The developed scaling approach shows that the necklace formation, structure, and dimensions are governed by the statistics of the opposite charges. Our findings provide a better insight into sequence-encoded conformations and conformational transitions in synthetic PAs and their biological analogs, intrinsically disordered proteins (IDPs).