Charge Arrangement Determines the Sensitivity of Aggregation Patterns between Peptide-Chains to the Surrounding Ionic Environment.

The molecular basis for the liquid-liquid phase separation (LLPS) behavior of various biomolecular components in the cell is the formation of multivalent and low-affinity interactions. When the content of these components exceeds a certain critical concentration, the molecules will spontaneously coalesce to form a new liquid phase; i.e., LLPS occurs. Intrinsically disordered proteins (IDPs) are usually rich in amino acids with charged side-chains, and thus, LLPS-involving interactions between their side-chains are of great interest. However, the molecular details of the coalescence of such charged IDPs in a salt solution are still lacking. Here, we focus on two types of peptide-chains with oppositely charged amino acids in extreme arrangements and investigate their aggregation patterns in various ionic environments. The results show that the interaction patterns between peptide-chains with nonuniform charge arrangement sequences are more sensitive to the surrounding cationic environment, and Na⁺ ions are more likely to cause aggregation of ASP residues compared to Mg²⁺ ions. As the ionic concentration increases, the electrostatic interactions between oppositely charged residues are gradually converted into a negative-negative amino acid interaction network bridged by Na⁺ ions, while the positive charge-rich regions are more strongly inclined to be exposed to the solvent environment and gain greater freedom of movement. Simultaneously, this effect will reach saturation with a further increase of salt concentration. The present study enriches insights into the electrostatic dominant factors in phase separation phenomena at the atomic level, which will hopefully inspire the design and application of targeted LLPS in the future.