Biphasic Coacervation Controlled by Kinetics as Studied by De Novo-Designed Peptides

Coacervation is generally treated as a liquid-liquid phase separation process and is controlled mainly by thermodynamics. However, kinetics could make a dominant contribution, especially in systems containing multiple interactions. In this work, using peptides of (XXLY)6SSSGSS to tune the charge density and the degree of hydrophobicity, as well as to introduce secondary structures, we evaluated the effect of kinetics on biphasic coacervates formed by peptides with single-stranded oligonucleotides and quaternized dextran at varying pH values. Only in the case where the charge density is constant and the electrostatic interaction is the major driving force for Coacervation is the effect of kinetics negligible. When pH-dependent electrostatic interaction and hydrophobic interaction are involved or the peptides form secondary structures, the Coacervation process is then path-dependent, indicating that the kinetics controls the phase separation process. The Coacervation by combining two different peptides suggests that the peptide with a higher charge density plays a leading role in the early stage, while the cooperation of both peptides takes over afterward. Our work demonstrates that it is normal to observe coacervates with different morphologies and functions due to kinetic control, especially in living cells. Peptides with minimized sequences are a practical approach to reveal the mechanism of Coacervation processes controlled by kinetics.