Formation of Single Double-Layered Coacervate of Poly(N,N-diethylacrylamide) in Water by a Laser Tweezer

Multiphasic Coacervates Assembled by Hydrogen Bonding and Hydrophobic Interactions

Coacervation has emerged as a prevalent mechanism to compartmentalize biomolecules in living cells. Synthetic coacervates help in understanding the assembly process and mimic the functions of biological coacervates as simplified artificial systems. Though the molecular mechanism and mesoscopic properties of coacervates formed from charged coacervates have been well investigated, the details of the assembly and stabilization of nonionic coacervates remain largely unknown. Here, we describe a library of coacervate-forming polyesteramides and show that the water-tertiary amide bridging hydrogen bonds and hydrophobic interactions stabilize these nonionic, single-component coacervates. Analogous to intracellular biological coacervates, these coacervates exhibit "liquid-like" features with low viscosity and low interfacial energy, and form coacervates with as few as five repeating units. By controlling the temperature and engineering the molar ratio between hydrophobic interaction sites and bridging hydrogen bonding sites, we demonstrate the tuneability of the viscosity and interfacial tension of polyesteramide-based coacervates. Taking advantage of the differences in the mesoscopic properties of these nonionic coacervates, we engineered multiphasic coacervates with core-shell architectures similar to those of intracellular biological coacervates, such as nucleoli and stress granule-p-body complexes. The multiphasic structures produced from these synthetic nonionic polyesteramide coacervates may serve as a valuable tool for investigating physicochemical principles deployed by living cells to spatiotemporally control cargo partitioning, biochemical reaction rates, and interorganellar signal transport.

Formation of a core-shell droplet in a thermo-responsive ionic liquid/water mixture by using optical tweezers

Many chemical and biological processes involve phase separation; however, controlling this is challenging. Here, we demonstrate local phase separation using optical tweezers in a thermo-responsive ionic liquid/water solution. Upon near-infrared laser irradiation, a single droplet is formed at the focal spot. The droplet has a core consisting of highly concentrated ionic liquid. The mechanism of the core-shell droplet formation is discussed in view of the spatial distribution of optical and thermal potentials.

Diffusion of LLPS Droplets Consisting of Poly(PR) Dipeptide Repeats and RNA on Chemically Modified Glass Surface

The liquid-liquid phase separation (LLPS) of proteins and RNA molecules has emerged in recent years as an important physicochemical process to explain the organization of membrane-less organelles in living cells and cellular functions and even some fatal neurodegenerative diseases, such as Amyotrophic Lateral Sclerosis (ALS) due to the spontaneous condensation and growth of LLPS droplets. In general, the characterization of LLPS droplets has been performed by optical microscopy, where we need transparent substrates. By virtue of the liquid and wetting properties of LLPS droplets on a glass surface, there have been some technical protocols recommended to immobilize droplets on the surfaces. However, interactions between LLPS droplets and glass surfaces still remain unclear. Here, we investigated the surface diffusion of LLPS droplets on the glass surface to understand the interactions of droplets in a dynamic manner, and employed chemically modified glass surface with charges to investigate their Coulombic interaction with the surface. Using the single-particle tracking method, we first analyzed the diffusion of droplets on an untreated glass surface. Then, we compared the diffusion modes of LLPS droplets on each substrate and found that there were two major states of droplets on a solid surface: fix and diffusion mode for the LLPS droplet diffusion. While untreated glass showed a diffusion of droplets mainly, chemically modified glass with positive charges exhibited droplets fixed on the surface. It could arise from the Coulombic interaction between droplets and solid surface, where LLPS droplets have a negative ζ-potential. Our findings on the dynamics of LLPS at the solid/liquid interface could provide a novel insight to advance fundamental studies for understanding the LLPS formation.