Revealing nanoscale structure and interfaces of protein and polymer condensates via cryo-electron microscopy.

Liquid-liquid phase separation (LLPS) is a ubiquitous demixing phenomenon observed in various molecular solutions, including in polymer and protein solutions. Demixing of solutions results in condensed, phase separated droplets which exhibit a range of liquid-like properties driven by transient intermolecular interactions. Understanding the organization within these condensates is crucial for deciphering their material properties and functions. This study explores the distinct nanoscale networks and interfaces in the condensate samples using a modified cryo-electron microscopy (cryo-EM) method. The method involves initiating condensate formation on electron microscopy grids to limit droplet growth as large droplet sizes are not ideal for cryo-EM imaging. The versatility of this method is demonstrated by imaging three different classes of condensates. We further investigate the condensate structures using cryo-electron tomography which provides 3D reconstructions, uncovering porous internal structures, unique core-shell morphologies, and inhomogeneities within the nanoscale organization of protein condensates. Comparison with dry-state transmission electron microscopy emphasizes the importance of preserving the hydrated structure of condensates for accurate structural analysis. We correlate the internal structure of protein condensates with their amino acid sequences and material properties by performing viscosity measurements that support that more viscous condensates exhibit denser internal assemblies. Our findings contribute to a comprehensive understanding of nanoscale condensate structure and its material properties. Our approach here provides a versatile tool for exploring various phase-separated systems and their nanoscale structures for future studies.

Liquid-liquid phase separation induced auto-confinement.

Confinement allows macromolecules and biomacromolecules to attain arrangements typically unachievable through conventional self-assembly processes. In the field of block copolymers, confinement has been achieved by preparing thin films and controlled solvent evaporation through the use of emulsions. A significant advantage of the confinement-driven self-assembly process is its ability to enable block copolymers to form particles with complex internal morphologies, which would otherwise be inaccessible. Here, we show that liquid-liquid phase separation (LLPS) can induce confinement during the self-assembly of a model block copolymer system. Since this confinement is driven by the block copolymers' tendency to undergo LLPS, we define this confinement type as auto-confinement. This study adds to the growing understanding of how LLPS influences block copolymer self-assembly and provides a new method to achieve confinement driven self-assembly.

End-of-life upcycling of polyurethanes using a room temperature, mechanism-based degradation.

A major challenge in developing recyclable polymeric materials is the inherent conflict between the properties required during and after their life span. In particular, materials must be strong and durable when in use, but undergo complete and rapid degradation, ideally under mild conditions, as they approach the end of their life span. We report a mechanism for degrading polymers called cyclization-triggered chain cleavage (CATCH cleavage) that achieves this duality. CATCH cleavage features a simple glycerol-based acyclic acetal unit as a kinetic and thermodynamic trap for gated chain shattering. Thus, an organic acid induces transient chain breaks with oxocarbenium ion formation and subsequent intramolecular cyclization to fully depolymerize the polymer backbone at room temperature. With minimal chemical modification, the resulting degradation products from a polyurethane elastomer can be repurposed into strong adhesives and photochromic coatings, demonstrating the potential for upcycling. The CATCH cleavage strategy for low-energy input breakdown and subsequent upcycling may be generalizable to a broader range of synthetic polymers and their end-of-life waste streams.

Discrete Libraries of Amphiphilic Poly(ethylene glycol) Graft Copolymers: Synthesis, Assembly, and Bioactivity.

Poly(ethylene glycol) (PEG) is an important and widely used polymer in biological and pharmaceutical applications for minimizing nonspecific binding while improving blood circulation for therapeutic/imaging agents. However, commercial PEG samples are polydisperse, which hampers detailed studies on chain length-dependent properties and potentially increases antibody responses in pharmaceutical applications. Here, we report a practical and scalable method to prepare libraries of discrete PEG analogues with a branched, nonlinear structure. These lipid-PEG derivatives have a monodisperse backbone with side chains containing a discrete number of ethylene glycol units (3 or 4) and unique functionalizable chain ends. Significantly, the branched, nonlinear structure is shown to allow for efficient nanoparticle assembly while reducing anti-PEG antibody recognition when compared to commercial polydisperse linear systems, such as DMG-PEG2000. By enabling the scalable synthesis of a broad library of graft copolymers, fundamental self-assembly properties can be understood and shown to directly correlate with the total number of PEG units, nature of the chain ends, and overall backbone length. These results illustrate the advantages of discrete macromolecules when compared to traditional disperse materials.

Observation of Liquid-Liquid-Phase Separation and Vesicle Spreading during Supported Bilayer Formation via Liquid-Phase Transmission Electron Microscopy.

Liquid-phase transmission electron microscopy (LP-TEM) enables the real-time visualization of nanoscale dynamics in solution. This technique has been used to study the formation and transformation mechanisms of organic and inorganic nanomaterials. Here, we study the formation of block-copolymer-supported bilayers using LP-TEM. We observe two formation pathways that involve either liquid droplets or vesicles as intermediates toward supported bilayers. Quantitative image analysis methods are used to characterize vesicle spread rates and show the origin of defect formation in supported bilayers. Our results suggest that bilayer assembly methods that proceed via liquid droplet intermediates should be beneficial for forming pristine supported bilayers. Furthermore, supported bilayers inside the liquid cells may be used to image membrane interactions with proteins and nanoparticles in the future.

A Close Look at Molecular Self-Assembly with the Transmission Electron Microscope.

Molecular self-assembly is pervasive in the formation of living and synthetic materials. Knowledge gained from research into the principles of molecular self-assembly drives innovation in the biological, chemical, and materials sciences. Self-assembly processes span a wide range of temporal and spatial domains and are often unintuitive and complex. Studying such complex processes requires an arsenal of analytical and computational tools. Within this arsenal, the transmission electron microscope stands out for its unique ability to visualize and quantify self-assembly structures and processes. This review describes the contribution that the transmission electron microscope has made to the field of molecular self-assembly. An emphasis is placed on which TEM methods are applicable to different structures and processes and how TEM can be used in combination with other experimental or computational methods. Finally, we provide an outlook on the current challenges to, and opportunities for, increasing the impact that the transmission electron microscope can have on molecular self-assembly.

pH-Triggered Release from Polyamide Microcapsules Prepared by Interfacial Polymerization of a Simple Diester Monomer.

The majority of current pH-triggered release systems is designed to respond to either low or high pH. Encapsulants based on polyampholytes are an example of materials that can respond to both acidic and basic pH. However, polyampholyte-based encapsulants generally possess a low loading capacity and have difficulty retaining their small-molecule cargo. The current work utilizes interfacial polymerization between polyamines and a pyromellitic diester diacid chloride to form high capacity "liquid core-shell" polyamide microcapsules that are stable in a dry or nonpolar environment but undergo steady, controlled release at pH 7.4 and accelerated release at pH 5 and pH 10. The rate of release can be tuned by adjusting the amine cross-linker feed ratio, which varies the degree of cross-linking in the polymer shell. The thin-shell microcapsule exhibited suitable barrier properties and tunable dual acid/base-triggered release, with applications in a wide range of pH environments.