Liquid-liquid Demixing Research Articles

Structural basis for the phase separation of the chromosome passenger complex.

The physical basis of phase separation is thought to consist of the same types of bonds that specify conventional macromolecular interactions yet is unsatisfyingly often referred to as 'fuzzy'. Gaining clarity on the biogenesis of membraneless cellular compartments is one of the most demanding challenges in biology. Here, we focus on the chromosome passenger complex (CPC), that forms a chromatin body that regulates chromosome segregation in mitosis. Within the three regulatory subunits of the CPC implicated in phase separation - a heterotrimer of INCENP, Survivin, and Borealin - we identify the contact regions formed upon droplet formation using hydrogen/deuterium exchange mass spectrometry (HXMS). These contact regions correspond to some of the interfaces seen between individual heterotrimers within the crystal lattice they form. A major contribution comes from specific electrostatic interactions that can be broken and reversed through initial and compensatory mutagenesis, respectively. Our findings reveal structural insight for interactions driving liquid-liquid demixing of the CPC. Moreover, we establish HXMS as an approach to define the structural basis for phase separation.

Polycomb repression of Hox genes involves spatial feedback but not domain compaction or phase transition.

Polycomb group proteins have a critical role in silencing transcription during development. It is commonly proposed that Polycomb-dependent changes in genome folding, which compact chromatin, contribute directly to repression by blocking the binding of activating complexes. Recently, it has also been argued that liquid-liquid demixing of Polycomb proteins facilitates this compaction and repression by phase-separating target genes into a membraneless compartment. To test these models, we used Optical Reconstruction of Chromatin Architecture to trace the Hoxa gene cluster, a canonical Polycomb target, in thousands of single cells. Across multiple cell types, we find that Polycomb-bound chromatin frequently explores decompact states and partial mixing with neighboring chromatin, while remaining uniformly repressed, challenging the repression-by-compaction or phase-separation models. Using polymer simulations, we show that these observed flexible ensembles can be explained by 'spatial feedback'-transient contacts that contribute to the propagation of the epigenetic state (epigenetic memory), without inducing a globular organization.

Chromosomal passenger complex condensates generate parallel microtubule bundles in vitro

The mitotic spindle contains many bundles of microtubules (MT) including midzones and kinetochore fibers, but little is known about how bundled structures are formed. Here, we show that the Chromosomal Passenger Complex (CPC) purified from E. coli undergoes liquid-liquid demixing (LLD) in vitro. An emergent property of the resultant condensates is to generate parallel MT bundles when incubated with free tubulin and GTP in vitro. We demonstrate MT bundles emerge from CPC droplets with protruding minus-ends that then grow into long, tapered MT structures. During this growth, we found the CPC in these condensates apparently reorganize to coat and bundle the resulting MT structures. CPC mutants attenuated for LLD or MT binding prevented the generation of parallel MT bundles in vitro and reduced the number of MTs present at spindle midzones in HeLa cells. Our data demonstrate that an in vitro biochemical activity to produce microtubule bundles emerges after the concentration of the CPC and provides models for how cells generate parallel-bundled MT structures that are important for the assembly of the mitotic spindle. Moreover, these data suggest that cells contain microtubule organizing centers that generate microtubule bundles that emerge with the opposite polarity from centrosomes.

Role of natural deep eutectic solvents (NADESs) in coagulation bath for PVDF-based membranes on enhanced permeation and separation of rare earth ions

Efficient separation of single rare earth element such as neodymium (Nd) has become an important research topic due to the supply crisis which seriously restricts the application and promotion of rare earth permanent magnets. PVDF-based separation membranes prepared by non-solvent induced phase separation (NIPS) method usually exhibited hydrophobic surface properties resulting in performance attenuation and operation cost increase. This article applied an easily available NADES from betaine and lactic acid to regulate and control the coagulation bath of a polymer inclusion membrane (PIM) for the separation of Nd, Sm and Dy ions. Evidences for the formation of DES were characterized via 1H NMR and FT-IR, and betaine showed more obvious changes than lactic acid during the formation of DES via hydrogen bonds. With the help of DES, pores on the surface were much larger and richer, and the migration of PO and P–O groups to the lower epidermal layer of the membrane made the surface layer become more hydrophilic. DES accelerated the liquid-liquid demixing process and led to faster membrane-forming speed and typical finger-like macrovoids and short vertical penetration channels, which could be observed at high DES contents, and a DES content of 5 wt% was found the most optimal considering the overall permeability and separation performances. As an effective means to improve membrane's performances, adding hydrophilic NADESs at a proper ratio in the coagulation bath would be conducive to form more porous structures, higher surface hydrophilicity, optimized permeability and separation performances.

Kinetic and Thermodynamic Studies in Cellulose Acetate-Polybutylene Succinate(CA-PBS)/Single Solvent/Water System for Desalination Membrane

The most important part of the membrane synthesis process so that it has the desired pores is the solidification process of the membrane, the process begins with a change from one liquid phase into two liquid phases (liquid-liquid demixing). At a certain period during demixing, the polymer-rich phase solidifies; thus, a dense membrane matrix is formed. Parameters that determine the mechanism of membrane formation are based on thermodynamics including phase separation of Solvent-Polymer-Non-solvent which is explained through a phase diagram (Flory-Huggins Theory). This study aims to determine the initial prediction of the formation of CA-PBS membranes with various solvents used and variations of non-solvents in the best system, which is proven by its characteristics and performance when applied to desalination membranes which include ternary diagrams using cloud point data, solubility parameters with Hansesn solubility, the solvent-non-solvent diffusivity using the Tyn Calus Equation approach and the morphological proofing of the membrane through SEM photos, and the performance of the resulting membrane through salt rejection and permeate flux. The results of the difference in solubility parameters are can be predicted that using DMF solvent on the CA-PBS membrane can reduce the pore size and eliminate voids and macrovoids in the membrane morphology.

Oxaliplatin disrupts nucleolar function through biophysical disintegration.

Platinum (Pt) compounds such as oxaliplatin are among the most commonly prescribed anti-cancer drugs. Despite their considerable clinical impact, the molecular basis of platinum cytotoxicity and cancer specificity remain unclear. Here we show that oxaliplatin, a backbone for the treatment of colorectal cancer, causes liquid-liquid demixing of nucleoli at clinically relevant concentrations. Our data suggest that this biophysical defect leads to cell-cycle arrest, shutdown of Pol I-mediated transcription, and ultimately cell death. We propose that instead of targeting a single molecule, oxaliplatin preferentially partitions into nucleoli, where it modifies nucleolar RNA and proteins. This mechanism provides a general approach for drugging the increasing number of cellular processes linked to biomolecular condensates.

Hydrophobic-cationic peptides modulate RNA polymerase ribozyme activity by accretion

Accretion and the resulting increase in local concentration is a widespread mechanism in biology to enhance biomolecular functions (for example, in liquid-liquid demixing phases). Such macromolecular aggregation phases (e.g., coacervates, amyloids) may also have played a role in the origin of life. Here, we report that a hydrophobic-cationic RNA binding peptide selected by phage display (P43: AKKVWIIMGGS) forms insoluble amyloid-containing aggregates, which reversibly accrete RNA on their surfaces in an RNA-length and Mg2+-concentration dependent manner. The aggregates formed by P43 or its sequence-simplified version (K2V6: KKVVVVVV) inhibited RNA polymerase ribozyme (RPR) activity at 25 mM MgCl2, while enhancing it significantly at 400 mM MgCl2. Our work shows that such hydrophobic-cationic peptide aggregates can reversibly concentrate RNA and enhance the RPR activity, and suggests that they could have aided the emergence and evolution of longer and functional RNAs in the fluctuating environments of the prebiotic earth.

Polymer Concentration and Liquid-Liquid Demixing Time Correlation with Porous Structure of Low Dielectric Polyimide in Diffusion-Driven Phase Separation.

Porous polyimide (PI) films are a promising low-k dielectric material for high-frequency data transmission with low signal attenuation. Pores are generated by non-solvent induced phase separation (NIPS) during phase inversion of polymer solution via non-solvent accumulation and solvent diffusion. In this study, aromatic PI was employed as a matrix for NIPS, and the influence of polymer concentration and liquid—liquid demixing time on the morphology of pores in the PI films was investigated. This ensured control over the porous structure of the PI film and provided desirable dielectric properties in a broad frequency range of 100 Hz–30 MHz (1.99 at 30 MHz) and thermal stability (Td5% > 576 °C, Tg > 391 °C). This study addresses the effect of polymer concentration and coagulation time on the morphology and physical properties of PI sponge films and provides guidance on the design and optimization of architectures for polymeric materials requiring pore modification.

Modeling Diffusive Mixing in Antisolvent Crystallization.

Diffusion controls local concentration profiles at interfaces between segregated fluid elements during mixing processes. This is important for antisolvent crystallization, where it is intuitively argued that local concentration profiles at interfaces between solution and antisolvent fluid elements can result in significant supersaturation overshoots over and above that at the final mixture composition, leading to poorly controlled nucleation. Previous work on modeling diffusive mixing in antisolvent crystallization has relied on Fickian diffusion, where concentration gradients are the driving force for diffusion. This predicts large overshoots in the supersaturation at interfaces between solution and antisolvent, as is often intuitively expected. However, chemical potential gradients provide a more physically realistic driving force for diffusion, and in highly nonideal solutions, such as those in antisolvent crystallization, this leads to nonintuitive behavior. In particular, as solute diffusion toward antisolvent is severely hindered, it can diffuse against its concentration gradient away from antisolvent. We apply thermodynamically consistent diffusion model based on the multicomponent Maxwell–Stefan formulation to examine diffusive mixing in a nonideal antisolvent crystallization system. Large supersaturation overshoots above that at the final mixture composition are not found when a thermodynamically consistent approach is used, demonstrating that these overshoots are modeling artifacts and are not expected to be present in physical systems. In addition, for certain conditions, localized liquid–liquid spinodal demixing is predicted to occur during the diffusive mixing process, even when the final mixture composition is outside the liquid–liquid phase separation region. Intermittent spinodal demixing driven by diffusive mixing may provide a novel explanation for differences of nucleation behaviors among various antisolvents.

Decoupling Complex Multi-Length-Scale Morphology in Non-Fullerene Photovoltaics with Nitrogen K-Edge Resonant Soft X-ray Scattering.

Complex morphology in organic photovoltaics (OPVs) and other functional soft materials commonly dictates performance. Such complexity in OPVs originates from the mesoscale kinetically trapped non-equilibrium state, which governs device charge generation and transport. Resonant soft X-ray scattering (RSoXS) has been revolutionary in the exploration of OPV morphology in the past decade due to its chemical and orientation sensitivity. However, for non-fullerene OPVs, RSoXS analysis near the carbon K-edge is challenging, due to the chemical similarity of the materials used in active layers. An innovative approach is provided by nitrogen K-edge RSoXS (NK-RSoXS), utilizing the spatial and orientational contrasts from the cyano groups in the acceptor materials, which allows for determination of phase separation. NK-RSoXS clearly visualizes the combined feature sizes in PM6:Y6 blends from crystallization and liquid-liquid demixing, while PM6:Y6:Y6-BO ternary blends with reduced phase-separation size and enhanced material crystallization can lead to current amplification in devices. Nitrogen is common in organic semiconductors and other soft materials, and the strong and directional N 1s → π* resonances make NK-RSoXS a powerful tool to uncover the mesoscale complexity and open opportunities to understand heterogeneous systems.

Opposite Effects Induced by Cholinium-Based Ionic Liquid Electrolytes in the Formation of Aqueous Biphasic Systems Comprising Polyethylene Glycol and Sodium Polyacrylate.

Cholinium-based ionic liquids ([Ch]-based ILs) were investigated as electrolytes in the formation of aqueous biphasic systems (ABS) composed of polyethylene glycol (PEG) and sodium polyacrylate (NaPA) polymers. Both enhancement and decrease in the liquid-liquid demixing ability induced by electrolytes in PEG-NaPA aqueous biphasic systems were observed. It is shown that the ILs that most extensively partition to the PEG-rich phase tend to act as inorganic salts enhancing the two-phase formation ability, while those that display a more significant partition to the NaPA-rich phase decrease the ABS formation capacity. The gathered results allowed us to confirm the tailoring ability of ILs and to identify, for the first time, opposite effects induced by electrolytes on the PEG-NaPA ABS formation ability. The distribution of the electrolyte ions between the coexisting phases and the polyelectrolyte ion compartmentalization are key factors behind the formation of PEG-NaPA-based ABS.

Minimal Microscopic Model for Liquid Polyamorphism and Waterlike Anomalies.

Liquid polyamorphism is the intriguing possibility for a single component substance to exist in multiple liquid phases. We propose a minimal model for this phenomenon. Starting with a binary lattice model with critical azeotropy and liquid-liquid demixing, we allow interconversion of the two species, turning the system into a single-component fluid with two states differing in energy and entropy. Unveiling the phase diagram of the noninterconverting binary mixture gives unprecedented insight on the phase behaviors accessible to the interconverting fluid, such as a liquid-liquid transition with a critical point, or a singularity-free scenario, exhibiting thermodynamic anomalies without polyamorphism. The model provides a unified theoretical framework to describe supercooled water and a variety of polyamorphic liquids with waterlike anomalies.

Amino-acid-based chiral ionic liquids characterization and application in aqueous biphasic systems

By using amino acids as anions, ten chiral ionic liquids (CILs) composed of tetrabutylammonium or cholinium as cations were synthesized by neutralization reactions and further characterized by assessing their optical rotation, thermophysical properties (melting and decomposition temperatures, density, viscosity and refractive index) and ecotoxicity against the marine bacterium Aliivibrio fischeri. The CILs are shown to display, in general, high thermal stability (> 439 K) and low to moderate toxicities (86-217 mg·L−1). It was found that the cation plays an important role in the density and viscosity of the CILs. Additionally, the effect of CILs optical configuration on these properties was evaluated by comparing the tetrabutylammonium D/L-phenylalaninate ([N4444][D/L-Phe]) and cholinium D/L-phenylalaninate ([N1112(OH)][D/L-Phe]) pairs. Finally, the CILs potential to form aqueous biphasic systems with sodium sulfate, citrate buffer and phosphate buffer was assessed and the ternary phase diagrams were determined. These allowed to infer the impact of the CILs’ cation, anion, and salt on the aqueous biphasic system formation. It was shown that the cation has a more pronounced impact on the aqueous biphasic system formation than the anion. Cholinium-based CILs failed to form aqueous biphasic systems with sodium sulfate under the tested conditions, contrary to the more hydrophobic tetrabutylammonium-based CILs. The ability of the tested salt and buffers to induce liquid-liquid demixing shows that citrate buffer and sodium sulfate represent the weakest and the strongest salting-out agents, respectively.

A co-casting route enables the formation of skinless, hydrophobic poly(vinylidene fluoride) membranes for DCMD

Unlike the dual-layer membranes in which dual-layers attached tightly after co-casting, the attainment of independent poly (vinylidene fluoride) (PVDF) membranes with uniformly rough surface structures necessitates the delamination of top-layer membranes from the underlying PVDF membranes. The coverage of top-layer membranes via co-casting is expected to reduce the solvent/nonsolvent exchange rate for the underlying PVDF membranes during a phase inversion process. In this context, a polyethersulfone (PES) membrane was co-cast onto a PVDF membrane followed by phase inversion in a water bath to fabricate PVDF membranes for membrane distillation (MD). The spontaneous delamination of PES from PVDF membranes occurs once the membrane solidifies. Due to the combined effect of the additional PES layer and the solvent retained at the PES/PVDF interface, delayed liquid-liquid (L-L) demixing and crystallization of the PVDF solution take place during the phase separation process, resulting in the formation of a porous surface with symmetric cross-section. Results show that the PVDF concentration significantly affects the membrane morphology. Increasing the PVDF content from 16 wt% to 20 wt% leads to a marked morphological change for membrane cross-section from cellular pores to PVDF particles, which indicates that the phase inversion mechanism varies from delayed L-L demixing to solid-liquid (S-L) demixing; this is mainly affected by the solution viscosity. Due to the roughness increment by the particulate surfaces, the hydrophobicity of co-cast PVDF membranes (PVDF-co) is distinctly enhanced compared to the single-layer PVDF membranes. Furthermore, the pore sizes of the PVDF-co membranes are enlarged, and the thicknesses are reduced. Therefore, the PVDF-co membranes have much higher fluxes than the single-layer PVDF membranes, especially for PVDF-co-20 membranes composed of large PVDF particles which show superior pore interconnectivity than the cellular pores. A long-term stability test is conducted with the PVDF-20-21 membrane for 48 h using a 3.5 wt% NaCl solution at the feed and permeate temperatures of 60 °C and 20 °C, respectively. Fairly steady fluxes are observed over the testing course, as well as a continuous high salt rejection of above 99.95%

Oxaliplatin Kills Cells via Liquid-Liquid Demixing of Nucleoli

Platinum (Pt) compounds such as oxaliplatin are amongst the most commonly prescribed anti-cancer drugs. Despite their considerable clinical impact, the molecular basis of platinum cytotoxicity and cancer specificity remain unclear. Here, we show that oxaliplatin, a backbone for the treatment of colorectal cancer, causes liquid-liquid demixing of nucleoli at clinically-relevant concentrations by interfering with the interaction networks that organize nucleoli. This biophysical defect leads to cell cycle arrest, impaired rRNA processing and shutdown of PolI-mediated transcription, ultimately resulting in cell death. We propose that instead of targeting a single molecule, oxaliplatin preferentially partitions into nucleoli where it modifies nucleolar RNA and proteins. This mechanism provides a blueprint for drugging the increasing number of cellular processes linked to biomolecular condensates.

Physical stability of hydroxypropyl methylcellulose-based amorphous solid dispersions: Experimental and computational study.

The preparation of an amorphous solid dispersion (ASD) is a promising strategy for improving the poor oral bioavailability of many active pharmaceutical ingredients (APIs). However, poor predictability of ASD long-term physical stability remains a prevalent problem. The purpose of this study was to evaluate and compare the predictive performance of selected models concerning solid-liquid equilibrium (SLE) curve and glass-transition temperature (Tg) line modeling of ibuprofen (IBU) in cellulosic polymers (i.e., hydroxypropyl methylcellulose (HPMC) and hydroxypropyl methylcellulose acetate succinate (HPMCAS)). For SLE curve modeling, an empiricalanalyticalapproach(Kyeremateng et al., 2014)and the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) equation of state (EOS) were chosen. Due to the unavailability of PC-SAFT parameters for both polymers, an approximation procedure for parametrization was applied. The Gordon-Taylor equation and Kwei equation were considered for Tg line determination. The impact of various computational set-ups (e.g., model parametrization or extrapolation length) on IBU solubility prediction at storage conditions was thoroughly investigated, assessed and confronted with the results from an 18-month physical stability study. IBU developed stable 20wt% API content ASDs with both HPMC and HPMCAS.The extrapolation behavior and subsequent ASD thermodynamic stability prediction at storage conditions deduced from the aforementioned models weresignificantly different. Overall, the PC-SAFT EOS predicted higher IBU solubility in both polymers and, thus, a lower recrystallization tendency when compared to the empirical analytical approach. At higherIBU concentrations, liquid-liquid demixing inIBU-polymer systems was predicted by the PC-SAFT EOS, which was in qualitative disagreement with experimental observation.

Purification of virus-like particles using aqueous biphasic systems composed of natural deep eutectic solvents

Virus-like particles (VLPs) mimic the structure of viral particles, presenting antigens in a repetitive and ordered structure, which makes them excellent vaccine immunogens. However, a major bottleneck in the VLPs industrial production process is the lack of robust purification methods, which can account for up to 80% of total manufacturing cost.Aqueous biphasic systems (ABS) are portrayed as viable and efficient platform in purification of wide variety of biomolecules, since they can be designed to be versatile, biocompatible and easy to scale-up, depending on the phase forming components. In the last years, natural deep eutectic solvents (NADES) have emerged and gained a lot of attention in several fields and different applications, including the ABS implementation, where they have been proposed as versatile, cost-effective and biocompatible phase forming compounds.In this work, a study of the liquid-liquid equilibria (LLE) of ABS composed of carbohydrates and carbohydrates-based NADES is reported and the mechanisms associated with phase equilibria discussed. The carbohydrates used in the NADES preparation are osmolytes, which can be regarded both as extractants and excipients that stabilize the final formulation of biopharmaceuticals, avoiding the tricky back extraction of VLPs. It is here shown that the use of NADES leads to an increase of two-phase region, when compared to the use of only one of the NADES components, thus offering liquid-liquid demixing at low concentrations of phase forming compounds. Furthermore, the cytotoxicity studies of these NADES and their individual components using two different human cell lines, HEK 293 and Huh-7, was evaluated, in order to conclude about the biocompatibility and suitability of proposed systems in vaccine development. Finally, the potential of the proposed ABS in purification of VLPs from bovine serum albumin (BSA) is demonstrated, while their structural properties are also assessed.

Epigenetic cell fate in Candida albicans is controlled by transcription factor condensates acting at super-enhancer-like elements

SummaryCell identity in eukaryotes is controlled by transcriptional regulatory networks (TRNs) that define cell type-specific gene expression. In the opportunistic fungal pathogen Candida albicans, TRNs regulate epigenetic switching between two alternative cell states, ‘white’ and ‘opaque’, that exhibit distinct host interactions. Here, we reveal that the transcription factors (TFs) regulating cell identity contain prion-like domains (PrLDs) that enable liquid-liquid demixing and the formation of phase-separated condensates. Multiple white-opaque TFs can co-assemble into complex condensates as observed on single DNA molecules. Moreover, heterotypic interactions between PrLDs supports the assembly of multifactorial condensates at a synthetic locus within live eukaryotic cells. Mutation of the Wor1 PrLD revealed that substitution of acidic residues abolished its ability to phase separate and to co-recruit other TFs in live cells, as well as its function in C. albicans cell fate determination. Together, these studies reveal that PrLDs support the assembly of TF complexes that control fungal cell identity and highlight parallels with the ‘super-enhancers’ that regulate mammalian cell fate.

Preparation and Characterization of Polyphenylsulfone (PPSU) Membranes for Biogas Upgrading.

Asymmetric polyphenylsulfone (PPSU) membranes were fabricated by a non-solvent induced phase inversion method. Glycerin and silica nanoparticles were added into the polymer solution to investigate their effects on the material properties and gas separation performance of prepared membranes. The morphology and structure of PPSU membranes were analyzed by scanning electron microscopy (SEM), the surface roughness of the selective layer was analyzed by atomic force microscopy (AFM), and the surface free energy was calculated based on the contact angle measurements by using various solvents. The gas separation performance of PPSU membranes was estimated by measuring the permeability of CO2 and CH4. The addition of glycerin as a nonsolvent into the polymer solution changed the cross-section structure from finger-like structure into sponge-like structure due to the delayed liquid-liquid demixing process, which was confirmed by SEM analysis. The incorporation of silica nanoparticles into PPSU membranes slightly increased the hydrophilicity, which was confirmed by water contact angle results. PPSU membrane fabricated from the polymer solution containing 10 wt.% glycerin showed the best CO2/CH4 selectivity of 3.86 and the CO2 permeability of 1044.01 Barrer. Mixed matrix PPSU membrane containing 0.1 wt.% silica nanoparticles showed the CO2/CH4 selectivity of 3.16 and the CO2 permeability of 1202.77 Barrer.

Kinetic modeling and simulation of non-solvent induced phase separation: Immersion precipitation of PVC-based casting solution in a finite salt coagulation bath

A mathematical model is developed to describe the phase inversion kinetics for the fabrication of porous PVC-based polymeric membranes under finite coagulation bath condition. The model accounts for the non-quasi-stationary diffusion of non-solvent across the solidified membrane, time-variant non-solvent concentration in the coagulation bath and swelling of the membrane while phase inversion. An analytical solution is obtained for the pseudo-steady-state model, whereas the unsteady-state phase inversion model is converted by finite integral transform to a set of first-order coupled ordinary differential equations, which are solved by the numerical means. The above models are tested with the real-time movement of the solidification front using PVC/N-methyl pyrrolidine (NMP)-based casting solution involving Pluronic F127 and bentonite additives, and CaCl2, NaCl and KCl salt coagulation bath as well as demineralized water. The proposed pseudo-steady-state and unsteady phase inversion models predict diffusivity of the non-solvent through membrane matrix and the diffusivity follows the following order for a given casting solution: KCl > NaCl > CaCl2 > water. The results reveal that the combined effect of Pluronic F127 and bentonite in PVC-NMP-based casting solution dominates over individual additives and it results from the higher non-solvent diffusivity with fast liquid-liquid demixing as well as the enhancement in membrane swelling.