Recent progress in the science of complex coacervation

Subcompartmentalization of polyampholyte species in organelle-like condensates is promoted by charge-pattern mismatch and strong excluded-volume interaction

Polyampholyte field theory and explicit-chain molecular dynamics models of sequence-specific phase separation of a system with two intrinsically disordered protein (IDP) species indicate consistently that a substantial polymer excluded volume and a significant mismatch of the IDP sequence charge patterns can act in concert, but not in isolation, to demix the two IDP species upon condensation. This finding reveals an energetic-geometric interplay in a stochastic, "fuzzy" molecular recognition mechanism that may facilitate subcompartmentalization of membraneless organelles.

Molecular Design of Chemically Fueled Peptide-Polyelectrolyte Coacervate-Based Assemblies

Complex coacervated-based assemblies form when two oppositely charged polyelectrolytes combine to phase separate into a supramolecular architecture. These architectures range from complex coacervate droplets, spherical and worm-like micelles, to vesicles. These assemblies are widely applied, for example, in the food industry, and as underwater or medical adhesives, but they can also serve as a great model for biological assemblies. Indeed, biology relies on complex coacervation to form so-called membraneless organelles, dynamic and transient droplets formed by the coacervation of nucleic acids and proteins. To regulate their function, membraneless organelles are dynamically maintained by chemical reaction cycles, including phosphorylation and dephosphorylation, but exact mechanisms remain elusive. Recently, some model systems also regulated by chemical reaction cycles have been introduced, but how to design such systems and how molecular design affects their properties is unclear. In this work, we test a series of cationic peptides for their chemically fueled coacervation, and we test how their design can affect the dynamics of assembly and disassembly of the emerging structures. We combine them with both homo- and block copolymers and study the morphologies of the assemblies, including morphological transitions that are driven by the chemical reaction cycle. We deduce heuristic design rules that can be applied to other chemically regulated systems. These rules will help develop membraneless organelle model systems and lead to exciting new applications of complex coacervate-based examples like temporary adhesives.

Temperature-Responsive Peptide-Nucleotide Coacervates

Coacervates are a type of liquid–liquid phase separated (LLPS) droplets that can serve as models of membraneless organelles (MLOs) in living cells. Peptide–nucleotide coacervates have been widely used to mimic properties of ribonucleoprotein (RNP) granules, but the thermal stability and the role of base stacking is still poorly understood. Here, we report a systematic investigation of coacervates formed by five different nucleoside triphosphates (NTPs) with poly-l-lysine and poly-l-arginine as a function of temperature. All studied combinations exhibit an upper critical solution temperature (UCST), and a temperature-dependent critical salt concentration, originating from a significant nonelectrostatic contribution to the mixing free energy. Both the enthalpic and entropic parts of this nonelectrostatic interaction decrease in the order G/A/U/C/T, in accordance with nucleobase stacking free energies. Partitioning of two dyes proves that the local hydrophobicity inside the peptide–nucleotide coacervates is different for every nucleoside triphosphate. We derive a simple relation between the temperature and salt concentration at the critical point based on a mean-field model of phase separation. Finally, when different NTPs are mixed with one common oppositely charged peptide, hybrid coacervates were formed, characterized by a single intermediate UCST and critical salt concentration. NTPs with lower critical salt concentrations can remain condensed in mixed coacervates far beyond their original critical salt concentration. Our results show that NTP-based coacervates have a strong temperature sensitivity due to base stacking interactions and that mixing NTPs can significantly influence the stability of condensates and, by extension, their bioavailability.

Dynamic Coupling in Unentangled Liquid Coacervates Formed by Oppositely Charged Polyelectrolytes

We develop a scaling theory that predicts the dynamics of symmetric and asymmetric unentangled liquid coacervates formed by solutions of oppositely-charged polyelectrolytes. Symmetric coacervates made from oppositely-charged polyelectrolytes consist of polycations and polyanions with equal and opposite charge densities along their backbones. These symmetric coacervates can be described as mixtures of polyelectrolytes in the quasi-neutral regime with a single correlation length. Asymmetric coacervates are made from polycations and polyanions with unequal charge densities. The difference in charge densities results in a double semidilute structure of asymmetric coacervates with two correlation lengths, one for the high-charge-density and the other for the low-charge-density polyelectrolytes. We predict that the double-semidilute structure in asymmetric coacervates results in a dynamic coupling which increases the friction of the high-charge-density polyelectrolyte. This dynamic coupling increases the contribution to the zero-shear viscosity of the high-charge-density polyelectrolyte. The diffusion coefficient of the high-charge-density polyelectrolyte is predicted to depend on the concentration and degree of polymerization of the low-charge-density polyelectrolyte in the coacervate if the size of the low-charge-density polymer is smaller than the correlation length of the high-charge-density polymer. We also predict a non-monotonic salt concentration dependence of the zero-shear viscosity of asymmetric coacervates.

Can oppositely charged polyelectrolyte stars form a gel? A simulational study

We present a Langevin molecular dynamics study of an equimolar mixture of monodispersed oppositely charged di-block four-armed polyelectrolyte stars. We used an implicit solvent coarse-grained representation of the polyelectrolyte stars, and varied the length of the terminal charged blocks that reside on each arm. By varying the polymer concentration, we computed PV diagrams and determined the free-swelling equilibrium concentration with respect to a pure water reservoir as a function of the charged block length. We investigated various structural properties of the resulting equilibrium structures, like the number of ionic bonds, dangling arms, isolated stars, and cluster sizes. The ionic bonds featured a broad distribution of the number of arms involved and also displayed a distribution of net charges peaked around the neutral ionic bond. Our main result is that for charged block length equal to 4 and 5 ionized beads the resulting macro-aggregate spans the box and forms a network phase. Furthermore, we investigated the restructuring dynamics of ionic bonds; the results suggested both short bond lifetimes and a high frequency of ballistic association/dissociation events. Bonds result strong enough to yield a stable gel phase, but they are still weak enough to allow network restructuring under thermal fluctuations.

100th Anniversary of Macromolecular Science Viewpoint: Attractive Soft Matter: Association Kinetics, Dynamics, and Pathway Complexity in Electrostatically Coassembled Micelles

Electrostatically coassembled micelles constitute a versatile class of functional soft materials with broad application potential as, for example, encapsulation agents for nanomedicine and nanoreactors for gels and inorganic particles. The nanostructures that form upon the mixing of selected oppositely charged (block co)polymers and other ionic species greatly depend on the chemical structure and physicochemical properties of the micellar building blocks, such as charge density, block length (ratio), and hydrophobicity. Nearly three decades of research since the introduction of this new class of polymer micelles shed significant light on the structure and properties of the steady-state association colloids. Dynamics and out-of-equilibrium processes, such as (dis)assembly pathways, exchange kinetics of the micellar constituents, and reaction-assembly networks, have steadily gained more attention. We foresee that the broadened scope will contribute toward the design and preparation of otherwise unattainable structures with emergent functionalities and properties. This Viewpoint focuses on current efforts to study such dynamic and out-of-equilibrium processes with greater spatiotemporal detail. We highlight different approaches and discuss how they reveal and rationalize similarities and differences in the behavior of mixed micelles prepared under various conditions and from different polymeric building blocks.

ATP mediated stimuli responsive supramolecular assembly

Adenosine triphosphate has been employed as a biomolecular building block to fabricate pH and enzyme responsive compartmentalized supramolecular assemblies sequestering silver nanoparticles (AgNPs) and doxorubicin in the core and increase the therapeutic efficacy. Detailed investigations reveal that meticulous design can integrate chemical enrichment, stimuli responsiveness and targeted delivery within compartmentalized models.

Decoupling salt- and polymer-dependent dynamics in polyelectrolyte complex coacervates via salt addition

In polyelectrolyte complex coacervates, changes in salt concentration and changes in polymer concentration are typically strongly coupled, complicating interpretation of the salt- and polymer-concentration-dependent dynamics of these materials. To address this problem, we developed a "salt addition" method for preparation of complex coacervates that allows the salt concentration of a coacervate sample to be varied without changing its polymer concentration. This method was used to prepare coacervates of poly(styrene sulfonate) (PSS) with poly(diallyldimethylammonium chloride) (PDADMAC) with salt concentrations between 1.2 and 2 M and volume fractions of polymer between 0.1 and 0.25. Characterization of these samples by small-amplitude oscillatory shear rheology revealed that the relaxation times scale significantly more strongly with polymer volume fraction than has been previously assumed, highlighting the need to account for both salt and polymer-dependent contributions to the dynamics of these complex materials.

Electrostatically Driven Topological Freezing of Polymer Diffusion at Intermediate Confinements

Breaking the paradigm that polymers in crowded aqueous media obey Einstein's law of diffusion, we report a localized nondiffusive hierarchical metastable state at intermediate confinements. Combining electrostatic and topological effects, we can tune the propensity of this new universality class in a quasicoacervate gel system consisting of guest polyamino acid chains inside an oppositely charged host hydrogel. Our observations offer strategies for controlled release and retention of macromolecules in aqueous crowded media, while opening a new direction for understanding topologically frustrated dynamics in polymers and other soft matter systems.

Designer Condensates: A Toolkit for the Biomolecular Architect

Protein phase separation has emerged as a novel paradigm to explain the biogenesis of membraneless organelles and other so-called biomolecular condensates. While the implication of this physical phenomenon within cell biology is providing us with novel ways for understanding how cells compartmentalize biochemical reactions and encode function in such liquid-like assemblies, the newfound appreciation of this process also provides immense opportunities for designing and sculpting biological matter. Here, we propose that understanding the cell's instruction manual of phase separation will enable bioengineers to begin creating novel functionalized biological materials and unprecedented tools for synthetic biology. We present FASE as the synthesis of the existing sticker-spacer framework, which explains the physical driving forces underlying phase separation, with quintessential principles of Scandinavian design. FASE serves both as a designer condensates catalogue and construction manual for the aspiring (membraneless) biomolecular architect. Our approach aims to inspire a new generation of bioengineers to rethink phase separation as an opportunity for creating reactive biomaterials with unconventional properties and to encode novel biological function in living systems. Although still in its infancy, several studies highlight how designer condensates have immediate and widespread potential applications in industry and medicine.

Encapsulation and sedimentation of nanomaterials through complex coacervation

HYPOTHESIS

Nanoparticles removal from seawage water is a health and environmental challenge, due to the increasing use of these materials of excellent colloidal stability. Herein we hypothesize to reach this objective through complex coacervation, a straightforward, low-cost process, normally accomplished with non-toxic and biodegradable macromolecules. Highly dense polymer-rich colloidal droplets (the coacervates) obtained from a reversible charge-driven phase separation, entrap suspended nanomaterials, allowing their settling and potential recovery.

EXPERIMENTS

In this work we apply this process to highly stable aqueous colloidal dispersions of different surface charge, size, type and state (solid or liquid). We systematically investigate the effects of the biopolymers excess and the nanomaterials concentration and charge on the encapsulation and sedimentation efficiency and rate. This strategy is also applied to real laboratory water-based wastes.

FINDINGS

Long-lasting colloidal suspensions are succesfully destabilized through coacervate formation, which ensures high nanomaterials encapsulation efficiencies (~85%), payloads and highly tranparent supernatants (%T ~90%), within two hours. Lower polymer excess induces faster clearance and less sediments, while preserving effective nanomaterials removal. Preliminary experiments also validate the method for the clearance of real water residuals, making complex coacervation a promising scalable, low-cost and ecofriendly alternative to concentrate, separate or recover suspended micro/nanomaterials from aqueous sludges.

Polysaccharide nanoparticles: from fabrication to applications

The present review highlights the developments in polysaccharide nanoparticles with a particular focus on applications in biomedicine, cosmetics and food.

Efficient Synthesis of Stable Polyelectrolyte Complex Nanoparticles by Electrostatic Assembly Directed Polymerization

Polyelectrolyte complex nanoparticles with integrated advances of coacervate complexes and nanomaterials have attracted considerable attention as soft templates and functional nano-carriers. Herein, a facile and robust strategy, namely electrostatic assembly directed polymerization (EADP), for efficient and scalable preparation of stable coacervate nanoparticles is presented. With homo-polyelectrolyte PAA (polyacrylic acid) as template and out of charge stoichiometry, the cationic monomers are polymerized together with cross-linkers, which creates coacervate nanoparticles featuring high stability against salt through one-pot synthesis. The particle size can be tuned by varying the cross-linker amount and salt concentrations during the polymerization and the composition of nanoparticles, as well as the corresponding properties can be regulated by combining different charged blocks from both strong and weak ionic monomers. The strategy can tolerate both high monomer concentrations and increased volume of up to l L, which is favorable for scaled-up preparations. Moreover, the coacervate nanoparticles can be freeze-dried to produce a product in powder form, which can be redispersed without any effect on the particle size and size distribution. Finally, the obtained nanoparticles loaded with enzyme and Au nanoparticles exhibit enhanced catalytic performance, demonstrating a great potential for exploring various applications of coacervate particles as soft and functional nano-carriers.

Liquid-Liquid Phase Separation of Peptide/Oligonucleotide Complexes in Crowded Macromolecular Media

The membraneless organelles (MLOs) and coacervates of oppositely charged polyelectrolytes are both formed by liquid-liquid phase separation. To reveal how the crowded cell interior regulates the MLOs, we chose the coacervates formed by peptide S5 and single-stranded oligonucleotide (ss-oligo) at 1:1 charge ratio and investigated the phase separation processes in polyacrylamide (PAM) and poly(ethylene oxide) (PEO) media at varying concentrations. Results show that the droplet formation unit is the neutral primary complex, instead of individual S5 or ss-oligo. Therefore, the coacervation process can be described by the classic theory of nucleation and growth. The dynamic scaling relationships show that S5/ss-oligo coacervation undergoes in sequence the heterogeneous nucleation, diffusion-limited growth, and Brownian motion coalescence with time. The inert crowders generate multiple effects, including accelerating the growth of droplets, weakening the electrostatic attraction, and slowing down or even trapping the droplets in the crowder network. The overall effect is that both the size and size distribution of the droplets decrease with increasing crowder concentration, and the effect of PEO is stronger than that of PAM. Our study provides a further step toward a deeper understanding of the kinetics of MLOs in crowded living cells.

The role of hydrophobicity in supramolecular polymer/surfactant catalysts: An understandable model for enzymatic catalysis

Enzymes are highly significant catalysts, essential to biological systems, and a source of inspiration for the design of artificial enzymes. Although many models have been developed describing enzymatic catalysis, a deeper understanding of these biocatalysts remains a major challenge. Herein we detail the formation, characterization, performance, and catalytic mechanisms of a series of bio-inspired supramolecular polymer/surfactant complexes acting as artificial enzymes. The supramolecular complexes were characterized and exhibited exceptional catalytic efficiency for the dephosphorylation of an activated phosphate diester, the reaction rate being highly responsive to: (a) pH, (b) surfactant concentration, and (c) the length of the hydrophobic chain of the surfactant. Under optimal conditions (at pH > 8 for the more hydrophobic systems and at pre-micellar concentrations), enzyme-like rate enhancements of up to 6.0 × 10⁹-fold over the rate of the spontaneous hydrolysis reaction in water were verified. The catalytic performance is a consequence of synergy between the hydrophobicity of the aggregates and the catalytic functionalities of the polymer and the catalytic mechanism is modulated by the nature of the hydrophobic pockets of these catalysts, changing from a general base mechanism to a nucleophilic mechanism as the hydrophobicity increases. Taken as a whole, the present results provide fundamental insights, through an understandable model, which are highly relevant to the design of novel bioinspired enzyme surrogates with multifunctional potentialities for future practical applications.

Structure and stoichiometry of CTAB-DNA complexes

We have studied the structure of cetyltrimethylammonium bromide-DNA complexes using small angle x-ray diffraction and elemental analysis. These complexes exhibit a two-dimensional hexagonal phase. The diffraction data have been analyzed using electron density models based on two different structures of these complexes proposed in the literature, which differ in the micelle to DNA stoichiometry. The structure with a 1:2 micelle-DNA stoichiometry is found to be more consistent with the diffraction data. Furthermore, this structure is also supported by the stoichiometry deduced from elemental analysis. Madelung energies of the two structures, calculated from the electrostatic interaction between their cylindrical constituents, give insight into their relative stability.

Effect of polyampholyte net charge on complex coacervation between polyampholytes and inorganic polyoxometalate giant anions

The effect of net charge of zwitterionic polymers on the phase behavior and viscoelastic properties of hybrid polyampholyte-polyoxometalate (POM) complexes in salted aqueous solutions is investigated with polyampholyte copolymers consisting of both positively and negatively charged monomers. Zwitterionic polyampholytes of varied net charge, abbreviated as PA_xM_y, are synthesized by varying the feeding molar ratio of negatively charged 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) to positively charged [3-(methacryloylamino)propyl]trimethylammonium chloride (MAPTAC) monomers in aqueous solution. The coacervate formation between PA_xM_y and inorganic anionic metatungstate POM ({W₁₂}) in LiCl added aqueous solutions can be enhanced by increasing the molar fraction of positively charged MAPTAC monomer and LiCl concentration. The salt-broadened coacervation, clearly distinct from the salt-suppressed one between oppositely charged polyelectrolytes, suggests the account of zwitterion-anion pairing for PA_xM_y-{W₁₂} coacervate formation due to stronger binding of multivalent {W₁₂} giant ions with PA_xM_y than simple ions. Importantly, as AMPS or MAPTAC monomer fraction in polyampholytes is varied by merely ±5% from the effective net neutral case, the viscoelasticity of PA_xM_y-{W₁₂} coacervates can be modified by 4-5 folds, suggesting a new tuning parameter to fine control the macroionic interactions and material properties of biomimetic complex coacervates.

Super Gas Barrier of a Polyelectrolyte/Clay Coacervate Thin Film

Transparent polymeric thin films with high oxygen barrier are important for extending the shelf life of food and protecting flexible organic electronic devices. Polyelectrolyte/clay multilayer nanocoatings are shown to exhibit super gas barrier performance, but the layer-by-layer assembly process requires numerous deposition steps. In an effort to more quickly fabricate this type of barrier, a polyelectrolyte/clay coacervate composed of branched polyethyleneimine (PEI), poly(acrylic acid) (PAA), and kaolinite (KAO) clay is prepared and deposited in a single step, followed by humidity and thermal post-treatments. When deposited onto a 179 µm poly(ethylene terephthalate) (PET) film, a 4 µm coacervate coating reduces the oxygen transmission rate (OTR) by more than three orders of magnitude, while maintaining high transparency. This single-step deposition process uses only low-cost, water-based components and ambient conditions, which can be used to for sensitive food and electronics packaging.

Towards Decoding the Sequence-Based Grammar Governing the Functions of Intrinsically Disordered Protein Regions

A substantial portion of the proteome consists of intrinsically disordered regions (IDRs) that do not fold into well-defined 3D structures yet perform numerous biological functions and are associated with a broad range of diseases. It has been a long-standing enigma how different IDRs successfully execute their specific functions. Further putting a spotlight on IDRs are recent discoveries of functionally relevant biomolecular assemblies, which in some cases form through liquid-liquid phase separation. At the molecular level, the formation of biomolecular assemblies is largely driven by weak, multivalent, but selective IDR-IDR interactions. Emerging experimental and computational studies suggest that the primary amino acid sequences of IDRs encode a variety of their interaction behaviors. In this review, we focus on findings and insights that connect sequence-derived features of IDRs to their conformations, propensities to form biomolecular assemblies, selectivity of interaction partners, functions in the context of physiology and disease, and regulation of function. We also discuss directions of future research to facilitate establishing a comprehensive sequence-function paradigm that will eventually allow prediction of selective interactions and specificity of function mediated by IDRs.

Spatiotemporal Formation and Growth Kinetics of Polyelectrolyte Complex Micelles with Millisecond Resolution

We have directly observed the in situ self-assembly kinetics of polyelectrolyte complex (PEC) micelles by synchrotron time-resolved small-angle X-ray scattering, equipped with a stopped-flow device that provides millisecond temporal resolution. A synthesized neutral-charged diblock polycation and homopolyanion that we have previously investigated as a model charge-matched, core-shell micelle system were selected for this work. The initial micellization of the oppositely charged polyelectrolytes was completed within the dead time of mixing of 100 ms, followed by micelle growth and equilibration up to several seconds. By combining the structural evolution of the radius of gyration (R_g) with complementary molecular dynamics simulations, we show how the self-assemblies evolve incrementally in size over time through a two-step kinetic process: first, oppositely charged polyelectrolyte chains pair to form nascent aggregates that immediately assemble into spherical micelles, and second, these PEC micelles grow into larger micellar entities. This work has determined one possible kinetic pathway for the initial formation of PEC micelles, which provides useful physical insights for increasing fundamental understanding self-assembly dynamics, driven by polyelectrolyte complexation that occurs on ultrafast time scales.

Ion Pairing and the Structure of Gel Coacervates

A scaling model for the structure of coacervates is presented for mixtures of oppositely-charged polyelectrolytes of both symmetric and asymmetric charge-densities for different degrees of electrostatic strength and levels of added salt. At low electrostatic strengths, weak coacervates, with the energy of electrostatic interactions between charges less than the thermal energy, k B T, are liquid. At higher electrostatic strengths, strong coacervates are gels with crosslinks formed by ion pairs of opposite charges bound to each other with energy higher than k B T. Charge-symmetric coacervates are formed for mixtures of oppositely-charged polyelectrolytes with equal and opposite charge-densities. While charge-symmetric weak coacervates form a semidilute polymer solution with a correlation length equal to the electrostatic blob size, charge-symmetric strong coacervates form reversible gels with a correlation length on the order of the distance between bound ion pairs. Charge-asymmetric coacervates are formed from mixtures of oppositely-charged polyelectrolytes with different charge-densities. While charge-asymmetric weak coacervates form double solutions with two correlation lengths and qualitatively different chain conformations of polycations and polyanions, charge-asymmetric strong coacervates form bottlebrush and star-like gels. Unlike liquid coacervates, for which an increase in the concentration of added salt screens electrostatic interactions, causing structural rearrangement and eventually leads to their dissolution, the salt does not affect the structure of strong coacervates until ion pairs dissociate and the gel disperses.

Emerging trends in the dynamics of polyelectrolyte complexes

Polyelectrolyte complexes (PECs) are highly tunable materials that result from the phase separation that occurs upon mixing oppositely charged polymers. Over the years, they have gained interest due to their broad range of applications such as drug delivery systems, protective coatings, food packaging, and surface adhesives. In this review, we summarize the structure, phase transitions, chain dynamics, and rheological and thermal properties of PECs. Although most literature focuses upon the thermodynamics and application of PECs, this review highlights the fundamental role of salt and water on mechanical and thermal properties impacting the PEC's dynamics. A special focus is placed upon experimental results and techniques. Specifically, the review examines phase behaviour and salt partitioning in PECs, as well as different techniques used to measure diffusion coefficients, relaxation times, various superpositioning principles, glass transitions, and water microenvironments in PECs. This review concludes with future areas of opportunity in fundamental studies and best practices in reporting.

Ubiquity of complex coacervation of DNA and proteins in aqueous solution

We report complex coacervation between a primarily hydrophobic protein, elastin, and a strong polyanion DNA (2 kbp) in aqueous and salty solutions at room temperature, 25 °C. The associative interaction at fixed elastin and varying DNA concentration, thereby maintaining a mixing ratio of r = [DNA] : [elastin] = 0.0027 to 0.093, was probed. What distinguishes this study from protein-DNA coacervation reported earlier is that the protein used here was mostly a hydrophobic polyampholyte with low linear charge density, and its complementary polyelectrolyte, DNA, concentration was chosen to be extremely small (1-35 ppm). The interaction profile was found to be strongly hierarchical in the mixing ratio, defined by three distinct regions: (i) Region I (r < 0.02) was defined as the onset of primary binding leading to condensation of DNA; (ii) Region II (0.02 < r < 0.08) indicated secondary binding which led to the formation of fully charge neutralized complexes signaling the onset of coacervation; and (iii) Region III (0.08 < r < 0.12) revealed growth of insoluble complexes of large size facilitating liquid-solid phase separation. The degree of complex coacervation was suppressed in the presence of a monovalent salt implying that screened Coulomb interactions governed the binding. Small angle neutron scattering data attributed an amorphous structure to the coacervates. The elastin-DNA system belongs to a rare class of interacting biopolymers where very weak electrostatic interactions may drive coacervation, thereby implying that coacervation between DNA and proteins may be ubiquitous.

Liquid Crystal Peptide/DNA Coacervates in the Context of Prebiotic Molecular Evolution

Liquid–liquid phase separation (LLPS) phenomena are ubiquitous in biological systems, as various cellular LLPS structures control important biological processes. Due to their ease of in vitro assembly into membraneless compartments and their presence within modern cells, LLPS systems have been postulated to be one potential form that the first cells on Earth took on. Recently, liquid crystal (LC)-coacervate droplets assembled from aqueous solutions of short double-stranded DNA (s-dsDNA) and poly-L-lysine (PLL) have been reported. Such LC-coacervates conjugate the advantages of an associative LLPS with the relevant long-range ordering and fluidity properties typical of LC, which reflect and propagate the physico-chemical properties of their molecular constituents. Here, we investigate the structure, assembly, and function of DNA LC-coacervates in the context of prebiotic molecular evolution and the emergence of functional protocells on early Earth. We observe through polarization microscopy that LC-coacervate systems can be dynamically assembled and disassembled based on prebiotically available environmental factors including temperature, salinity, and dehydration/rehydration cycles. Based on these observations, we discuss how LC-coacervates can in principle provide selective pressures effecting and sustaining chemical evolution within partially ordered compartments. Finally, we speculate about the potential for LC-coacervates to perform various biologically relevant properties, such as segregation and concentration of biomolecules, catalysis, and scaffolding, potentially providing additional structural complexity, such as linearization of nucleic acids and peptides within the LC ordered matrix, that could have promoted more efficient polymerization. While there are still a number of remaining open questions regarding coacervates, as protocell models, including how modern biologies acquired such membraneless organelles, further elucidation of the structure and function of different LLPS systems in the context of origins of life and prebiotic chemistry could provide new insights for understanding new pathways of molecular evolution possibly leading to the emergence of the first cells on Earth.

Active coacervate droplets as a model for membraneless organelles and protocells

Membraneless organelles like stress granules are active liquid-liquid phase-separated droplets that are involved in many intracellular processes. Their active and dynamic behavior is often regulated by ATP-dependent reactions. However, how exactly membraneless organelles control their dynamic composition remains poorly understood. Herein, we present a model for membraneless organelles based on RNA-containing active coacervate droplets regulated by a fuel-driven reaction cycle. These droplets emerge when fuel is present, but decay without. Moreover, we find these droplets can transiently up-concentrate functional RNA which remains in its active folded state inside the droplets. Finally, we show that in their pathway towards decay, these droplets break apart in multiple droplet fragments. Emergence, decay, rapid exchange of building blocks, and functionality are all hallmarks of membrane-less organelles, and we believe that our work could be powerful as a model to study such organelles.

Membraneless organelles are liquid-liquid phase-separated droplets whose behaviour can be regulated by chemical reactions, but this process is poorly understood. Here, the authors report model membraneless organelles based on coacervate droplets that show fuel-driven dynamic behaviour and concentrate functional RNA.

Microphase separation and aggregate self-assembly in brushes of oppositely charged polyelectrolytes triggered by ion pairing

This work applies a molecular theory to study the formation of lateral self-assembled aggregates in mixed brushes composed of polyanion and polycation chains. In order to overcome the well-known limitations of mean-field electrostatics to capture polyelectrolyte complexation, the formation of ion pairs between anionic and cationic groups in the polyelectrolytes is explicitly modeled in our theory as an association reaction. This feature is essential to capture the microphase separation of the mixed brush and the formation of lateral aggregates triggered by polyelectrolyte complexation. The effects of solution pH and ionic strength, surface coverage, and chain length on the morphology of the mixed brush are systematically explored. It is shown that increasing salt concentration leads to the rupture of polyelectrolyte complexes and the stabilization of the homogeneous, non-aggregated brush, providing that the formation of ion pairs between the polyelectrolytes and the salt ions in solution is explicitly accounted for by the theory. The inclusion of ion-pairing association reactions between oppositely charged polyelectrolytes within a mean-field description of electrostatics emerges from this work as a useful and simple theoretical approach to capture the formation of polyelectrolyte complexes and their responsiveness to solution ionic strength and pH.

Heteroprotein complex coacervation: Focus on experimental strategies to investigate structure formation as a function of intrinsic and external physicochemical parameters for food applications

Proteins are important components of foods, because they are one of the essential food groups, they have many functional properties that are very useful for modifying the physicochemical and textural properties of processed foods and possess many biological activities that are beneficial to human health. The process of heteroprotein complex coacervation (HPCC) combines two or more proteins through long-range coulombic interaction and specific short-range forces, creating a liquid-liquid colloid, with highly concentrated protein in the droplet phase and much more diluted-protein in the bulk phase. Coacervates possess novel, modifiable, physicochemical characteristics, and often exhibit the combined biological activities of the protein components, which makes them applicable to formulated foods and encapsulation carriers. This review discusses research progress in the field of HPCC in three parts: (1) the basic and innovative experimental methods and simulation tools for understanding the physicochemical behavior of these heteroprotein supramolecular architectures; (2) the influence of environmental factors (pH, mixing ratio, salts, temperature, and formation time) and intrinsic factors (protein modifications, metal-binding, charge anisotropy, and polypeptide designs) on HPCC; (3) the potential applications of HPCC materials, such as encapsulation of nutraceuticals, nanogels, emulsion stabilization, and protein separation. The wide diversity of possible combinations of proteins with different properties, endows HPCC materials with great potential for development into highly-innovation functional food ingredients.

Adhesive Coacervates Driven by Hydrogen-Bonding Interaction

Coacervation plays a critical role in numerous biological activities such as constructing biological tissues and achieving robust wet adhesion of marine sessile organisms, which conventionally occurs when oppositely charged polyelectrolytes are mixed in aqueous solutions driven by electrostatic attraction. Here, a novel type of adhesive coacervate is reported, driven by hydrogen-bonding interactions, readily formed by mixing silicotungstic acid and nonionic polyethylene glycol in water, providing a new approach for developing coacervates from nonionic systems. The as-prepared coacervate is easily paintable underwater, show strong wet adhesion to diverse substrates, and has been successfully applied as a hemostatic agent to treat organ injuries without displaying hemolytic activity, while with inherent antimicrobial properties thus avoiding inflammations and infections due to microorganism accumulation. This work demonstrates that coacervation can occur in salt-free environments via non-electrostatic interactions, providing a new platform for engineering multifunctional coacervate materials as tissue glues, wound dressings and membrane-free cell systems.

Interface Engineering in Multiphase Systems toward Synthetic Cells and Organelles: From Soft Matter Fundamentals to Biomedical Applications

Synthetic cells have a major role in gaining insight into the complex biological processes of living cells; they also give rise to a range of emerging applications from gene delivery to enzymatic nanoreactors. Living cells rely on compartmentalization to orchestrate reaction networks for specialized and coordinated functions. Principally, the compartmentalization has been an essential engineering theme in constructing cell-mimicking systems. Here, efforts to engineer liquid-liquid interfaces of multiphase systems into membrane-bounded and membraneless compartments, which include lipid vesicles, polymer vesicles, colloidosomes, hybrids, and coacervate droplets, are summarized. Examples are provided of how these compartments are designed to imitate biological behaviors or machinery, including molecule trafficking, growth, fusion, energy conversion, intercellular communication, and adaptivity. Subsequently, the state-of-art applications of these cell-inspired synthetic compartments are discussed. Apart from being simplified and cell models for bridging the gap between nonliving matter and cellular life, synthetic compartments also are utilized as intracellular delivery vehicles for nuclei acids and nanoreactors for biochemical synthesis. Finally, key challenges and future directions for achieving the full potential of synthetic cells are highlighted.

The benefit of poor mixing: kinetics of coacervation

Complex coacervation has become a prominent area of research in the fields of food science, personal care, drug stabilization, and more. However, little has been reported on the kinetics of assembly of coacervation itself. Here, we describe a simple, low-cost way of looking at the kinetics of coacervation by creating poorly mixed samples. In particular, we examine how polymer chain length, the patterning and symmetry of charges on the oppositely charged polyelectrolytes, and the presence of salt and a zwitterionic buffer affect the kinetics of complex coacervation. Our results suggest an interesting relationship between the time for equilibration and the order of addition of polymers with asymmetric patterns of charge. Furthermore, we demonstrated that increasing polymer chain length resulted in a non-monotonic trend in the sample equilibration times as a result of opposing factors such as excluded volume and diffusion. We also observed differences in the rate of sample equilibration based on the presence of a neutral, zwitterionic buffer, as well as the presence and identity of added salt, consistent with previous reports of salt-specific effects on the rheology of complex coacervates. While not a replacement for more advanced characterization strategies, this turbidity-based method could serve as a screening tool to identify interesting and unique phenomena for further study.

Crossover from Rouse to Reptation Dynamics in Salt-Free Polyelectrolyte Complex Coacervates

Considerable interest in the dynamics and rheology of polyelectrolyte complex coacervates has been motivated by their industrial application as viscosity modifiers. A central question is the extent to which classical Rouse and reptation models can be applied to systems where electrostatic interactions play a critical role on the thermodynamics. By relying on molecular simulations, we present a direct analysis of the crossover from Rouse to reptation dynamics in salt-free complex coacervates as a function of chain length. This crossover shifts to shorter chain lengths as electrostatic interactions become stronger, which corresponds to the formation of denser coacervates. To distinguish the roles of Coulomb interactions and density, we compare the dynamics of coacervates to those of neutral, semidilute solutions at the same density. Both systems exhibit a universal dynamical behavior in the connectivity-dominated (subdiffusion and normal diffusion) regimes, but the monomer relaxation time in coacervates is much longer and increases with increasing Bjerrum length. This is similar to the cage effect observed in glass-forming polymers, but the local dynamical slowdown is caused here by strong Coulomb attractions (ion pairing) between oppositely charged monomers. Our findings provide a microscopic framework for the quantitative understanding of coacervate dynamics and rheology.

Dendrimicelles with pH-controlled aggregation number of core-dendrimers and stability

We present a simple way to build up well-controlled coacervate-core dendrimicelles by assembly of anionic PAMAM dendrimers with a cationic-neutral diblock copolymer. Upon increasing pH, the formation of micellar structures shows constant size but the number of dendrimer molecules incorporated in one micelle decreases, following the charge stoichiometry formation rules; concomitantly, the salt stability increases. This study shows the straightforward tuning of macromolecular core-units and related micelle properties.

Complex coacervates as artificial membraneless organelles and protocells

Complex coacervates are water droplets dispersed in water, which are formed by spontaneous liquid-liquid phase separation of an aqueous solution of two oppositely charged polyelectrolytes. Similar to the membraneless organelles that exist in biological cells, complex coacervate droplets are membraneless and have a myriad of features including easy formation, high viscosity, selective encapsulation of biomolecules, and dynamic behaviors in response to environmental stimuli, which make coacervates an excellent option for constructing artificial membraneless organelles. In this article, I first summarize recent advances in artificial compartments that are built from coacervates and their response to changes in the surrounding environment and then show the advantages of microfluidic techniques in the preparation of monodisperse coacervates and encapsulation of coacervates in droplets and liposomes to construct complex cell-like compartments, and finally discuss the future challenges of such membraneless aqueous compartments in cell mimics and origin of life.

Stimuli-Induced Nonequilibrium Phase Transitions in Polyelectrolyte-Surfactant Complex Coacervates

Polyelectrolyte-surfactant complexes (PESCs) are important soft colloids with applications in the fields of personal care, cosmetics, pharmaceutics, and much more. If their phase diagrams have long been studied under pseudoequilibrium conditions, and often inside the micellar or vesicular regions, understanding the effect of nonequilibrium conditions, applied at phase boundaries, on the structure of PESCs generates an increasing interest. In this work we cross the micelle-vesicle and micelle-fiber phase boundaries in an isocompositional surfactant-polyelectrolyte aqueous system through a continuous and rapid variation of pH. We employ two microbial glycolipid biosurfactants in the presence of polyamines, both systems being characterized by their responsiveness to pH. We show that complex coacervates (Co) are always formed in the micellar region of both glycolipids' phase diagram and that their phase behavior drives the PESC stability and structure. However, for glycolipid forming single-wall vesicles, we observe an isostructural and isodimensional transition between complex coacervates and a multilamellar walls vesicle (MLWV) phase. For the fiber-forming glycolipid, on the contrary, the complex coacervate disassembles into free polyelectrolyte coexisting with the equilibrium fiber phase. Last but not least, this work also demonstrates the use of microbial glycolipid biosurfactants in the development of sustainable PESCs.

Lower Critical Solution Temperature-Driven Self-Coacervation of Nonionic Polyester Underwater Adhesives

To enable attachment to underwater surfaces, aquatic fauna such as mussels and sandcastle worms utilize the advantages of coacervation to deliver concentrated protein-rich adhesive cocktails in an aqueous environment onto underwater surfaces. Recently, a mussel adhesive protein Mfp-3s, was shown to exhibit a coacervation-based adhesion mechanism. Current synthetic strategies to mimic Mfp-3s often involve complexation of oppositely charged polymers. Such complex coacervates are more sensitive to changes in pH and salt, thereby limiting their utility to narrow ranges of pH and ionic strength. In this study, by taking advantage of the lower critical solution temperature-driven coacervation, we have created mussel foot protein-inspired, tropoelastin-like, bioabsorbable, nonionic, self-coacervating polyesters for the delivery of photo-cross-linkable adhesives underwater and to overcome the challenges of adhesion in wet or underwater environments. We describe the rationale for their design and the underwater adhesive properties of these nonionic adhesives. Compared to previously reported coacervate adhesives, these "charge-free" polyesters coacervate in wide ranges of pH (3-12) and ionic strength (0-1 M NaCl) and rapidly (<300 s) adhere to substrates submerged underwater. The study introduces smart materials that mimic the self-coacervation and environmental stability of Mfp-3s and demonstrate the potential for biological adhesive applications where high water content, salts, and pH changes can be expected.

Distinct Cation-Anion Interactions in the UCST and LCST Behavior of Polyelectrolyte Complex Aqueous Solutions

Polyelectrolyte complexes (PECs) are recently observed to possess diversified thermoresponsive phase transition behaviors in aqueous solutions. Herein, by adjusting the initial polymer concentrations (C_pi) of poly(styrenesulfonate) (PSS)/poly(diallyldimethylammonium) (PDADMA) PEC in the same saline solution, in addition to previously reported lower critical solution temperature (LCST), we experimentally observed the upper critical solution temperature (UCST)-type phase transition behavior of PSS/PDADMA PECs at a lower polymer concentration. As elucidated by temperature-dependent Raman spectroscopy and two-dimensional correlation analysis, at temperatures lower than UCST, more hydrophobic polyelectrolyte chains lead to a high proportion of contact ion pairs (CIPs), contributing to UCST-type solid-liquid phase transition; however, at higher concentrations of PEC, the less hydrophobic polyelectrolyte chains correspond to a higher proportion of solvent-separated ion pairs (SIPs), which enables the occurrence of LCST-type liquid-liquid phase transition. With the spectroscopic indicator of CIPs/SIPs peak ratio and monitoring the hydration state of polymer chains, the complex interplays of PSS/PDADMA PECs are hereby interpreted at the molecular level, which lays the mechanistic foundation for designing other thermoresponsive PEC assemblies.

Time-Ionic Strength Superposition: A Unified Description of Chain Relaxation Dynamics in Polyelectrolyte Complexes

Addition of salt speeds up chain relaxation dynamics in polyelectrolyte complexes (PECs), and time-salt superposition (TSS) approaches to describe the linear viscoelastic response of PECs are well-established. However, TSS is carried out at fixed initial polyelectrolyte concentrations, and varying the initial polyelectrolyte concentration results in distinct TSS master curves. In this contribution, we show that accounting for the small ions that accompany the oppositely charged polyelectrolyte chains (designated as accompanying counterions) enables assimilation of these distinct TSS master curves into a single universal master curve. This approach, that we christen as time-ionic strength superposition (TISS), enables a unified description of the PEC viscoelastic response in terms of the solution ionic strength, that accounts for both the accompanying counterions and the added ions, and underlines the dynamic similarities between PECs and semidilute polymer solutions. The sticky electrostatic associations among the oppositely charged chains, however, contribute additional relaxation modes in the PECs. We demonstrate that the time scales of these additional relaxation modes are described quantitatively by a modified sticky Rouse model that accounts for the influence of solution ionic strength on electrostatic screening and chain friction.

Structural Polymorphism of Single pDNA Condensates Elicited by Cationic Block Polyelectrolytes

DNA folding is a core phenomenon in genome packaging within a nucleus. Such a phenomenon is induced by polyelectrolyte complexation between anionic DNA and cationic proteins of histones. In this regard, complexes formed between DNA and cationic polyelectrolytes have been investigated as models to gain insight into genome packaging. Upon complexation, DNA undergoes folding to reduce its occupied volume, which often results in multi-complex associated aggregates. However, when cationic copolymers comprising a polycation block and a neutral hydrophilic polymer block are used instead, DNA undergoes folding as a single molecule within a spontaneously formed polyplex micelle (PM), thereby allowing the observation of the higher-order structures that DNA forms. The DNA complex forms polymorphic structures, including globular, rod-shaped, and ring-shaped (toroidal) structures. This review focuses on the polymorphism of DNA, particularly, to elucidate when, how, and why DNA organizes into these structures with cationic copolymers. The interactions between DNA and the copolymers, and the specific nature of DNA in rigidity; i.e., rigid but foldable, play significant roles in the observed polymorphism. Moreover, PMs serve as potential gene vectors for systemic application. The significance of the controlled DNA folding for such an application is addressed briefly in the last part.

Structural and Biophysical Insights into the Function of the Intrinsically Disordered Myc Oncoprotein

Myc is a transcription factor driving growth and proliferation of cells and involved in the majority of human tumors. Despite a huge body of literature on this critical oncogene, our understanding of the exact molecular determinants and mechanisms that underlie its function is still surprisingly limited. Indubitably though, its crucial and non-redundant role in cancer biology makes it an attractive target. However, achieving successful clinical Myc inhibition has proven challenging so far, as this nuclear protein is an intrinsically disordered polypeptide devoid of any classical ligand binding pockets. Indeed, Myc only adopts a (partially) folded structure in some contexts and upon interacting with some protein partners, for instance when dimerizing with MAX to bind DNA. Here, we review the cumulative knowledge on Myc structure and biophysics and discuss the implications for its biological function and the development of improved Myc inhibitors. We focus this biophysical walkthrough mainly on the basic region helix-loop-helix leucine zipper motif (bHLHLZ), as it has been the principal target for inhibitory approaches so far.