The Effect of Salt on the Complex Coacervation of Vinyl Polyelectrolytes

Constrained dynamics of DNA oligonucleotides in phase-separated droplets

Understanding the dynamics of biomolecules in complex environments is crucial for elucidating the effect of condensed and heterogeneous environments on their functional properties. A relevant environment-and one that can also be mimicked easily in vitro-is that of phase-separated droplets. While phase-separated droplet systems have been shown to compartmentalize a wide range of functional biomolecules, the effects of internal structuration of droplets on the dynamics and mobility of internalized molecules remain poorly understood. Here, we use fluorescence correlation spectroscopy to measure the dynamics of short oligonucleotides encapsulated within two representative kinds of uncharged and charged phase-separated droplets. We find that the internal structuration controls the oligonucleotide dynamics in these droplets, revealed by measuring physical parameters at high spatiotemporal resolution. By varying oligonucleotide length and salt concentrations (and thereby charge screening), we found that the dynamics are significantly affected in the noncharged droplets compared to the charged system. Our work lays the foundation for unraveling and quantifying the physical parameters governing biomolecular transport in the condensed environment.

Counter Anion Type Influences the Glass Transition Temperature of Polyelectrolyte Complexes

Salt acts as a plasticizer in polyelectrolyte complexes (PECs), which impacts the physical, thermal, and mechanical properties, thus having implications in applications, such as drug delivery, energy storage, and smart coatings. Added salt disrupts polycation-polyanion intrinsic ion pairs, lowering a hydrated PEC's glass transition temperature (Tg). However, the relative influence of counterion type on the PEC's Tg is not well understood. Here, the effect of anion type (NaCl, NaBr, NaNO3, and NaI) on the Tg of solid-like, hydrated PECs composed of poly(diallydimethylammonium) (PDADMA)-poly(styrenesulfonate) (PSS) is investigated. With increasing the chaotropic nature of the salt anion, the Tg decreases. The relative differences are attributed to the doping level, the amount of bound water, the mobility of water molecules within the PECs, and the strength of interactions between the PEs. For all studied salt concentrations and salt types, the Tg followed the scaling of -1/Tg ≈ ln([IP]/[H2O]), in which [IP]/[H2O] is the ratio of intrinsic pairs to water. The scaling estimates that about 7 to 17% of the intrinsic ion pairs should be weakened for the PEC to partake in a glass transition. Put together, this study highlights that the Tg in PECs is impacted by the salt anion, but the mechanism of the glass transition remains unchanged.

Molecular Dynamics Simulations of Polyelectrolyte Complexes

Polyelectrolyte complexes (PECs) are currently of great interest due to their applications toward developing new adaptive materials and their relevance in membraneless organelles. These complexes emerge during phase separation when oppositely charged polymers are mixed in aqueous media. Peptide-based PECs are particularly useful toward developing new drug delivery methods due to their inherent biocompatibility. The underlying peptide sequence can be tuned to optimize specific material properties of the complex, such as interfacial tension and viscosity. Given their applicability, it would be advantageous to understand the underlying sequence-dependent phase behavior of oppositely charged peptides. Here, we report microsecond molecular dynamic simulations to characterize the effect of hydrophobicity on the sequence-dependent peptide conformation for model polypeptide sequences that were previously reported by Tabandeh et al. These sequences are designed with alternating chirality of the peptide backbone. We present microsecond simulations of six oppositely charged peptide pairs, characterizing the sequence-dependent effect on peptide size, degree of hydrogen bonding, secondary structure, and conformation. This analysis recapitulates sensible trends in peptide conformation and degree of hydrogen bonding, consistent with experimentally reported results. Ramachandran plots reveal that backbone conformation at the single amino acid level is highly influenced by the neighboring sequence in the chain. These results give insight into how subtle changes in hydrophobic side chain size and chirality influence the strength of hydrogen bonding between the chains and, ultimately, the secondary structure. Furthermore, principal component analysis reveals that the minimum energy structures may be subtly modulated by the underlying sequence.

Sequestration of Small Ions and Weak Acids and Bases by a Polyelectrolyte Complex Studied by Simulation and Experiment

Mixing of oppositely charged polyelectrolytes can result in phase separation into a polymer-poor supernatant and a polymer-rich polyelectrolyte complex (PEC). We present a new coarse-grained model for the Grand-reaction method that enables us to determine the composition of the coexisting phases in a broad range of pH and salt concentrations. We validate the model by comparing it to recent simulations and experimental studies, as well as our own experiments on poly(acrylic acid)/poly(allylamine hydrochloride) complexes. The simulations using our model predict that monovalent ions partition approximately equally between both phases, whereas divalent ones accumulate in the PEC phase. On a semiquantitative level, these results agree with our own experiments, as well as with other experiments and simulations in the literature. In the sequel, we use the model to study the partitioning of a weak diprotic acid at various pH values of the supernatant. Our results show that the ionization of the acid is enhanced in the PEC phase, resulting in its preferential accumulation in this phase, which monotonically increases with the pH. Currently, this effect is still waiting to be confirmed experimentally. We explore how the model parameters (particle size, charge density, permittivity, and solvent quality) affect the measured partition coefficients, showing that fine-tuning of these parameters can make the agreement with the experiments almost quantitative. Nevertheless, our results show that charge regulation in multivalent solutes can potentially be exploited in engineering the partitioning of charged molecules in PEC-based systems at various pH values.

Design Rules for the Sequestration of Viruses into Polypeptide Complex Coacervates

Encapsulation is a strategy that has been used to facilitate the delivery and increase the stability of proteins and viruses. Here, we investigate the encapsulation of viruses via complex coacervation, which is a liquid-liquid phase separation resulting from the complexation of oppositely charged polymers. In particular, we utilized polypeptide-based coacervates and explored the effects of peptide chemistry, chain length, charge patterning, and hydrophobicity to better understand the effects of the coacervating polypeptides on virus incorporation. Our study utilized two nonenveloped viruses, porcine parvovirus (PPV) and human rhinovirus (HRV). PPV has a higher charge density than HRV, and they both appear to be relatively hydrophobic. These viruses were compared to characterize how the charge, hydrophobicity, and patterning of chemistry on the surface of the virus capsid affects encapsulation. Consistent with the electrostatic nature of complex coacervation, our results suggest that electrostatic effects associated with the net charge of both the virus and polypeptide dominated the potential for incorporating the virus into a coacervate, with clustering of charges also playing a significant role. Additionally, the hydrophobicity of a virus appears to determine the degree to which increasing the hydrophobicity of the coacervating peptides can enhance virus uptake. Nonintuitive trends in uptake were observed with regard to both charge patterning and polypeptide chain length, with these parameters having a significant effect on the range of coacervate compositions over which virus incorporation was observed. These results provide insights into biophysical mechanisms, where sequence effects can control the uptake of proteins or viruses into biological condensates and provide insights for use in formulation strategies.

Self-Assembling Polypeptides in Complex Coacervation

Intracellular compartmentalization plays a pivotal role in cellular function, with membrane-bound organelles and membrane-less biomolecular "condensates" playing key roles. These condensates, formed through liquid-liquid phase separation (LLPS), enable selective compartmentalization without the barrier of a lipid bilayer, thereby facilitating rapid formation and dissolution in response to stimuli. Intrinsically disordered proteins (IDPs) or proteins with intrinsically disordered regions (IDRs), which are often rich in charged and polar amino acid sequences, scaffold many condensates, often in conjunction with RNA.Comprehending the impact of IDP/IDR sequences on phase separation poses a challenge due to the extensive chemical diversity resulting from the myriad amino acids and post-translational modifications. To tackle this hurdle, one approach has been to investigate LLPS in simplified polypeptide systems, which offer a narrower scope within the chemical space for exploration. This strategy is supported by studies that have demonstrated how IDP function can largely be understood based on general chemical features, such as clusters or patterns of charged amino acids, rather than residue-level effects, and the ways in which these kinds of motifs give rise to an ensemble of conformations.Our laboratory has utilized complex coacervates assembled from oppositely charged polypeptides as a simplified material analogue to the complexity of liquid-liquid phase separated biological condensates. Complex coacervation is an associative LLPS that occurs due to the electrostatic complexation of oppositely charged macro-ions. This process is believed to be driven by the entropic gains resulting from the release of bound counterions and the reorganization of water upon complex formation. Apart from their direct applicability to IDPs, polypeptides also serve as excellent model polymers for investigating molecular interactions due to the wide range of available side-chain functionalities and the capacity to finely regulate their sequence, thus enabling precise control over interactions with guest molecules.Here, we discuss fundamental studies examining how charge patterning, hydrophobicity, chirality, and architecture affect the phase separation of polypeptide-based complex coacervates. These efforts have leveraged a combination of experimental and computational approaches that provide insight into molecular level interactions. We also examine how these parameters affect the ability of complex coacervates to incorporate globular proteins and viruses. These efforts couple directly with our fundamental studies into coacervate formation, as such "guest" molecules should not be considered as experiencing simple encapsulation and are instead active participants in the electrostatic assembly of coacervate materials. Interestingly, we observed trends in the incorporation of proteins and viruses into coacervates formed using different chain length polypeptides that are not well explained by simple electrostatic arguments and may be the result of more complex interactions between globular and polymeric species. Additionally, we describe experimental evidence supporting the potential for complex coacervates to improve the thermal stability of embedded biomolecules, such as viral vaccines.Ultimately, peptide-based coacervates have the potential to help unravel the physics behind biological condensates, while paving the way for innovative methods in compartmentalization, purification, and biomolecule stabilization. These advancements could have implications spanning medicine to biocatalysis.

Effect of Polyelectrolyte Charge Density on the Linear Viscoelastic Behavior and Processing of Complex Coacervate Adhesives

It is well-known that the phase behavior and physicochemical and adhesive properties of complex coacervates are readily tuneable with the salt concentration of the medium. For toxicity reasons, however, the maximum applicable salt concentration in biomedical applications is typically low. Consequently, other strategies must be implemented in order to optimize the properties of the resulting complex coacervates. In this work, the effect of the charge density of a strong polyanion on the properties of complex coacervates was studied. To control this charge density, statistical anionic/charge-neutral hydrophilic copolymers were synthesized by means of an elegant protection/deprotection strategy and subsequently complexed with a strong polycation. The resulting complexes were observed to have an increasing water content as well as faster relaxation dynamics, with either increasing salt concentration or decreasing charge density. Time-salt and time-salt-charge density superpositions could be performed and showed that the relaxation mechanism of the complex coacervates remained unchanged. When the charge density was decreased, lower salt concentration complexes became suitable for viscoelastic adhesion with improved injectability. Such complex coacervates are promising candidates for injectable biomedical adhesives.

Drug Encapsulation via Peptide-Based Polyelectrolyte Complexes

Peptide-based polyelectrolyte complexes are biocompatible materials that can encapsulate molecules with different polarities due to their ability to be precisely designed. Here we use UV-Vis spectroscopy, fluorescence microscopy, and infrared spectroscopy to investigate the encapsulation of model drugs, doxorubicin (DOX) and methylene blue (MB) using a series of rationally designed polypeptides. For both drugs, we find an overall higher encapsulation efficiency with sequences that have higher charge density, highlighting the importance of ionic interactions between the small molecules and the peptides. However, comparing molecules with the same charge density, illustrated that the most hydrophobic sequence pairs had the highest encapsulation of both DOX and MB molecules. The phase behavior and stability of DOX-containing complexes did not change compared to the complexes without drugs. However, MB encapsulation caused changes in the stabilities of the complexes. The sequence pair with the highest charge density and hydrophobicity had the most dramatic increase in stability, which coincided with a phase change from liquid to solid. This study illustrates how multiple types of molecular interactions are required for efficient encapsulation of poorly soluble drugs and provides insights into the molecular design of delivery carriers.

Antifouling Coatings from Glassy Polyelectrolyte Complex Films

Coatings that prevent or decrease fouling are sought for many applications, including those that inhibit the attachment of organisms in aquatic environments. To date, antifouling coatings have mostly followed design criteria assembled over decades: surfaces should be well/strongly hydrated, possess low net charge, and maintain a hydrophilic character when exposed to the location of use. Thus, polymers based on ethylene glycol or zwitterionic repeat units have been shown to be highly effective. Unfortunately, hydrated materials can be quite soft, limiting their use in some environments. In a major paradigm shift, this work describes glassy antifouling films made from certain complexes of positive and negative polyelectrolytes. The dense network of electrostatic interactions yields tough materials below the glass transition temperature, Tg, in normal use, while the highly ionic character of these polyelectrolyte complexes ensures strong hydration. The proximity of equal numbers of opposite charges within these complexes mimics zwitterionic structures. Films, assembled layer-by-layer from aqueous solutions, contained sulfonated poly(ether ether ketone), SPEEK, a rigid polyelectrolyte that binds strongly to a selection of quaternary ammonium polycations. Layer-by-layer buildup of SPEEK and polycations was linear, indicating strong complexes between polyelectrolytes. Calorimetry also showed that complex formation was exothermic. Surfaces coated with these films in the 100 nm thickness range completely resisted adhesion of the common flagellate green algae, Chlamydomonas reinhardtii, which were removed from surfaces at a minimum applied flow rate of 0.8 cm s-1. The total surface charge density of adsorbed cations, determined with a sensitive radioisotopic label, was very low, around 10% of a monolayer, which minimized adsorption driven by counterion release from the surface. The viscoelastic properties of the complexes, which were stable even in concentrated salt solutions, were explored using rheology of bulk samples. When fully hydrated, their Tg values were observed to be above 75 °C.

Fluctuations, structure, and size inside coacervates

Aqueous solutions of oppositely charged macromolecules exhibit the ubiquitous phenomenon of coacervation. This subject is of considerable current interest due to numerous biotechnological applications of coacervates and the general premise of biomolecular condensates. Towards a theoretical foundation of structural features of coacervates, we present a field-theoretic treatment of coacervates formed by uniformly charged flexible polycations and polyanions in an electrolyte solution. We delineate different regimes of polymer concentration fluctuations and structural features of coacervates based on the concentrations of polycation and polyanion, salt concentration, and experimentally observable length scales. We present closed-form formulas for correlation length of polymer concentration fluctuations, scattering structure factor, and radius of gyration of a labelled polyelectrolyte chain inside a concentrated coacervate. Using random phase approximation suitable for concentrated polymer systems, we show that the inter-monomer electrostatic interaction is screened by interpenetration of all charged polymer chains and that the screening length depends on the individual concentrations of the polycation and the polyanion, as well as the salt concentration. Our calculations show that the scattering intensity decreases monotonically with scattering wave vector at higher salt concentrations, while it exhibits a peak at intermediate scattering wave vector at lower salt concentrations. Furthermore, we predict that the dependence of the radius of gyration of a labelled chain on its degree of polymerization generally obeys the Gaussian chain statistics. However, the chain is modestly swollen, the extent of which depending on polyelectrolyte composition, salt concentration, and the electrostatic features of the polycation and polyanion such as the degree of ionization.

Effect of Dynamically Arrested Domains on the Phase Behavior, Linear Viscoelasticity and Microstructure of Hyaluronic Acid - Chitosan Complex Coacervates

Complex coacervates make up a class of versatile materials formed as a result of the electrostatic associations between oppositely charged polyelectrolytes. It is well-known that the viscoelastic properties of these materials can be easily altered with the ionic strength of the medium, resulting in a range of materials from free-flowing liquids to gel-like solids. However, in addition to electrostatics, several other noncovalent interactions could influence the formation of the coacervate phase depending on the chemical nature of the polymers involved. Here, the importance of intermolecular hydrogen bonds on the phase behavior, microstructure, and viscoelasticity of hyaluronic acid (HA)-chitosan (CHI) complex coacervates is revealed. The density of intermolecular hydrogen bonds between CHI units increases with increasing pH of coacervation, which results in dynamically arrested regions within the complex coacervate, leading to elastic gel-like behavior. This pH-dependent behavior may be very relevant for the controlled solidification of complex coacervates and thus for polyelectrolyte material design.

Coacervates Forming Coexisting Phases on a Mineral Surface

Minerals played a crucial role in the chemical evolution of small molecules into biopolymers. Yet, it is still not clear how the minerals are related to the formation and the evolution of protocells on early Earth. In this work, using the coacervate formed by quaternized dextran (Q-dextran) and single-stranded oligonucleotides (ss-oligo) as the protocell model, we systematically studied the phase separation of Q-dextran and ss-oligo on the muscovite surface. Serving as rigid and 2D polyelectrolytes, the muscovite surface can be treated by Q-dextran to become negatively charged, neutral, or positively charged. We observed that Q-dextran and ss-oligo form uniform coacervates on naked and neutral muscovite surfaces, while they form biphasic coacervates containing Q-dextran-rich phases and ss-oligo-rich phases on positively or negatively charged muscovite surfaces that were pretreated by Q-dextran. The evolution of the phases is caused by the redistribution of the components as the coacervate touches the surface. Our study indicates that the mineral surface could be a potential driving force for the formation of protocells with hierarchical structures and desirable functions on prebiotic Earth.

The Counterion-Mediated Controllable Coacervation of Nano-Ions with Polyelectrolytes

Nano-ions can complex with polyelectrolytes for coacervates with hierarchical structures; however, the rational design of functional coacervations is still rare due to the poor understanding of their structure-property relationship from their complex interaction. Herein, 1 nm anionic metal oxide clusters, PW₁₂O₄₀^3-, with well-defined, mono-disperse structures are applied to complex with cationic polyelectrolyte and the system shows tunable coacervation via the alternation of counterions (H⁺ and Na⁺) of PW₁₂O₄₀^3-. Suggested from Fourier transform infrared spectroscopy (FT-IR) and isothermal titration studies, the interaction between PW₁₂O₄₀^3- and cationic polyelectrolytes can be modulated by the bridging effect of counterions via hydrogen bonding or ion-dipole interaction to carbonyl groups of polyelectrolytes. The condensed structures of the complexed coacervates are explored by small angle X-ray and neutron scattering techniques, respectively. The coacervate with H⁺ as counterions shows both crystallized and discrete PW₁₂O₄₀^3- clusters, with a loose polymer-cluster network in comparison to the system of Na⁺ which shows a dense packing structure with aggregated nano-ions filling the meshes of polyelectrolyte networks. The bridging effect of counterions helps understand the super-chaotropic effect observed in nano-ion system and provides avenues for the design of metal oxide cluster-based functional coacervates.

Capturing coacervate formation and protein partition by molecular dynamics simulation

Biomolecules localize and function in microenvironments where their local concentration, spatial organization, and biochemical reactivity are regulated. To compartmentalize and control the local properties of the native microenvironment, cellular mimics and artificial bioreactors have been developed to study the properties of membraneless organelles or mimic the bio-environment for life origin. Here, we carried out molecular dynamics simulation with the Martini 3.0 model to reproduce the experimental salt concentration and pH dependency of different complex coacervates. We showed that coacervates inside vesicles are able to change their shape. In addition, we used these coacervate systems to explore the partitioning of the ubiquitous cytoskeletal protein actin and found that actin spontaneously partitions to all the coacervate peripheries. Therefore, we believe that our study can provide a better understanding of the versatile coacervate platform, where biomolecules partition and gather to fulfill their biological duties.

Activation of L-lactate oxidase by the formation of enzyme assemblies through liquid-liquid phase separation

The assembly state of enzymes is gaining interest as a mechanism for regulating the function of enzymes in living cells. One of the current topics in enzymology is the relationship between enzyme activity and the assembly state due to liquid-liquid phase separation. In this study, we demonstrated enzyme activation via the formation of enzyme assemblies using L-lactate oxidase (LOX). LOX formed hundreds of nanometer-scale assemblies with poly-L-lysine (PLL). In the presence of ammonium sulfate, the LOX-PLL clusters formed micrometer-scale liquid droplets. The enzyme activities of LOX in clusters and droplets were one order of magnitude higher than those in the dispersed state, owing to a decrease in K_M and an increase in k_cat. Moreover, the clusters exhibited a higher activation effect than the droplets. In addition, the conformation of LOX changed in the clusters, resulting in increased enzyme activation. Understanding enzyme activation and assembly states provides important information regarding enzyme function in living cells, in addition to biotechnology applications.

Polyelectrolyte complexation of two oppositely charged symmetric polymers: A minimal theory

Interplay of Coulomb interaction energy, free ion entropy, and conformational elasticity is a fascinating aspect in polyelectrolytes (PEs). We develop a theory for complexation of two oppositely charged PEs, a process known to be the precursor to the formation of complex coacervates in PE solutions, to explore the underlying thermodynamics of complex formation, at low salts. The theory considers general degrees of solvent polarity and dielectricity within an implicit solvent model, incorporating a varying Coulomb strength. Explicit calculation of the free energy of complexation and its components indicates that the entropy of free counterions and salt ions and the Coulomb enthalpy of bound ion-pairs dictate the equilibrium of PE complexation. This helps decouple the self-consistent dependency of charge and size of the uncomplexed parts of the polyions, derive an analytical expression for charge, and evaluate the free energy components as functions of chain overlap. Complexation is observed to be driven by enthalpy gain at low Coulomb strengths, driven by entropy gain of released counterions but opposed by enthalpy loss due to reduction of ion-pairs at moderate Coulomb strengths, and progressively less favorable due to enthalpy loss at even higher Coulomb strengths. The total free energy of the system is found to decrease linearly with an overlap of chains. Thermodynamic predictions from our model are in good quantitative agreement with simulations in literature.

A mini-review on bio-inspired polymer self-assembly: single-component and interactive polymer systems

Biology demonstrates meticulous ways to control biomaterials self-assemble into ordered and disordered structures to carry out necessary bioprocesses. Empowering the synthetic polymers to self-assemble like biomaterials is a hallmark of polymer physics studies. Unlike protein engineering, polymer science demystifies self-assembly by purposely embedding particular functional groups into the backbone of the polymer while isolating others. The polymer field has now entered an era of advancing materials design by mimicking nature to a very large extend. For example, we can make sequence-specific polymers to study highly ordered mesostructures similar to studying proteins, and use charged polymers to study liquid-liquid phase separation as in membraneless organelles. This mini-review summarizes recent advances in studying self-assembly using bio-inspired strategies on single-component and multi-component systems. Sequence-defined techniques are used to make on-demand hybrid materials to isolate the effects of chirality and chemistry in synthetic block copolymer self-assembly. In the meantime, sequence patterning leads to more hierarchical assemblies comprised of only hydrophobic and hydrophilic comonomers. The second half of the review discusses complex coacervates formed as a result of the associative charge interactions of oppositely charged polyelectrolytes. The tunable phase behavior and viscoelasticity are unique in studying liquid macrophase separation because the slow polymer relaxation comes primarily from charge interactions. Studies of bio-inspired polymer self-assembly significantly impact how we optimize user-defined materials on a molecular level.

Polymer/Enzyme Composite Materials—Versatile Catalysts with Multiple Applications

A significant interest was granted lately to enzymes, which are versatile catalysts characterized by natural origin, with high specificity and selectivity for particular substrates. Additionally, some enzymes are involved in the production of high-valuable products, such as antibiotics, while others are known for their ability to transform emerging contaminates, such as dyes and pesticides, to simpler molecules with a lower environmental impact. Nevertheless, the use of enzymes in industrial applications is limited by their reduced stability in extreme conditions and by their difficult recovery and reusability. Rationally, enzyme immobilization on organic or inorganic matrices proved to be one of the most successful innovative approaches to increase the stability of enzymatic catalysts. By the immobilization of enzymes on support materials, composite biocatalysts are obtained that pose an improved stability, preserving the enzymatic activity and some of the support material’s properties. Of high interest are the polymer/enzyme composites, which are obtained by the chemical or physical attachment of enzymes on polymer matrices. This review highlights some of the latest findings in the field of polymer/enzyme composites, classified according to the morphology of the resulting materials, following their most important applications.

Driving force and pathway in polyelectrolyte complex coacervation

There is notable discrepancy between experiments and coarse-grained model studies regarding the thermodynamic driving force in polyelectrolyte complex coacervation: experiments find the free energy change to be dominated by entropy, while simulations using coarse-grained models with implicit solvent usually report a large, even dominant energetic contribution in systems with weak to intermediate electrostatic strength. Here, using coarse-grained, implicit-solvent molecular dynamics simulation combined with thermodynamic analysis, we study the potential of mean force (PMF) in the two key stages on the coacervation pathway for symmetric polyelectrolyte mixtures: polycation-polyanion complexation and polyion pair-pair condensation. We show that the temperature dependence in the dielectric constant of water gives rise to a substantial entropic contribution in the electrostatic interaction. By accounting for this electrostatic entropy, which is due to solvent reorganization, we find that under common conditions (monovalent ions, room temperature) for aqueous systems, both stages are strongly entropy-driven with negligible or even unfavorable energetic contributions, consistent with experimental results. Furthermore, for weak to intermediate electrostatic strengths, this electrostatic entropy, rather than the counterion-release entropy, is the primary entropy contribution. From the calculated PMF, we find that the supernatant phase consists predominantly of polyion pairs with vanishingly small concentration of bare polyelectrolytes, and we provide an estimate of the spinodal of the supernatant phase. Finally, we show that prior to contact, two neutral polyion pairs weakly attract each other by mutually induced polarization, providing the initial driving force for the fusion of the pairs.

An Overview of Coacervates: The Special Disperse State of Amphiphilic and Polymeric Materials in Solution

Individual amphiphiles, polymers, and colloidal dispersions influenced by temperature, pH, and environmental conditions or interactions between their oppositely charged pairs in solvent medium often produce solvent-rich and solvent-poor phases in the system. The solvent-poor denser phase found either on the top or the bottom of the system is called coacervate. Coacervates have immense applications in various technological fields. This review comprises a concise introduction, focusing on the types of coacervates, and the influence of different factors in their formation, structures, and stability. In addition, their physicochemical properties, thermodynamics of formation, and uses and multifarious applications are also concisely presented and discussed.

J-aggregation of 5, 10, 15, 20-tetraphenyl-21H, 23H-porphinetetrasulfonic acid in a molecular crowding environment simulated using dextran

In a molecular crowding environment, different thermodynamics is often observed in a dilute solution. One such example is the promotion of the formation of amyloids, which are causal agents of Alzheimer's disease. Although a considerable number of molecular crowding studies have been reported, its effect remains unclear. In this study, we investigated a J-aggregation of a porphyrin derivative, 5, 10, 15, 20-tetraphenyl-21H,23H-porphinetetrasulfonic acid (TPPS), in a molecular crowding environment simulated by dextran (Dex) in HClO₄, HCl, and NaCl solutions. The changes in the number of monomers in the J-aggregate (n) with the concentration of Dex (C_Dex) depended on the type of solution. No change in n was observed in the NaCl solution, which indicated that the Dex solution did not affect the J-aggregation because of the ionic strength effect. In the HCl solution, the aggregation behavior changed with the pH. Further, at a low pH, the electrostatic interactions promoted J-aggregation by the volume exclusion of Dex, while the aggregation was suppressed at a high pH owing to steric hindrance. A different aggregation mechanism, involving the hydrogen bonding between NH in the center of the TPPS macrocyclic frame and the SO₃H and ClO₄^- functional groups, was responsible for the J-aggregation in the HClO₄ solution. Moreover, the n value increased owing to the volume exclusion effect. We expect that this study will be useful for further elucidation of the molecular crowding effect.

Self-Assembly of Miktoarm Star Polyelectrolytes in Solutions with Various Ionic Strengths

We studied the self-assembly of miktoarm star polyelectrolytes with different numbers of arms in solutions with various ionic strengths using coarse-grained molecular dynamic simulations. Spherical micelles are obtained for star polyelectrolytes with fewer arms, whereas wormlike clusters are obtained for star polyelectrolytes with more arms at a low ionic strength environment, with hydrophilic arms showing a stretched conformation. The number of clusters shows an overall decreasing tendency with increasing the number of arms in star polyelectrolytes due to strong electrostatic coupling between polycations and polyanions. The formation of wormlike clusters follows an overall stepwise pathway with an intermittent association-dissociation process for star polyelectrolytes with weak electrostatic coupling. These computational results can provide relevant physical insights to understand the self-assembly mechanism of star polyelectrolytes in solvents with various ionic strengths and to design star polyelectrolytes with functional groups that can fine-tune self-assembled structures for specific applications.

Coacervation in polyzwitterion-polyelectrolyte systems and their potential applications for gastrointestinal drug delivery platforms

Traditionally, complex coacervation is regarded as a process whereby two oppositely charged polyelectrolytes self-assemble into spherical droplets. Here, we introduce the polyzwitterionic complex, “pZC”, formed by the liquid-liquid phase separation of a polyzwitterion and a polyelectrolyte, and elucidate a mechanism by which such complexes can assemble using theory and experimental evidence. This system exhibits orthogonal phase behavior-it remains intact in acidic conditions, but disassembles as the pH increases, a process governed by the acid-base equilibria of the constituent chains. We relate the observed phase behavior to physiological conditions within the gastrointestinal tract with a simulation of the gastroduodenal junction, and demonstrate using video microscopy the viability of polyzwitterionic coacervates as technologies for the pH-triggered release of cargo. Such a system is envisaged to tackle imminent problems of drug transport via the oral route and serve as a packaging solution to increase uptake efficiency.

Coacervation is widely studied as potential drug delivery platform, but formation of coacervates at lower pH ranges, such as found in the gastrointestinal tract, remains challenging. Here, using theory and experimental methods, the authors demonstrate the formation of a polyzwitterionic complex, formed by coacervation of a polyzwitterion and a polyelectrolyte, exhibiting orthogonal phase behavior under physiological low pH conditions.

Temperature-dependent reentrant phase transition of RNA-polycation mixtures

Liquid-liquid phase separation (LLPS) of multivalent biopolymers is a ubiquitous process in biological systems and is of importance in bio-mimetic soft matter design. The phase behavior of biomolecules, such as proteins and nucleic acids, is typically encoded by the primary chain sequence and regulated by solvent properties. One of the most important physical modulators of LLPS is temperature. Solutions of proteins and/or nucleic acids have been shown to undergo liquid-liquid phase separation either upon cooling (with an upper critical solution temperature, UCST) or upon heating (with a lower critical solution temperature, LCST). However, many theoretical frameworks suggest the possibility of more complex temperature-dependent phase behaviors, such as an hourglass or a closed-loop phase diagram with concurrent UCST and LCST transitions. Here, we report that RNA-polyamine mixtures undergo a reentrant phase separation with temperature. Specifically, at low temperatures, RNA-polyamine mixtures form a homogenous phase. Increasing the temperature leads to the formation of RNA-polyamine condensates. A further increase in temperature leads to the dissolution of condensates, rendering a reentrant homogenous phase. This dual-response phase separation of RNA is not unique to polyamines but also observed with short cationic peptides. The immiscibility gap is controlled by the charge of the polycation, salt concentration, and mixture composition. Based on the existing theories of complex coacervation, our results point to a complex interplay between desolvation entropy, ion-pairing, and electrostatic interactions in dictating the closed-loop phase behavior of RNA-polycation mixtures.

Biocatalytic self-assembled synthetic vesicles and coacervates: From single compartment to artificial cells

Compartmentalization is an intrinsic feature of living cells that allows spatiotemporal control over the biochemical pathways expressed in them. Over the years, a library of compartmentalized systems has been generated, which includes nano to micrometer sized biomimetic vesicles derived from lipids, amphiphilic block copolymers, peptides, and nanoparticles. Biocatalytic vesicles have been developed using a simple bag containing enzyme design of liposomes to multienzymes immobilized multi-vesicular compartments for artificial cell generation. Additionally, enzymes were also entrapped in membrane-less coacervate droplets to mimic the cytoplasmic macromolecular crowding mechanisms. Here, we have discussed different types of single and multicompartment systems, emphasizing their recent developments as biocatalytic self-assembled structures using recent examples. Importantly, we have summarized the strategies in the development of the self-assembled structure to improvise their adaptivity and flexibility for enzyme immobilization. Finally, we have presented the use of biocatalytic assemblies in mimicking different aspects of living cells, which further carves the path for the engineering of a minimal cell.

Non-equilibrium conditions inside rock pores drive fission, maintenance and selection of coacervate protocells

Key requirements for the first cells on Earth include the ability to compartmentalize and evolve. Compartmentalization spatially localizes biomolecules from a dilute pool and an evolving cell, which, as it grows and divides, permits mixing and propagation of information to daughter cells. Complex coacervate microdroplets are excellent candidates as primordial cells with the ability to partition and concentrate molecules into their core and support primitive and complex biochemical reactions. However, the evolution of coacervate protocells by fusion, growth and fission has not yet been demonstrated. In this work, a primordial environment initiated the evolution of coacervate-based protocells. Gas bubbles inside heated rock pores perturb the coacervate protocell distribution and drive the growth, fusion, division and selection of coacervate microdroplets. Our findings provide a compelling scenario for the evolution of membrane-free coacervate microdroplets on the early Earth, induced by common gas bubbles within heated rock pores.

A simple simulation model for complex coacervates

When oppositely charged polyelectrolytes mix in an aqueous solution, associative phase separation gives rise to coacervates. Experiments reveal the phase diagram for such coacervates, and determine the impact of charge density, chain length and added salt. Simulations often use hybrid MC-MD methods to produce such phase diagrams, in support of experimental observations. We propose an idealized model and a simple simulation technique to investigate coacervate phase behavior. We model coacervate systems by charged bead-spring chains and counterions with short-range repulsions, of size equal to the Bjerrum length. We determine phase behavior by equilibrating a slab of concentrated coacervate with respect to swelling into a dilute phase of counterions. At salt concentrations below the critical point, the counterion concentration in the coacervate and dilute phases are nearly the same. At high salt concentrations, we find a one-phase region. Along the phase boundary, the total concentration of beads in the coacervate phase is nearly constant, corresponding to a "Bjerrum liquid''. This result can be extended to experimental phase diagrams by assigning appropriate volumes to monomers and salts.

Complex Coacervation of Polymerized Ionic Liquids in Non-aqueous Solvents

Oppositely charged polymerized ionic liquids (PILs) were used to form complex coacervates in two different organic solvents, 2,2,2-trifluoroethanol (TFE) and hexafluoro-2-propanol (HFIP), and the corresponding phase diagrams were constructed using UV-vis, NMR, and turbidity experiments. While previous studies on complex coacervates have focused almost exclusively on aqueous environments, the use of PILs in the current work enabled studies in solvents with substantially lower dielectric constants (27.0 for TFE, 16.7 for HFIP). The critical salt concentration required to induce complete miscibility was roughly 2-fold larger in HFIP compared with TFE, and two different PIL complexes, solidlike precipitates and liquidlike coacervates, were found in both systems. This study provides insight into the effects of low-dielectric-constant solvents on complex coacervation, which has not been widely studied because of the limited solubility of conventional polyelectrolytes in these media.

Electrospinnable, Neutral Coacervates for Facile Preparation of Solid Phenolic Bioadhesives

Liquid-liquid phase separation in an aqueous polymer solution is a unique physicochemical phenomenon, and the material present in the dense bottom layer is called a coacervate. A partial degree of water exclusion during coacervate formation often results in adhesive properties. The high viscosity makes coacervates incompatible with electrospinning processes. Coacervates can be electrospinnable only when the viscosity level of coacervates is adjusted. Electrospinning of coacervates results in a liquid-to-solid phase transition, addressing a long-term stability issue of coacervates. The preserved electrospun membranes can always be reconverted to a coacervate state by dissolution. Herein, we fabricate a spinnable coacervate solution using cosolvents. For neutral, hydrogen bond-dominated coacervates, such as those composed of poly(vinyl alcohol) (PVA) and phenolic tannic acid (TA), the use of a polar cosolvent system such as methanol-water results in an electrospinnable coacervate solution. The spun PVA-TA porous mats are a physicochemically stable solid, and the materials are converted back to an adhesive state upon wetting with body fluid. Considering the emerging studies related to coacervate adhesives, this study suggests that electrospinning a coacervate solution can be a strategy to dramatically increase the material stability and functionality.

Charge Density and Hydrophobicity-Dominated Regimes in the Phase Behavior of Complex Coacervates

The role of hydrophobicity, and particularly of nonionic hydrophobic comonomers, in the phase behavior of polyelectrolyte complex coacervates is not well-understood. Here, we address this problem by synthesizing a library of polymers with a wide range of charge densities and nonionic hydrophobic side chain lengths, and characterizing their phase behavior by optical turbidity. The polymers were prepared by postpolymerization modification of poly(N-acryloxy succinimide), targeting charge densities between 40 and 100% and nonionic aliphatic side chains with lengths from 0 to 12 carbons long. Turbidity measurements on pairs of polycations and polyanions with matched charge densities and nonionic side chain lengths revealed a complex salt response with distinct charge density-dominated and hydrophobicity-dominated regimes. The polymer solubilities were not directly correlated with the phase behavior of the coacervates, indicating the difficulty of understanding the coacervate phase behavior in terms of the polymer-water interaction parameter. This result suggests that there is significant room for further work to understand the mechanisms by which specific molecular-scale interactions moderate the phase behavior of complex coacervates.

Transfer Matrix Model of pH Effects in Polymeric Complex Coacervation

Oppositely charged polyelectrolytes can undergo an associative phase separation, in a process known as polymeric complex coacervation. This phenomenon is driven by the electrostatic attraction between polyanion and polycation species, leading to the formation of a polymer-dense coacervate phase and a coexisting polymer-dilute supernatant phase. There has been significant recent interest in the physical origin and features of coacervation; yet notably, experiments often use weak polyelectrolytes the charge state of which depends on solution pH, while theoretical or computational efforts typically assume strong polyelectrolytes that remain fully charged. There have been only a few efforts to address this limitation, and thus there has been little exploration of how pH can affect complex coacervation. In this paper, we modify a transfer matrix theory of coacervation to account for acid-base equilibria, taking advantage of its ability to directly account for some local ion correlations that will affect monomer charging. We show that coacervation can stabilize the charged state of a weak polyelectrolyte via the proximity of oppositely charged monomers, and can lead to asymmetric phase diagrams where the positively and negatively charged polyelectrolytes exhibit different behaviors near the pK_a of either chain. Specifically, there is a partitioning of one of the salt species to a coacervate to maintain electroneutrality when one of the polyelectrolytes is only partially charged. This results in the depletion of the same salt species in the supernatant, and overall can suppress phase separation. We also demonstrate that, when one of the species is only partially charged, mixtures that are off-stoichiometric in volume fraction but stoichiometric in charge exhibit the greatest propensity to form coacervate phases.

The effects of protein charge patterning on complex coacervation

The complex coacervation of proteins with other macromolecules has applications in protein encapsulation and delivery and for determining the function of cellular coacervates. Theoretical or empirical predictions for protein coacervates would enable the design of these coacervates with tunable and predictable structure-function relationships; unfortunately, no such theories exist. To help establish predictive models, the impact of protein-specific parameters on complex coacervation were probed in this study. The complex coacervation of sequence-specific, polypeptide-tagged, GFP variants and a strong synthetic polyelectrolyte was used to evaluate the effects of protein charge patterning on phase behavior. Phase portraits for the protein coacervates demonstrated that charge patterning dictates the protein's binodal phase boundary. Protein concentrations over 100 mg mL-1 were achieved in the coacervate phase, with concentrations dependent on the tag polypeptide sequence covalently attached to the globular protein domain. In addition to shifting the binodal phase boundary, polypeptide charge patterning provided entropic advantages over isotropically patterned proteins. Together, these results show that modest changes of only a few amino acids in the tag polypeptide sequence alter the coacervation thermodynamics and can be used to tune the phase behavior of polypeptides or proteins of interest.

Preparation of Poly(acrylate)/Poly(diallyldimethylammonium) Coacervates without Small Counterions and Their Phase Behavior upon Salt Addition towards Poly-Ions Segregation

In this work, we report the phase behavior of polyelectrolyte complex coacervates (PECs) of poly(acrylate) (PA-) and poly(diallyldimethylammonium) (PDADMA+) in the presence of inorganic salts. Titrations of the polyelectrolytes in their acidic and alkaline forms were performed to obtain the coacervates in the absence of their small counterions. This approach was previously applied to the preparation of polymer-surfactant complexes, and we demonstrate that it also succeeded in producing complexes free of small counterions with a low extent of Hofmann elimination. For phase behavior studies, two different molar masses of poly(acrylate) and two different salts were employed over a wide concentration range. It was possible to define the regions at which associative and segregative phase separation take place. The latter one was exploited in more details because the segregation phenomenon in mixtures of oppositely charged polyelectrolytes is scarcely reported. Phase composition analyses showed that there is a strong segregation for both PA- and PDADMA+, who are accompanied by their small counterions. These results demonstrate that the occurrence of poly-ion segregation in these mixtures depends on the anion involved: in this case, it was observed with NaCl, but not with Na2SO4.

Looping-in complexation and ion partitioning in nonstoichiometric polyelectrolyte mixtures

A wide variety of intracellular membraneless compartments are formed via liquid-liquid phase separation of charged proteins and nucleic acids. Understanding the stability of these compartments, while accounting for the compositional heterogeneity intrinsic to cellular environments, poses a daunting challenge. We combined experimental and theoretical efforts to study the effects of nonstoichiometric mixing on coacervation behavior and accurately measured the concentrations of polyelectrolytes and small ions in the coacervate and supernatant phases. For synthetic polyacrylamides and polypeptides/DNA, with unequal mixing stoichiometry, we report a general "looping-in" phenomenon found around physiological salt concentrations, where the polymer concentrations in the coacervate initially increase with salt addition before subsequently decreasing. This looping-in behavior is captured by a molecular model that considers reversible ion binding and electrostatic interactions. Further analysis in the low-salt regime shows that the looping-in phenomenon originates from the translational entropy of counterions that are needed to neutralize nonstoichiometric coacervates.