Effects of polyelectrolyte complex micelles and their components on the enzymatic activity of lipase

Robust Electrostatic-Templated Polymerization for Controllable Synthesis of Stable and Permeable Polyelectrolyte Vesicles

Polymer vesicles are of profound interest for designing delivery vehicles and nanoreactors toward a variety of biomedical and catalytic applications, yet robust synthesis of stable and permeable vesicles remains challenging. Here, we propose an electrostatic-templated polymerization that enables fabrication of polyelectrolyte vesicles with simultaneously regulated stability and permeability. In our design, cationic monomers were copolymerized with cross-linkers in the presence of a polyanionic-neutral diblock copolymer as a template. By properly choosing the block length ratio of the template, we fabricated a type of polyion complex vesicle consisting of a cross-linked cationic membrane, electrostatically assembled with the template copolymer which can be removed by sequential dissociation and separation under concentrated salt. We finally obtained stable polyelectrolyte vesicles of regulated size, membrane permeability, and response properties by tuning the synthesis factors including ionic strength, cross-linker type, and fraction as well as different monomers and concentrations. As a proof-of-concept, lipase was loaded in the designed cationic vesicles, which exhibited enhanced enzyme stability and activity. Our study has developed a novel and robust strategy for controllable synthesis of a new class of stable and permeable polymer (polyelectrolyte) vesicles that feature great potential applications as functional delivery carriers and nanoreactors.

Impact of a Lightly Branched Star Polyelectrolyte Architecture on Polyelectrolyte Complexes

The effect of charge density in blocky and statistical linear polyelectrolytes on polyelectrolyte complex (PEC) properties has been studied with the finding that increased charge density in a polyelectrolyte tends to increase the salt resistance and modulus of a PEC across various polyelectrolyte pairs. Here, we demonstrate the ability to orthogonally alter PEC salt resistance while maintaining rheological properties and internal structure by going from linear to lightly branched architectures with similar total degrees of polymerization. Using a model system built around glycidyl methacrylate (GMA) and thiol-epoxy "click" functionalization, we create a library of homologous linear, 4-armed, 6-armed, and 8-armed star polyelectrolytes. The PECs formed from these model polyelectrolyte pairs are then characterized via optical microscopy, rheology, and small-angle X-ray scattering to evaluate their salt resistance, mechanical properties, and internal structure. We argue that our results are due to the difference between linear charge density or charge per unit length along backbone segments for each polyelectrolyte and spatial charge density, the number of charges per unit volume of the polyelectrolyte prior to complexation. Our findings suggest that linear charge density is the dominant factor in determining intermolecular interactions of the complex, leading to identical rheological and structural behavior, whereas the spatial charge density primarily influences the stability of the complexes. These distinct mechanisms for altering various sought-after PEC properties offer greater potential applications in precision design of polyelectrolyte complex materials.

Electrostatically Mediated In Situ Polymerization for Enzyme Immobilization and Activation

Enzyme immobilization in nanoparticles is of interest for boosting their catalytic applications, yet rational approaches to designs achieving both high enzyme loading and activation remain a challenge. Herein, we report an electrostatically mediated in situ polymerization strategy that simultaneously realizes enzyme immobilization and activation. This was achieved by copolymerizing cationic monomers with a cross-linker in the presence of the enzyme lipase (anionic) as the template, which produces enzyme-loaded nanogels. The effects of different control factors such as pH, lipase dosage, and cross-linker fraction on nanogel formation are investigated systematically, and optimal conditions for enzyme loading and activation have been determined. A central finding is that the cationic polymer network of the nanogel creates a favorable environment that not only protects the enzyme but also boosts enzymatic activity nearly 2-fold as compared to free lipase. The nanogels improve the stability of the lipase to tolerate a broader working range of pH (5.5-8.5) and temperature (25-70 °C) and allow recycling such that after six cycles of reaction, 70% of the initial activity is conserved. The established fabrication strategy can be applied generally to different cationic monomers, and most of these nanogels exhibit adequate immobilization and activation of lipase. Our study confirms that in situ polymerization based on electrostatic interaction provides a facile and robust strategy for enzyme immobilization and activation. The wide variety of ionic monomers, therefore, features great potential for developing functional platforms toward satisfying enzyme immobilization and demanding applications.

Assessing the Structure and Equilibrium Conditions of Complex Coacervate Core Micelles by Varying Their Shell Composition and Medium Ionic Strength

Complex coacervates result from an associative phase separation commonly involving oppositely charged polyelectrolytes. When this associative interaction occurs between charged-neutral diblock copolymers and oppositely charged homopolymers, a nanometric aggregate called a complex coacervate core micelle, C3M, is formed. Recent studies have addressed the issue of their thermodynamic or kinetic stability but without a clear consensus. To further investigate this issue, we have studied C3Ms formed by the combination of poly(diallyldimethylammonium) and copolymer poly(acrylamide)-b-poly(acrylate) using different preparation protocols. Dynamic light scattering and small-angle X-ray scattering measurements suggest that these structures are in an equilibrium condition because the aggregates do not vary with different preparation protocols or upon aging. In addition, their stability and structures are critically dependent on several parameters such as the density of neutral blocks in their shell and the ionic strength of the medium. Decreasing the amount of copolymer in the system and, hence, the density of neutral blocks in the shell results in an increase in the aggregate size because of the core growth, although their globular shape is retained. On the other hand, larger clusters of micelles were formed at higher ionic strengths. Partially replacing 77% of the copolymer with a homopolymer of the same charge or increasing the ionic strength of the system (above 100 mmol L-1 NaCl) leads to a metastable state, after which phase separation is eventually observed. SAXS analyses reveal that this phase separation above a certain salt concentration occurs due to the coagulation of individual micelles that seem to retain their individual globular structures. Overall, these results confirm earlier claims that equilibrium C3Ms are achieved close to 1:1 charge stoichiometry but also reveal that these conditions may vary at different shell densities or higher ionic strengths, which constitute vital information for envisioning future applications of C3Ms.

Formation of Polyion Complex Aggregate Formed from a Cationic Block Copolymer and Anionic Polysaccharide

Block copolymers (PmMn; P20M101 and P100M98) comprising poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC, P) containing biocompatible phosphorylcholin pendants and cationic poly((3-acryloylaminopropyl) trimethylammonium chloride) (PMAPTAC, M) were synthesized via a controlled radical polymerization method. The degrees of polymerization of the PMPC and PMAPTAC segments are denoted by subscripts (PmMn). The mixture of cationic PmMn and anionic sodium chondroitin sulfate C (CS) with the pendant anionic carboxylate and sulfonate groups formed polyion complex (PIC) aggregates in phosphate-buffered saline. A charge-neutralized mixture of P20M101 with CS formed P20M101/CS PIC vesicles with a hydrodynamic radius (Rh) of 97.2 nm, zeta potential of ca. 0 mV, and aggregation number (Nagg) of 23,044. PMPC shells covered the surface of the PIC vesicles. The mixture of P100M98 and CS formed PIC spherical micelles with the PIC core and hydrophilic PMPC shells. The Rh, zeta potential, and Nagg of the PIC micelles were 26.4 nm, ca. 0 mV, and 404, respectively. At pH < 4, the carboxylate anions in CS were protonated. Thus, the charge balance in the PIC micelles shifted to decrease the core density owing to the electrostatic repulsions of the excess cations in the core. The PIC micelles dissociated at a NaCl concentration ≥0.6 M owing to the charge screening effect. The positively charged PIC micelles with excess P100M98 can encapsulate anionic dyes owing to electrostatic interaction.

Noncovalent PEGylation of protein and peptide therapeutics

Clinical applications of protein therapeutics-an advanced generation of drugs characterized by high biological specificity-are rapidly expanding. However, their development is often impeded by unfavorable pharmacokinetic profiles and largely relies on the use of drug delivery systems to prolong their in vivo half-life and suppress undesirable immunogenicity. Although a commercially established PEGylation technology based on protein conjugation with poly(ethylene glycol) (PEG)-protective steric shield resolves some of the challenges, the search for alternatives continues. Noncovalent PEGylation, which mainly relies on multivalent (cooperative) interactions and high affinity (host-guest) complexes formed between protein and PEG offers a number of potential advantages. Among them are dynamic or reversible protection of the protein with minimal loss of biological activity, drastically lower manufacturing costs, "mix-and-match" formulations approaches, and expanded scope of PEGylation targets. While a great number of innovative chemical approaches have been proposed in recent years, the ability to effectively control the stability of noncovalently assembled protein-PEG complexes under physiological conditions presents a serious challenge for the commercial development of the technology. In an attempt to identify critical factors affecting pharmacological behavior of noncovalently linked complexes, this Review follows a hierarchical analysis of various experimental techniques and resulting supramolecular architectures. The importance of in vivo administration routes, degradation patterns of PEGylating agents, and a multitude of potential exchange reactions with constituents of physiological compartments are highlighted. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease.

Preparation of Water-Soluble Polyion Complex (PIC) Micelles with Random Copolymers Containing Pendant Quaternary Ammonium and Sulfonate Groups

Cationic random copolymers (PCm) consisting of 2-(methacryloyloxy)ethyl phosphorylcholine (MPC; P) with methacroylcholine chloride (MCC; C) and anionic random copolymers (PSn) consisting of MPC and potassium 3-(methacryloyloxy)propanesulfonate (MPS; S) were prepared via a reversible addition-fragmentation chain transfer method. "m" and "n" represent the compositions (mol %) of the MCC and MPS units in the copolymers, respectively. The degrees of polymerization for the copolymers were 93-99. Water-soluble MPC unit contains a pendant zwitterionic phosphorylcholine group whose charges are neutralized in pendant groups. MCC and MPS units contain the cationic quaternary ammonium and anionic sulfonate groups, respectively. The stoichiometrically charge-neutralized mixture of a matched pair of PCm and PSn aqueous solutions resulted in the spontaneous formation of water-soluble PCm/PSn polyion complex (PIC) micelles. These PIC micelles have the MPC-rich surface and MCC/MPS core. These PIC micelles were characterized using ¹H NMR, dynamic and static light scattering, and transmission electron microscopic measurements. The hydrodynamic radius of these PIC micelles depends on the mixing ratio of the oppositely charged random copolymers. The charge-neutralized mixture formed maximum-size PIC micelles.

Biomolecular Liquid-Liquid Phase Separation for Biotechnology

The liquid-liquid phase separation (LLPS) of biomolecules induces condensed assemblies called liquid droplets or membrane-less organelles. In contrast to organelles with lipid membrane barriers, the liquid droplets induced by LLPS do not have distinct barriers (lipid bilayer). Biomolecular LLPS in cells has attracted considerable attention in broad research fields from cellular biology to soft matter physics. The physical and chemical properties of LLPS exert a variety of functions in living cells: activating and deactivating biomolecules involving enzymes; controlling the localization, condensation, and concentration of biomolecules; the filtration and purification of biomolecules; and sensing environmental factors for fast, adaptive, and reversible responses. The versatility of LLPS plays an essential role in various biological processes, such as controlling the central dogma and the onset mechanism of pathological diseases. Moreover, biomolecular LLPS could be critical for developing new biotechnologies such as the condensation, purification, and activation of a series of biomolecules. In this review article, we introduce some fundamental aspects and recent progress of biomolecular LLPS in living cells and test tubes. Then, we discuss applications of biomolecular LLPS toward biotechnologies.

Comb Polyelectrolytes Stabilize Complex Coacervate Microdroplet Dispersions

Complex coacervate microdroplets are membraneless compartments that selectively sequester biological molecules from their surroundings and enhance bioreactions. Yet, their use as protocell models and bioreactors has been limited owing to a lack of feasible strategies to prevent their uncontrolled coalescence. Herein, we introduce an approach to mitigate coalescence of complex coacervate microdroplets using comb polyelectrolytes as stabilizers, creating complex coacervate dispersions with months-long stabilities. Tunability of microdroplet size and stability is achieved by the regulation of comb polyelectrolyte concentration and molecular weight. Importantly, the comb polyelectrolyte-stabilized coacervate microdroplets spontaneously sequester and retain proteins over extended periods. Moreover, enhanced catalytic activity of proteins and significant (up to 10-fold) acceleration of bioreactions are achieved in stabilized complex coacervate dispersions, even when stored for up to 48 h. Our findings are expected to expand the utility of complex coacervate microdroplets as artificial protocells, encapsulants, and bioreactors and also facilitate their use in pharmaceutical, agricultural, food, and cosmetics formulations.

Enhanced stability of complex coacervate core micelles following different core-crosslinking strategies

Complex coacervate core micelles (C3Ms) are formed by mixing aqueous solutions of a charged (bio)macromolecule with an oppositely charged-neutral hydrophilic diblock copolymer. The stability of these structures is dependent on the ionic strength of the solution; above a critical ionic strength, the micelles will completely disintegrate. This instability at high ionic strengths is the main drawback for their application in, e.g., drug delivery systems or protein protection. In addition, the stability of C3Ms composed of weak polyelectrolytes is pH-dependent as well. The aim of this study is to assess the effectiveness of covalent crosslinking of the complex coacervate core to improve the stability of C3Ms. We studied the formation of C3Ms using a quaternized and amine-functionalized cationic-neutral diblock copolymer, poly(2-vinylpyridine)-block-poly(ethylene oxide) (QP2VP-b-PEO), and an anionic homopolymer, poly(acrylic acid) (PAA). Two different core-crosslinking strategies were employed that resulted in crosslinks between both types of polyelectrolyte chains in the core (i.e., between QP2VP and PAA) or in crosslinks between polyelectrolyte chains of the same type only (i.e., QP2VP). For these two strategies we used the crosslinkers 1-ethyl-3-(3'-dimethylaminopropyl)carbodiimide hydrochloride (EDC) and dimethyl-3,3'-dithiopropionimidate dihydrochloride (DTBP), respectively. EDC provides permanent crosslinks, while DTBP crosslinks can be broken by a reducing agent. Dynamic light scattering showed that both approaches significantly improved the stability of C3Ms against salt and pH changes. Furthermore, reduction of the disulphide bridges in the DTBP core-crosslinked micelles largely restored the original salt-stability profile. Therefore, this feature provides an excellent starting point for the application of C3Ms in controlled release formulations.

Synthesis of Anionic Nanogels for Selective and Efficient Enzyme Encapsulation

Polyelectrolyte nanogels containing cross-linked ionic polymer networks feature both soft environment and intrinsic charges which are of great potential for enzyme encapsulation. In this work, well-defined poly(acrylic acid) (PAA) nanogels have been synthesized based on a facile strategy, namely, electrostatic assembly directed polymerization (EADP). Specifically, AA monomers are polymerized together with a cross-linker in the presence of a cationic-neutral diblock copolymer as the template. Effects of control factors including pH, salt concentration, and cross-linking degree have been investigated systematically, based on which the optimal preparation of PAA nanogels has been established. The obtained nanogel features not only compatible pocket for safely loading enzymes without disturbing their structures, but also abundant negative charges which enable selective and efficient encapsulation of cationic enzymes. The loading capacities of PAA nanogels for cytochrome (cyt c) and lysozyme are 100 and 125 μg/mg (enzyme/nanogel), respectively. More notably, the PAA network seems to modulate a favorable microenvironment for cyt c and induces 2-fold enhanced activity for the encapsulated enzymes, as indicated by the steady-state kinetic assay. Our study reveals the control factors of EADP for optimal synthesis of anionic nanogels and validates their distinctive advances with respect to efficient loading and activation of cationic enzymes.

Charged Polypeptide Tail Boosts the Salt Resistance of Enzyme-Containing Complex Coacervate Micelles

Encapsulation of proteins can have advantages for their protection, stability, and delivery purposes. One of the options to encapsulate proteins is to incorporate them in complex coacervate core micelles (C3Ms). This can easily be achieved by mixing aqueous solutions of the protein and an oppositely charged neutral-hydrophilic diblock copolymer. However, protein-containing C3Ms often suffer from salt-inducible disintegration due to the low charge density of proteins. The aim of this study is to improve the salt stability of protein-containing C3Ms by increasing the net charge of the protein by tagging it with a charged polypeptide. As a model protein, we used CotA laccase and generated variants with 10, 20, 30, and 40 glutamic acids attached at the C-terminus of CotA using genetic engineering. Micelles were obtained by mixing the five CotA variants with poly(N-methyl-2-vinyl-pyridinium)-block-poly(ethylene oxide) (PM2VP128-b-PEO477) at pH 10.8. Hydrodynamic radii of the micelles of approximately 31, 27, and 23 nm for native CotA, CotA-E20, and CotA-E40, respectively, were determined using dynamic light scattering (DLS) and fluorescence correlation spectroscopy (FCS). The encapsulation efficiency was not affected using enzymes with a polyglutamic acid tail but resulted in more micelles with a smaller number of enzyme molecules per micelle. Furthermore, it was shown that the addition of a polyglutamic acid tail to CotA indeed resulted in improved salt stability of enzyme-containing C3Ms. Interestingly, the polyglutamic acid CotA variants showed an enhanced enzyme activity. This study demonstrates that increasing the net charge of enzymes through genetic engineering is a promising strategy to improve the practical applicability of C3Ms as enzyme delivery systems.

Extraction of Lysozyme from Chicken Albumen Using Polyelectrolyte Complexes

Cells use droplet-like membrane-less organelles (MLOs) to compartmentalize and selectively take-up molecules, such as proteins, from their internal environment. These membraneless organelles can be mimicked by polyelectrolyte complexes (PECs) consisting of oppositely charged polyelectrolytes. Previous research has demonstrated that protein uptake strongly depends on the PEC composition. This suggests that PECs can be used to selectively extract proteins from a multi-protein mixture. With this in mind, the partitioning of the protein lysozyme in four PEC systems consisting of different weak and strong polyelectrolyte combinations is investigated. All systems show similar trends in lysozyme partitioning as a function of the complex composition. The release of lysozyme from complexes at their optimal lysozyme uptake composition is investigated by increasing the salt concentration to 500 mm NaCl or lowering the pH from 7 to 4. Complexes of poly(allylamine hydrochloride) and poly(acrylic acid) have the best uptake and release properties. These are used for selective extraction of lysozyme from a hen-egg white protein matrix. The (back)-extracted lysozyme retains its enzymatic activity, showing the capability of PECs to function as extraction media for proteins.

Regulated Polyelectrolyte Nanogels for Enzyme Encapsulation and Activation

Polyelectrolyte (PE) nanogels consisting of cross-linked PE networks integrate the advanced features of both nanogels and PEs. The soft environment and abundant intrinsic charges are of special interest for enzyme immobilization. However, the crucial factors that regulate enzyme encapsulation and activation remain obscure to date. Herein, we synthesized cationic poly (dimethyl aminoethyl methacrylate), PDMAEMA, nanogels with well-defined size and cross-link degrees and fully investigated the effects of different control factors on lipase immobilization. We demonstrate that the cationic PDMAEMA nanogels indeed enable efficient and safe loading of anionic lipase without disturbing their structures. Strong charge interaction achieved by tuning pH and larger particle size are favorable for lipase loading, while the enhanced enzymatic activity demands nanogels with smaller size and a moderate cross-link degree. As such, PDMAEMA nanogels with a hydrodynamic radius of 35 nm and 30% cross-linker fraction display the optimal catalytic efficiency, which is fourfold of that of free lipase. Moreover, the immobilization endows enhanced enzymatic activity in a broad scope of pH, ionic strength, and temperature, demonstrating effective protection and activation of lipase by the designed nanogels. Our study validates the crucial controls of the size and structure of PE nanogels on enzyme encapsulation and activation, and the revealed findings shall be helpful for designing functional PE nanogels and boosting their applications for enzyme immobilization.

Complex coacervates as extraction media

Various solvents such as ionic liquids, deep eutectic solvents, and aqueous two phase systems have been suggested as greener alternatives to existing extraction processes. We propose to add macroscopic complex coacervates to this list. Complex coacervates are liquid-like forms of polyion condensates and consist of a complex of oppositely charged polyions and water. Previous research focussing on the biological significance of these polyion-rich phases has shown that polyion condensates have the ability to extract certain solutes from water and back-extract them by changing parameters such as ionic strength and pH. In this study, we present the distribution coefficients of five commonly used industrial chemicals, namely lactic acid, butanol, and three types of lipase enzymes in poly(ethylenimine)/poly(acrylic acid) complex coacervates. It was found that the distribution coefficients can vary strongly upon variation of tunable parameters such as polyion ratio, ionic strength, polyion and compound concentrations, and temperature. Distribution coefficients ranged from approximately 2 to 50 depending on the tuning of the system parameters. It was also demonstrated that a temperature-swing extraction is possible, with back-extraction of butanol from complex coacervates with a recovery of 21.1%, demonstrating their potential as extraction media.

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.

On Complex Coacervate Core Micelles: Structure-Function Perspectives

The co-assembly of ionic-neutral block copolymers with oppositely charged species produces nanometric colloidal complexes, known, among other names, as complex coacervates core micelles (C3Ms). C3Ms are of widespread interest in nanomedicine for controlled delivery and release, whilst research activity into other application areas, such as gelation, catalysis, nanoparticle synthesis, and sensing, is increasing. In this review, we discuss recent studies on the functional roles that C3Ms can fulfil in these and other fields, focusing on emerging structure-function relations and remaining knowledge gaps.

Balancing Enzyme Encapsulation Efficiency and Stability in Complex Coacervate Core Micelles

Encapsulation of charged proteins into complex coacervate core micelles (C3Ms) can be accomplished by mixing them with oppositely charged diblock copolymers. However, these micelles tend to disintegrate at high ionic strength. Previous research showed that the addition of a homopolymer with the same charge sign as the protein improved the stability of protein-containing C3Ms. In this research, we used fluorescence correlation spectroscopy (FCS) and dynamic light scattering (DLS) to study how the addition of the homopolymer affects the encapsulation efficiency and salt stability of the micelles. We studied the encapsulation of laccase spore coat protein A (CotA), a multicopper oxidase, using a strong cationic-neutral diblock copolymer, poly(N-methyl-2-vinyl-pyridinium iodide)-block-poly(ethylene oxide) (PM2VP128-b-PEO477), and a negatively charged homopolymer, poly(4-styrenesulfonate) (PSS215). DLS indeed showed an improved stability of this three-component C3M system against the addition of salt compared to a two-component system. Remarkably, FCS showed that the release of CotA from a three-component C3M system occurred at a lower salt concentration and over a narrower concentration range than the dissociation of C3Ms. In conclusion, although the addition of the homopolymer to the system leads to micelles with a higher salt stability, CotA is excluded from the C3Ms already at lower ionic strengths because the homopolymer acts as a competitor of the enzyme for encapsulation.

Effects of pH on the Formation of PIC Micelles from PAMAM Dendrimers

Dendrimer-based PIC micelles are novel nanostructures from the assembly of dendrimers with polyion-neutral diblock copolymers. Because of the branched and three-dimensional structure of dendrimers, understanding the electrostatic assembly is challenging yet essential for manipulating the formation and property of the PIC micelles. Herein, we present the pH effects on the assembly of amine-terminated PAMAM dendrimers with PSS₉₂-b-PEO₁₁₃ diblock copolymers. The step-wise protonation of primary and tertiary amine groups of PAMAM allows us to manipulate the number of the positive charges by tuning pH. We find that the assembly based on the surface charges of PAMAM from G2 to G7 at pH 7 leads to well-defined micelles with high stability against salt. At pH 3, both the interior and surface charges contribute to the assembly, and the formed micelles are sensitive to ionic strength, namely, increasing salt concentration results in the formation of elongated (G2-G5) or bigger (G7) aggregates. Our study reveals the pH manipulation on the assembly of PAMAM dendrimers with linear polyelectrolytes and displays new findings that shall be helpful for understanding the assembly of asymmetric polyelectrolytes, as well as for designing new PIC micelles and functional soft nanocarriers.

Charge Matters: Electrostatic Complexation As a Green Approach to Assemble Advanced Functional Materials

We report on electrostatically complexed materials bearing advanced functions that are not possible for other assemblies. The fundamentals of electrostatic association between oppositely charged polyelectrolytes and colloidal particles are introduced together with the conditions needed for complexation, including those related to ionic strength, pH, and hydration. Related considerations allow us to control the properties of the formed complexes and to develop features such as self-healing and underwater adhesion. In contrast to assemblies produced by typical hydrophobic and chemical interactions, electrostatic complexation leads to reversible systems. A state-of-the-art account of the field of electrostatically complexed materials is provided, including those formed from biomolecules and for salt-controlled rheology, underwater adhesiveness, and interfacial spinning. Finally, we present an outlook of electrostatic complexation from the colloidal chemistry perspective.

Supramolecular crosslinks enable PIC micelles with tuneable salt stability and diverse properties

The stability of polyion complex (PIC) nanoparticles, like PIC micelles or PICsomes, in water is typically affected by added salt because salt screens the electrostatic driving force. This lack of salt stability seriously hampers numerous potential applications and a remedy is needed. Extending an earlier idea, we develop here a general strategy for preparing PIC micelles, with not only tuneable salt stability but also built-in functions. Using two different dipicolinic (DPA)-based ligands (a linear bis-ligand and a branched tris-ligand), as well as various metal ions we obtain anionic coordination polymers that subsequently co-assemble with a polycationic-neutral diblock copolymer to form PIC micelles. By a judicious choice of the metal ions and/or an appropriate mixture of the ligands we can create micellar cores with two types of reversible cross-links. In this way, we construct PIC micelles with not only tuneable and enhanced salt stability, but also tuned metal-derived properties, such as luminescence or magnetic relaxation. This non-covalent cross-link strategy, exclusively based on building block composition, is generally applicable with different metal ions and ligand combinations, and is therefore a robust approach for preparing stable and functional PIC micelles. Extension to other types of assemblies such as 'PICsomes' is possible, and therefore a range of applications becomes feasible.

Charge-Based Separation of Proteins Using Polyelectrolyte Complexes as Models for Membraneless Organelles

Membraneless organelles are liquid compartments within cells with different solvent properties than the surrounding environment. This difference in solvent properties is thought to result in function-related selective partitioning of proteins. Proteins have also been shown to accumulate in polyelectrolyte complexes, but whether the uptake in these complexes is selective has not been ascertained yet. Here, we show the selective partitioning of two structurally similar but oppositely charged proteins into polyelectrolyte complexes. We demonstrate that these proteins can be separated from a mixture by altering the polyelectrolyte complex composition and released from the complex by lowering the pH. Combined, we demonstrate that polyelectrolyte complexes can separate proteins from a mixture based on protein charge. Besides providing deeper insight into the selective partitioning in membraneless organelles, potential applications for selective biomolecule partitioning in polyelectrolyte complexes include drug delivery or extraction processes.

Catalytic Biosensors from Complex Coacervate Core Micelle (C3M) Thin Films

Enzymes have been applied to a variety of industrially and medically relevant chemistries as both catalysts and sensors. Incorporation of proteins and enzymes into complex coacervates has been demonstrated to improve the thermal, chemical, and temporal stability of enzymes in solution. In this work, a neutral-cationic block copolymer and an enzyme, alkaline phosphatase, are incorporated into complex coacervate core micelles (C3Ms) and coated onto a solid substrate to create a biocatalytic film from aqueous solution. The incorporation of photo-cross-linkable groups into the neutral block of the polymer allows the film to be cross-linked under ultraviolet light, rendering it insoluble. The morphology of the film is shown to depend most strongly on the protein loading within the film, while solvent annealing is shown to have a minimal effect. These films are then demonstrated as specific sensors for Zn²⁺ in solution in the presence of other metals, a model reaction for ion-selective heavy metal biosensing useful in environmental monitoring. They are shown to have low leaching and maintain sufficient activity and response for sensing for 1 month after aging under ambient conditions and at 40 °C and 50% relative humidity. The C3M immobilization method demonstrated can be applied to a wide variety of proteins with minimal chemical or genetic modification and could be used for immobilization of charged macromolecules in general to produce a wide variety of thin-film devices.

Protein-Polyelectrolyte Complexes and Micellar Assemblies

In this review, we highlight the recent progress in our understanding of the structure, properties and applications of protein-polyelectrolyte complexes in both bulk and micellar assemblies. Protein-polyelectrolyte complexes form the basis of the genetic code, enable facile protein purification, and have emerged as enterprising candidates for simulating protocellular environments and as efficient enzymatic bioreactors. Such complexes undergo self-assembly in bulk due to a combined influence of electrostatic interactions and entropy gains from counterion release. Diversifying the self-assembly by incorporation of block polyelectrolytes has further enabled fabrication of protein-polyelectrolyte complex micelles that are multifunctional carriers for therapeutic targeted delivery of proteins such as enzymes and antibodies. We discuss research efforts focused on the structure, properties and applications of protein-polyelectrolyte complexes in both bulk and micellar assemblies, along with the influences of amphoteric nature of proteins accompanying patchy distribution of charges leading to unique phenomena including multiple complexation windows and complexation on the wrong side of the isoelectric point.

Biomolecular Chemistry in Liquid Phase Separated Compartments

Biochemical processes inside the cell take place in a complex environment that is highly crowded, heterogeneous, and replete with interfaces. The recently recognized importance of biomolecular condensates in cellular organization has added new elements of complexity to our understanding of chemistry in the cell. Many of these condensates are formed by liquid-liquid phase separation (LLPS) and behave like liquid droplets. Such droplet organelles can be reproduced and studied in vitro by using coacervates and have some remarkable features, including regulated assembly, differential partitioning of macromolecules, permeability to small molecules, and a uniquely crowded environment. Here, we review the main principles of biochemical organization in model membraneless compartments. We focus on some promising in vitro coacervate model systems that aptly mimic part of the compartmentalized cellular environment. We address the physicochemical characteristics of these liquid phase separated compartments, and their impact on biomolecular chemistry and assembly. These model systems enable a systematic investigation of the role of spatiotemporal organization of biomolecules in controlling biochemical processes in the cell, and they provide crucial insights for the development of functional artificial organelles and cells.

Macro- and Microphase Separated Protein-Polyelectrolyte Complexes: Design Parameters and Current Progress

Protein-containing polyelectrolyte complexes (PECs) are a diverse class of materials, composed of two or more oppositely charged polyelectrolytes that condense and phase separate near overall charge neutrality. Such phase-separation can take on a variety of morphologies from macrophase separated liquid condensates, to solid precipitates, to monodispersed spherical micelles. In this review, we present an overview of recent advances in protein-containing PECs, with an overall goal of defining relevant design parameters for macro- and microphase separated PECs. For both classes of PECs, the influence of protein characteristics, such as surface charge and patchiness, co-polyelectrolyte characteristics, such as charge density and structure, and overall solution characteristics, such as salt concentration and pH, are considered. After overall design features are established, potential applications in food processing, biosensing, drug delivery, and protein purification are discussed and recent characterization techniques for protein-containing PECs are highlighted.

Metallo-polyelectrolytes as a class of ionic macromolecules for functional materials

The fields of soft polymers and macromolecular sciences have enjoyed a unique combination of metals and organic frameworks in the name of metallopolymers or organometallic polymers. When metallopolymers carry charged groups, they form a class of metal-containing polyelectrolytes or metallo-polyelectrolytes. This review identifies the unique properties and functions of metallo-polyelectrolytes compared with conventional organo-polyelectrolytes, in the hope of shedding light on the formation of functional materials with intriguing applications and potential benefits. It concludes with a critical perspective on the challenges and hurdles for metallo-polyelectrolytes, especially experimental quantitative analysis and theoretical modeling of ionic binding.

Electric field-induced circulation and vacuolization regulate enzyme reactions in coacervate-based protocells

Artificial protocells operating under non-equilibrium conditions offer a new approach to achieve dynamic features with life-like properties. Using coacervate micro-droplets comprising polylysine (PLL) and a short single-stranded oligonucleotide (ss-oligo) as a membrane-free protocell model, we demonstrate that circulation and vacuolization can occur simultaneously inside the droplet in the presence of an electric field. The circulation is driven by electrohydrodynamics and applies specifically to the major components of the protocell (PLL and ss-oligo). Significantly, under low electric fields (E = 10 V cm^-1) the circulation regulates the movement of the vacuoles, while high levels of vacuolization produced at higher electric fields can deform or reshape the circulation. By taking advantage of the interplay between vacuolization and circulation, we achieve dynamic localization of an enzyme cascade reaction at specific droplet locations. In addition, the spatial distribution of the enzyme reaction is globalized throughout the droplet by tuning the coupling of the circulation and vacuolization processes. Overall, our work provides a new strategy to create non-equilibrium dynamic behaviors in molecularly crowded membrane-free synthetic protocells.

Encapsulation into complex coacervate core micelles promotes EGFP dimerization

Complex coacervate core micelles (C3Ms) are colloidal structures useful for encapsulation of biomacromolecules. We previously demonstrated that enhanced green fluorescent protein (EGFP) can be encapsulated into C3Ms using the diblock copolymer poly(2-methyl-vinyl-pyridinium)₄₁-b-poly(ethylene-oxide)₂₀₅. This packaging resulted in deviating spectroscopic features of the encapsulated EGFP molecules. Here we show that for monomeric EGFP variant (mEGFP) micellar encapsulation affects the absorption and fluorescence properties to a much lesser extent, and that changes in circular dichroism characteristics are specific for encapsulated EGFP. Time-resolved fluorescence anisotropy of encapsulated (m)EGFP established the occurrence of homo-FRET (Förster resonance energy transfer) with larger transfer correlation times in the case of EGFP. Together, these findings support that EGFP dimerizes whereas the mEGFP mainly remains as a monomer in the densely packed C3Ms. We propose that dimerization of encapsulated EGFP causes a reorientation of Glu222, resulting in a pK_a shift of the chromophore, which is fully reversible after release of EGFP from the C3Ms at a high ionic strength.

Complex Coacervate Core Micelles for the Dispersion and Stabilization of Organophosphate Hydrolase in Organic Solvents

Organophosphate (OP) nerve agents are a class of chemical warfare agents (CWAs) that exist as bulk stocks in current and past war zones. Thus, a technology that can perform on-site decontamination in a safe and timely fashion is desirable. Here, complex coacervate core micelles (C3Ms) were used to encapsulate organophosphate hydrolase (OPH) and chemostabilize it to maintain activity after exposure to organophosphate simulants ethanol and dimethyl methylphosphonate (DMMP). C3Ms were formed by two polymers-poly(acrylic acid) (PAA) and poly(oligo(ethylene glycol) methacrylate)-b-poly(4-vinyl N-methylpyridyl iodide), (POEGMA-b-qP4VP). Complexes of the coacervate micelles with the enzyme OPH were investigated by small angle neutron scattering (SANS), dynamic light scattering (DLS), and transmission electron microscopy (TEM), demonstrating the formation of micellar structures in solution. The activity of OPH against methyl paraoxon in these C3Ms under aqueous conditions was assayed after heat treatment for 3 days at 37 °C. The OPH in C3Ms retained 88 ± 7% of its initial activity, as compared to the 48 ± 3% activity retained by OPH alone, indicating that the C3Ms were able to stabilize the enzyme to heat treatment. C3Ms transferred into the two organic solvents formed larger structures than inverse micelles formed by the block copolymer alone. The addition of OPH to the C3Ms in organic solvents did not significantly change their structure. The activity of OPH (again, against methyl paraoxon) after 24 h of incubation at 4 °C was measured and compared to that of OPH in C3Ms. While OPH alone retained less than 5% of its activity after this incubation in both solvents, OPH in C3Ms retained 35 ± 3% of its activity in DMMP and 26 ± 1% of its activity in ethanol.

Formation of Polyion Complex (PIC) Micelles and Vesicles with Anionic pH-Responsive Unimer Micelles and Cationic Diblock Copolymers in Water

A random copolymer (p(A/MaU)) of sodium 2-(acrylamido)-2-methylpropanesulfonate (AMPS) and sodium 11-methacrylamidoundecanate (MaU) was prepared via conventional radical polymerization, which formed a unimer micelle under acidic conditions due to intramolecular hydrophobic interactions between the pendant undecanoic acid groups. Under basic conditions, unimer micelles were opened up to an expanded chain conformation by electrostatic repulsion between the pendant sulfonate and undecanoate anions. A cationic diblock copolymer (P163M99) consisting of poly(3-(methacrylamido)propyl)trimethylammonium chloride (PMAPTAC) and hydrophilic polybetaine, 2-(methacryloyloxy)ethylphosphorylcholine (MPC), blocks was prepared via controlled radical polymerization. Mixing of p(A/MaU) and P163M99 in 0.1 M aqueous NaCl under acidic conditions resulted in the formation of spherical polyion complex (PIC) micelles and vesicles, depending on polymer concentration before mixing. Shapes of the PIC micelles and vesicles changed under basic conditions due to collapse of the charge balance between p(A/MaU) and P163M99. The PIC vesicles can incorporate nonionic hydrophilic guest molecules, and the PIC micelles and vesicles can accept hydrophobic guest molecules in the hydrophobic core formed from p(A/MaU).

Complex coacervation of supercharged proteins with polyelectrolytes

Complexation of proteins with polyelectrolytes or block copolymers can lead to phase separation to generate a coacervate phase or self-assembly of coacervate core micelles. However, many proteins do not coacervate at conditions near neutral pH and physiological ionic strength. Here, protein supercharging is used to systematically explore the effect of protein charge on the complex coacervation with polycations. Four model proteins were anionically supercharged to varying degrees as quantified by mass spectrometry. Proteins phase separated with strong polycations when the ratio of negatively charged residues to positively charged residues on the protein (α) was greater than 1.1-1.2. Efficient partitioning of the protein into the coacervate phase required larger α (1.5-2.0). The preferred charge ratio for coacervation was shifted away from charge symmetry for three of the four model proteins and indicated an excess of positive charge in the coacervate phase. The composition of protein and polymer in the coacervate phase was determined using fluorescently labeled components, revealing that several of the coacervates likely have both induced charging and a macromolecular charge imbalance. The model proteins were also encapsulated in complex coacervate core micelles and micelles formed when the protein charge ratio α was greater than 1.3-1.4. Small angle neutron scattering and transmission electron microscopy showed that the micelles were spherical. The stability of the coacervate phase in both the bulk and micelles improved to increased ionic strength as the net charge on the protein increased. The micelles were also stable to dehydration and elevated temperatures.

Folding Behaviors of Protein (Lysozyme) Confined in Polyelectrolyte Complex Micelle

The folding/unfolding behavior of proteins (enzymes) in confined space is important for their properties and functions, but such a behavior remains largely unexplored. In this article, we reported our finding that lysozyme and a double hydrophilic block copolymer, methoxypoly(ethylene glycol)5K-block-poly(l-aspartic acid sodium salt)10 (mPEG(5K)-b-PLD10), can form a polyelectrolyte complex micelle with a particle size of ∼30 nm, as verified by dynamic light scattering and transmission electron microscopy. The unfolding and refolding behaviors of lysozyme molecules in the presence of the copolymer were studied by microcalorimetry and circular dichroism spectroscopy. Upon complex formation with mPEG(5K)-b-PLD10, lysozyme changed from its initial native state to a new partially unfolded state. Compared with its native state, this copolymer-complexed new folding state of lysozyme has different secondary and tertiary structures, a decreased thermostability, and significantly altered unfolding/refolding behaviors. It was found that the native lysozyme exhibited reversible unfolding and refolding upon heating and subsequent cooling, while lysozyme in the new folding state (complexed with the oppositely charged PLD segments of the polymer) could unfold upon heating but could not refold upon subsequent cooling. By employing the heating-cooling-reheating procedure, the prevention of complex formation between lysozyme and polymer due to the salt screening effect was observed, and the resulting uncomplexed lysozyme regained its proper unfolding and refolding abilities upon heating and subsequent cooling. Besides, we also pointed out the important role the length of the PLD segment played during the formation of micelles and the monodispersity of the formed micelles. Furthermore, the lysozyme-mPEG(5K)-b-PLD10 mixtures prepared in this work were all transparent, without the formation of large aggregates or precipitates in solution as frequently observed in other protein-polyelectrolyte systems. Hence, the present protein-PEGylated poly(amino acid) mixture provides an ideal water-soluble model system to study the important role of electrostatic interaction in the complexation between proteins and polymers, leading to important new knowledge on the protein-polymer interactions. Moreover, the polyelectrolyte complex micelle formed between protein and PEGylated polymer may provide a good drug delivery vehicle for therapeutic proteins.

Accumulation of small protein molecules in a macroscopic complex coacervate

To obtain insight into the accumulation of proteins into macroscopic complex coacervate phases, the lysozyme concentration in complex coacervates containing the cationic polyelectrolyte poly-(N,N dimethylaminoethyl methacrylate) and the anionic polyelectrolyte polyacrylic acid was investigated as a function of the mixing ratio, protein concentration and ionic strength. Maximal protein enrichment of the complex coacervate phase was observed to require the presence of all three macromolecules. Under optimized conditions the protein concentrations in the complex coacervate were as high as 200 g L(-1). Such high concentrations are comparable to the protein concentration in the cytosol, suggesting that these interesting liquid phases may serve a suitable model system for the phase behavior of the cytosol and genesis and function of membrane-less organelles. The high stability of the complexes and the salt dependent uptake of protein suggest that complex coacervates may provide a way to store hydrated proteins at high concentrations and might therefore be of interest in the formulation of high protein foods.

Multi-responsive protein nanocarriers from an anionic dynamic covalent copolymer

PIC micelles were formed through electrostatic interactions between the anionic dynamer and lysozyme, and the micelles possessed pH-, salt-, and enzyme-responsive features.

Preparation and characterization of polyion complex micelles with phosphobetaine shells

A pair of oppositely charged diblock copolymers, poly(2-(methacryloyloxy)ethyl phosphorylcholine)-block-poly((3-(methacryloylamino)propyl)trimethylammonium chloride) (PMPC-b-PMAPTAC) and poly(2-(methacryloyloxy)ethyl phosphorylcholine)-block-poly(sodium 2-(acrylamido)-2-methylpropanesulfonate) (PMPC-b-PAMPS), was prepared via reversible addition-fragmentation chain transfer radical polymerization using a PMPC-based macro chain transfer agent. The pendant phosphorylcholine group in the hydrophilic PMPC block has anionic phosphate and cationic quaternary amino groups, which are neutralized within the pendant group. Therefore, the mixing of aqueous solutions of PMPC-b-PMAPTAC and PMPC-b-PAMPS leads to the spontaneous formation of simple core-shell spherical polyion complex (PIC) micelles comprising of a segregated PIC core and PMPC shells. The PIC micelles were characterized using (1)H NMR spin-spin (T2) and spin-lattice relaxation times (T1), diffusion-ordered NMR spectroscopy, static light scattering, dynamic light scattering (DLS), and transmission electron microscopy techniques. The hydrodynamic size of the PIC micelle depended on the mixing ratio of PMPC-b-PMAPTAC and PMPC-b-PAMPS; the maximum size occurred at the mixing ratio yielding stoichiometric charge neutralization. The PIC micelles disintegrated to become unimers with the addition of salts.

Stabilization and enhanced reactivity of actinorhodin polyketide synthase minimal complex in polymer-nucleotide coacervate droplets

Compartmentalization of the minimal complex of actinorhodin polyketide synthase in coacervate liquid droplets produces enhanced yields of shunt polyketides under conditions of low and high ionic strength.

Antibacterial polyelectrolyte micelles for coating stainless steel

In this study, we report on the original synthesis and characterization of novel antimicrobial coatings for stainless steel by alternating the deposition of aqueous solutions of positively charged polyelectrolyte micelles doped with silver-based nanoparticles with a polyanion. The micelles are formed by electrostatic interaction between two oppositely charged polymers: a polycation bearing 3,4-dihydroxyphenylalanine units (DOPA, a major component of natural adhesives) and a polyanion (poly(styrene sulfonate), PSS) without using any block copolymer. DOPA units are exploited for their well-known ability to anchor to stainless steel and to form and stabilize biocidal silver nanoparticles (Ag(0)). The chlorine counteranion of the polycation forms and stabilizes biocidal silver chloride nanoparticles (AgCl). We demonstrate that two layers of micelles (alternated by PSS) doped with silver particles are enough to impart to the surface strong antibacterial activity against gram-negative E. coli. Moreover, micelles that are reservoirs of biocidal Ag(+) can be easily reactivated after depletion. This novel water-based approach is convenient, simple, and attractive for industrial applications.

Microgels and microcapsules in peptide and protein drug delivery

The present review focuses on the interaction of microgels and microcapsules with biological macromolecules, particularly peptides and proteins, as well as drug delivery applications of such systems. Results from recent studies on factors affecting peptide/protein binding to, and release from, microgels and related systems are discussed, including effects of network properties, as well as protein aggregation, peptide length, hydrophobicity and charge (distributions), secondary structure, and cyclization. Effects of ambient conditions (pH, ionic strength, temperature, etc.) are also discussed, all with focus on factors of importance for the performance of microgel and microcapsule delivery systems for biomacromolecular drugs.

The model Lysozyme-PSSNa system for electrostatic complexation: Similarities and differences with complex coacervation

We review, based on structural information, the mechanisms involved when putting in contact two nano-objects of opposite electrical charge, in the case of one negatively charged polyion, and a compact charged one. The central case is mixtures of PSS, a strong flexible polyanion (the salt of a strong acid, and with high linear charge density), and Lysozyme, a globular protein with a global positive charge. A wide accurate and consistent set of information in different situations is available on the structure at local scales (5-1000Å), due to the possibility of matching, the reproducibility of the system, its well-defined electrostatics features, and the well-defined structures obtained. We have related these structures to the observations at macroscopic scale of the phase behavior, and to the expected mechanisms of coacervation. On the one hand, PSS/Lysozyme mixtures show accurately many of what is expected in PEL/protein complexation, and phase separation, as reviewed by de Kruif: under certain conditions some well-defined complexes are formed before any phase separation, they are close to neutral; even in excess of one species, complexes are only modestly charged (surface charges in PEL excess). Neutral cores are attracting each other, to form larger objects responsible for large turbidity. They should lead the system to phase separation; this is observed in the more dilute samples, while in more concentrated ones the lack of separation in turbid samples is explained by locking effects between fractal aggregates. On the other hand, although some of the features just listed are the same required for coacervation, this phase transition is not really obtained. The phase separation has all the macroscopic aspects of a fluid (undifferentiated liquid/gas phase) - solid transition, not of a fluid-fluid (liquid-liquid) one, which would correspond to real coacervation). The origin of this can be found in the interaction potential between primary complexes formed (globules), which agrees qualitatively with a potential shape of the type repulsive long range attractive very short range. Finally we have considered two other systems with accurate structural information, to see whether other situations can be found. For Pectin, the same situation as PSS can be found, as well as other states, without solid precipitation, but possibly with incomplete coacervation, corresponding to differences in the globular structure. It is understandable that these systems show smoother interaction potential between the complexes (globules) likely to produce liquid-liquid transition. Finally, we briefly recall new results on Hyaluronan/Lysozyme, which present clear signs of coacervation in two liquid phases, and at the same time the existence of non-globular complexes, of specific geometry (thin rods) before any phase separation. These mixtures fulfill many of the requirements for complex coacervation, while other theories should also be checked like the one of Shklovskii et al.