Protein Purification by Polyelectrolyte Coacervation: Influence of Protein Charge Anisotropy on Selectivity

Selective protein complexation and coacervation by polyelectrolytes

This review discusses the possible relationship between protein charge anisotropy, protein binding affinity, polymer structure, and selective phase separation. We hope that a fundamental understanding of primarily electrostatically driven protein-polyelectrolyte (PE) interactions can enable the prediction of selective protein binding, and hence selective coacervation through non-specific electrostatics. Such research will partially challenge the assumption that specific binding has to be realized through specific binding sites with a variety of short-range interactions and some geometric match. More specifically, the recent studies on selective binding of proteins by polyelectrolytes were examined from different assemblies in addition to the electrostatic features of proteins and PEs. At the end, the optimization of phase separation based on binding affinity for selective coacervation and some considerations relevant to using PEs for protein purification were also overviewed.

Thermodynamics of complex coacervation

Isothermal titration calorimetry has routinely been used to understand the thermodynamic characteristics of complexation and coacervation. Most commonly, built-in models that assume independent binding sites have been employed in these studies. However, the non-covalent nature of interactions and steric effects accompanying macromolecules require (i) usage of new models such as overlapping binding sites and Satake-Yang's two-state binding models and (ii) reformed interpretations of the data as two-stage structuring. Fitting data with these models, forces driving the interaction of polyelectrolytes with oppositely charged polyelectrolytes, surfactants, and proteins have been identified as electrostatics and/or counterion release with possible contributions from hydrogen bonding and hydrophobic interactions. Additionally, for surfactant-polyelectrolyte coacervation, ITC signals indicated separate regions for formation of polymer-induced micelles and free micelles. Regardless of the type of the coacervation system, thermodynamics of coacervation is affected by the following parameters: pH and ionic strength of the medium, charge density, molecular weight of the polyelectrolyte, concentration, and mixing order of macroions. Lastly, we present a brief comparison between ITC on one hand and surface plasmon resonance or capillary electrophoresis on the other regarding their application in coacervation.

Application of Monte Carlo simulation in addressing key issues of complex coacervation formed by polyelectrolytes and oppositely charged colloids

This paper reviews the recent advance of Monte Carlo (MC) simulation in addressing key issues of complex coacervation between polyelectrolytes and oppositely charged colloids. Readers were first supplied with a brief overview of current knowledge and experimental strategies in the study of complex coacervation. In the next section, the general MC simulation procedures as well as representative strategies applied in complex coacervation were summarized. The unique contributions of MC simulation in either capturing delicate features, easing the experimental trials or proving the concept were then elucidated through the following aspects: i) identify phase boundary and decouple interaction contributions; ii) clarify composition distribution and internal structure; iii) predict the influences of physicochemical conditions on complex coacervation; iv) delineate the mechanisms for "binding on the wrong side of the isoelectric point". Finally, current challenges as well as prospects of MC simulation in complex coacervation are also discussed. The ultimate goal of this review is to provide readers with basic guideline for synergistic design of experiments in combination with MC simulation, and deliver convincing interpretation and reliable prediction for the structure and behavior in polyelectrolyte-macroion complex coacervation.

Heteroprotein complex coacervation: A generic process

Proteins exhibit a rich diversity of functional, physico-chemical and biodegradable properties which makes them appealing for various applications in the food and non-food sectors. Such properties are attributed to their ability to interact and assemble into a diversity of supramolecular structures. The present review addresses the updated research progress in the recent field of complex coacervation made from mixtures of oppositely charged proteins (i.e. heteroprotein systems). First, we describe briefly the main proteins used for heteroprotein coacervation. Then, through some selected examples, we illustrate the particularity and specificity of each heteroprotein system and the requirements that drive optimal assembly into coacervates. Finally, possible and promising applications of heteroprotein coacervates are mentioned.

Rational design of food-grade polyelectrolyte complex coacervate for encapsulation and enhanced oral delivery of oenothein B

Controlled release of OeB through GI tract using CPP–CS nanoparticles cross-linked with genipin was achievable.

Liquid-liquid and liquid-solid phase separation in protein-polyelectrolyte systems

The coacervation of systems containing colloids (e.g. proteins or micelles) and polyelectrolytes (notably ionic polysaccharides) is often accompanied by precipitation. This can introduce inhomogeneity, irreversibility and irreproducible kinetics in applications in food science and bioengineering, with negative impact on texture and stability of food products, and unpredictable delivery of active "payloads." The relationship between coacervation and precipitation is obscure in that coacervates might be intermediates in the formation of precipitates, or else the two phenomena might proceed by different but possibly simultaneous mechanisms. This review will summarize the recent literature on coacervation/precipitation in protein-polyelectrolyte systems for which reports are most abundant, particularly in the context of food science. We present current findings and opinions about the relationship between the two types of phase separation. Results vary considerably depending not only on the protein-polyelectrolyte pairs chosen, but also on conditions including macromolecular concentrations and ionic strength. Nevertheless, we offer some general approaches that could explain a variety of observations.

Linear viscoelasticity of complex coacervates

Rheology is a powerful method for material characterization that can provide detailed information about the self-assembly, structure, and intermolecular interactions present in a material. Here, we review the use of linear viscoelastic measurements for the rheological characterization of complex coacervate-based materials. Complex coacervation is an electrostatically and entropically-driven associative liquid-liquid phase separation phenomenon that can result in the formation of bulk liquid phases, or the self-assembly of hierarchical, microphase separated materials. We discuss the need to link thermodynamic studies of coacervation phase behavior with characterization of material dynamics, and provide parallel examples of how parameters such as charge stoichiometry, ionic strength, and polymer chain length impact self-assembly and material dynamics. We conclude by highlighting key areas of need in the field, and specifically call for the development of a mechanistic understanding of how molecular-level interactions in complex coacervate-based materials affect both self-assembly and material dynamics.

Structure, thermodynamic and kinetic signatures of a synthetic polyelectrolyte coacervating system

While many studies on coacervation have targeted biomacromolecules, we review in this article the key structure, thermodynamic and kinetic features of a fully synthetic coacervating system based on polyacrylic acid (PAA) and poly(diallyldimethylammonium chloride) (PDADMAC) oppositely charged polyelectrolytes at pH10, where PAA chains are fully deprotonated. Among the main points of interest, we can highlight (i) the presence of polyelectrolyte complex (PEC) nanoparticles that, unexpectedly, coexist with a certain amount of coacervate droplets in a large range of compositions, even far from stoichiometry; (ii) the fact that these PEC nanoparticles are likely precursors of the coacervation occurring at stoichiometry; (iii) the formation of soluble PECs only in a certain range of physicochemical conditions; (iv) the equilibrium properties of the system; (v) and last but not least a distinctive kinetic signature at stoichiometry evidenced by a peak in light scattering at very short times (~100ms). Some of these results can be rationalized on the basis of weak interaction unfolding between oppositely charged PAA and PDADMAC chains as revealed by microcalorimetry measurements.

Protein Selectivity Controlled by Polymer Charge Density and Protein Yield: Carboxylated Polysaccharides versus Sulfated Polysaccharides

The effect of polymer charge density on protein selectivity in the presence of carboxylated polysaccharides (CPS) and sulfated polysaccharides (SPS) was investigated for Kunitz trypsin inhibitor/Bowman-Birk protease inhibitor (KTI/BBI, KBM). To determine the conditions for coacervation or precipitation as a function of polymer charge densities, turbidimetric titrations and Tricine-SDS-PAGE were used. Polymer charge density as well as chain flexibility greatly influenced the strength of interactions and protein recovery. Although charge compensation must occur for CPS-KBM complexes, SPS-KBM systems did not require conservation of charge neutrality. Despite their similar isoelectric points, KTI bound preferentially to CPS and SPS due to its higher affinity compared to BBI. Complexation of KBM with the polysaccharide with the lowest charge density, arabic gum, expectedly cannot realize the purification of BBI under conditions where binding to more highly charged polysaccharides occurs. This work will be beneficial to selective purification of target proteins through control of protein-polysaccharide complexation.

Effect of the surface charge distribution on the fluid phase behavior of charged colloids and proteins

A generic but simple model is presented to evaluate the effect of the heterogeneous surface charge distribution of proteins and zwitterionic nanoparticles on their thermodynamic phase behavior. By considering surface charges as continuous "patches," the rich set of surface patterns that is embedded in proteins and charged patchy particles can readily be described. This model is used to study the fluid phase separation of charged particles where the screening length is of the same order of magnitude as the particle size. In particular, two types of charged particles are studied: dipolar fluids and protein-like fluids. The former represents the simplest case of zwitterionic particles, whose charge distribution can be described by their dipole moment. The latter system corresponds to molecules/particles with complex surface charge arrangements such as those found in biomolecules. The results for both systems suggest a relation between the critical region, the strength of the interparticle interactions, and the arrangement of charged patches, where the critical temperature is strongly correlated to the magnitude of the dipole moment. Additionally, competition between attractive and repulsive charge-charge interactions seems to be related to the formation of fluctuating clusters in the dilute phase of dipolar fluids, as well as to the broadening of the binodal curve in protein-like fluids. Finally, a variety of self-assembled architectures are detected for dipolar fluids upon small changes to the charge distribution, providing the groundwork for studying the self-assembly of charged patchy particles.

Spontaneous assembly of chemically encoded two-dimensional coacervate droplet arrays by acoustic wave patterning

The spontaneous assembly of chemically encoded, molecularly crowded, water-rich micro-droplets into periodic defect-free two-dimensional arrays is achieved in aqueous media by a combination of an acoustic standing wave pressure field and in situ complex coacervation. Acoustically mediated coalescence of primary droplets generates single-droplet per node micro-arrays that exhibit variable surface-attachment properties, spontaneously uptake dyes, enzymes and particles, and display spatial and time-dependent fluorescence outputs when exposed to a reactant diffusion gradient. In addition, coacervate droplet arrays exhibiting dynamical behaviour and exchange of matter are prepared by inhibiting coalescence to produce acoustically trapped lattices of droplet clusters that display fast and reversible changes in shape and spatial configuration in direct response to modulations in the acoustic frequencies and fields. Our results offer a novel route to the design and construction of ‘water-in-water' micro-droplet arrays with controllable spatial organization, programmable signalling pathways and higher order collective behaviour.

Isolated droplets can be used as micro-reactors, yet it is challenging to operate them functionally in solution and observe chemical exchanges between droplets. Here, Tian et al. use an acoustic trap to assemble water-based micro-droplets into periodic arrays, spontaneously separated from solution media.

RNA-Based Coacervates as a Model for Membraneless Organelles: Formation, Properties, and Interfacial Liposome Assembly

Liquid-liquid phase separation is responsible for formation of P granules, nucleoli, and other membraneless subcellular organelles composed of RNA and proteins. Efforts to understand the physical basis of liquid organelle formation have thus far focused on intrinsically disordered proteins (IDPs) as major components that dictate occurrence and properties. Here, we show that complex coacervates composed of low complexity RNA (polyuridylic acid, polyU) and short polyamines (spermine and spermidine) share many features of IDP-based coacervates. PolyU/polyamine coacervates compartmentalize biomolecules (peptides, oligonucleotides) in a sequence- and length-dependent manner. These solutes retain mobility within the coacervate droplets, as demonstrated by rapid recovery from photobleaching. Coacervation is reversible with changes in solution temperature due to changes in the polyU structure that impact its interactions with polyamines. We further demonstrate that lipid vesicles assemble at the droplet interface without impeding RNA entry/egress. These vesicles remain intact at the interface and can be released upon temperature-induced droplet dissolution.

Coacervation and precipitation in polysaccharide-protein systems

Precipitation poses a consistent problem for the growing applications of biopolymer coacervation, but the relationship between the two types of phase separation is not well understood. To clarify this relationship, we studied phase separation as a function of pH and ionic strength, in three systems of proteins with anionic polysaccharides: β-lactoglobulin (BLG)/hyaluronic acid (HA); BLG/tragacanthin (TG); and monoclonal antibody (mAb)/HA. We found that coacervation and precipitation are intrinsically different phenomena, responsive to different factors, but their simultaneity (for example with changing pH) may be confused with transitions from one state to another. We propose that coacervate does not literally turn into precipitate, but rather that both coacervate and precipitate are in equilibrium with free protein and polyanion, so that dissolution of one and formation of the other can overlap in time. While protein-polyanion complexes must achieve neutrality for coacervation, precipitation only requires tight binding which leads to the expulsion of counterions and water molecules. The pH-dependence of phase separation, considered in terms of protein and polyion charge, revealed that the electrostatic magnitude of the protein's polymer-binding site ("charge patch") plays a key role in the strength of interaction. These findings were supported by the inhibition of precipitation, seen when the bulky side chains of TG impede close protein-polymer interactions.

Production of alpha-amylase from Aspergillus oryzae for several industrial applications in a single step

A one-step method as a strategy of alpha-amylase concentration and purification was developed in this work. This methodology requires the use of a very low concentration of biodegradable polyelectrolyte (Eudragit(®) E-PO) and represents a low cost, fast, easy to scale up and non-polluting technology. Besides, this methodology allows recycling the polymer after precipitation. The formation of reversible soluble/insoluble complexes between alpha-amylase and the polymer Eudragit(®) E-PO was studied, and their precipitation in selected conditions was applied with bioseparation purposes. Turbidimetric assays allowed to determine the pH range where the complexes are insoluble (4.50-7.00); pH 5.50 yielded the highest turbidity of the system. The presence of NaCl (0.05M) in the medium totally dissociates the protein-polymer complexes. When the adequate concentration of polymer was added under these conditions to a liquid culture of Aspergillus oryzae, purification factors of alpha-amylase up to 7.43 and recoveries of 88% were obtained in a simple step without previous clarification. These results demonstrate that this methodology is suitable for the concentration and production of alpha-amylase from this source and could be applied at the beginning of downstream processing.

Whey protein isolate/gum arabic intramolecular soluble complexes improving the physical and oxidative stabilities of conjugated linoleic acid emulsions

Protein–polysaccharide intramolecular soluble complexes are proved to have superior emulsifying properties in stabilizing PUFAs-based emulsions.

Complex coacervation of hyaluronic acid and chitosan: effects of pH, ionic strength, charge density, chain length and the charge ratio

Hyaluronic acid (HA) and chitosan (CH) can form nanoparticles, hydrogels, microspheres, sponges, and films, all with a wide range of biomedical applications. This variety of phases reflects the multiple pathways available to HA/CH complexes. Here, we use turbidimetry, dynamic light scattering, light microscopy and zeta potential measurements to show that the state of the dense phase depends on the molar ratio of HA carboxyl to CH amines, and is strongly dependent on their respective degrees of ionization, α and β. Due to the strong charge complementarity between HA and CH, electrostatic self-assembly takes place at very acidic pH, but is almost unobservable at ionic strength (I) ≥ 1.5 M NaCl. All systems display discontinuity in the I-dependence of the turbidity, corresponding to a transition from coacervates to flocculates. An increase in either polymer chain length or charge density enhances phase separation. Remarkably, non-stoichiometric coacervate suspensions form at zeta potentials far away from zero. This result is attributed to the entropic effects of chain semi-flexibility as well as to the charge mismatch between the two biopolymers.

Interactions in globular proteins with polyampholyte: coacervation route for protein separation

Representative model of protein–protein separation in a BSA–GB–β-Lg aqueous solution.

Development of surfactant coacervation in aqueous solution

Coacervation is a phenomenon in which a colloidal dispersion separates into two immiscible liquid phases: a liquid rich in colloidal phase in equilibrium with another diluted liquid phase. Surfactant coacervation here refers to coacervation whose main components are surfactants with low molecular weights. Over the past two decades, surfactants have been greatly developed and studies on coacervation in systems of novel surfactants have been reported. This review summarizes the development of coacervation occurring in monomeric surfactants, one-head and two-tail surfactants, gemini surfactants and their mixtures. The effects of surfactant molecular structure and external conditions on critical conditions for coacervation, structures of precursors and coacervates, and their relationships are described. The effects of inorganic salts, alcohols and organic salts on surfactant coacervation are also reviewed.

Enhancement of enzymatic activity by magnetic spherical polyelectrolyte brushes: a potential recycling strategy for enzymes

Interactions between amyloglucosidase and magnetic spherical polyelectrolyte brushes (MSPB) were studied by turbidimetric titration, which reveals reversible and tunable behaviors of pH-dependent enzyme-SPB binding. Quantitative thermodyanmic parameters including binding affinity and stoichiometry between enzyme and SPBs were further measured by isothermal titration calorimetry (ITC). A large amount of enzyme can be loaded in MSPB without loss of MSPB stability. We demonstrated that the enzymatic activity of amyloglucosidase bound in MSPB could be greatly enhanced (catalytic reaction rate, k(bound) = 1.36k(free)) compared to free enzyme acitivity in solution. This is tremendous improvement from other carrier systems that usually lead to a significant decrease of enzymatic activity. Both the high enzyme loading capacity and the enhancement of the catalytic activity probably arise from the Coulombic interactions between the enzyme and MSPB. These findings provide a practical strategy for enhancement of enzyme activity and enzyme recycling by MSPB.

Soybean whey protein/chitosan complex behavior and selective recovery of kunitz trypsin inhibitor

Proteins in soybean whey were separated by Tricine-SDS-PAGE and identified by MALDI-TOF/TOF-MS. In addition to β-amylase, soybean agglutinin (SBA), and Kunitz trypsin inhibitor (KTI), a 12 kDa band was found to have an amino acid sequence similar to that of Bowman-Birk protease inhibitor (BBI) and showed both trypsin and chymotrypsin inhibitor activities. The complex behavior of soybean whey proteins (SWP) with chitosan (Ch) as a function of pH and protein to polysaccharide ratio (RSWP/Ch) was studied by turbidimetric titration and SDS-PAGE. During pH titration, the ratio of zeta potentials (absolute values) for proteins to chitosan (|ZSWP|/ZCh) at the initial point of phase separation (pHφ1) was equal to the reciprocal of their mass ratio (SWP/Ch), revealing that the electric neutrality conditions were fulfilled. The maximum protein recovery (32%) was obtained at RSWP/Ch = 4:1 and pH 6.3, whereas at RSWP/Ch = 20:1 and pH 5.5, chitosan consumption was the lowest (0.196 g Ch/g recovered proteins). In the protein-chitosan complex, KTI and the 12 kDa protein were higher in content than SBA and β-amylase. However, if soybean whey was precentrifuged to remove aggregated proteins and interacted with chitosan at the conditions of SWP/Ch = 100:1, pH 4.8, and low ionic strength, KTI was found to be selectively complexed. After removal of chitosan at pH 10, a high-purity KTI (90% by SEC-HPLC) could be obtained.

Prevention of thermally induced aggregation of IgG antibodies by noncovalent interaction with poly(acrylate) derivatives

Prevention of thermal aggregation of antibodies in aqueous solutions was achieved by noncovalent association with hydrophobically modified poly(acrylate) copolymers. Using a polyclonal immunoglobin G (IgG) as a model system for antibodies, we have studied the mechanisms by which this multidomain protein interacts with polyanions when incubated at physiological pH and at temperatures below and above the protein unfolding/denaturation temperature, in salt-free solutions and in 0.1 M NaCl solutions. The polyanions selected were sodium poly(acrylates), random copolymers of sodium acrylate and N-n-octadecylacrylamide (3 mol %), and a random copolymer of sodium acrylate, N-n-octylacrylamide (25 mol %), and N-isopropylacrylamide (40 mol %). They were derived from two poly(acrylic acid) parent chains of Mw 5000 and 150000 g·mol(-1). The IgG/polyanion interactions were monitored by static and dynamic light scattering, fluorescence correlation spectroscopy, capillary zone electrophoresis, and high sensitivity differential scanning calorimetry. In salt-free solutions, the hydrophilic PAA chains form complexes with IgG upon thermal unfolding of the protein (1:1 w/w IgG/PAA), but they do not interact with native IgG. The complexes exhibit a remarkable protective effect against IgG aggregation and maintain low aggregation numbers (average degree of oligomerization <12 at a temperature up to 85 °C). These interactions are screened in 0.1 M NaCl and, consequently, PAAs lose their protective effect. Amphiphilic PAA derivatives (1:1 w/w IgG/polymer) are able to prevent thermal aggregation (preserving IgG monomers) or retard aggregation of IgG (formation of oligomers and slow growth), revealing the importance of both hydrophobic interactions and modulation of the Coulomb interactions with or without NaCl present. This study leads the way toward the design of new formulations of therapeutic proteins using noncovalent 1:1 polymer/protein association that are transient and require a markedly lower additive concentration compared to conventional osmolyte protecting agents. They do not modify IgG permanently, which is an asset for applications in therapeutic protein formulations since the in vivo efficacy of the protein should not be affected.

Optimizing the selective recognition of protein isoforms through tuning of nanoparticle hydrophobicity

We demonstrate that ligand hydrophobicity can be used to increase affinity and selectivity of binding between monolayer-protected cationic gold nanoparticles and β-lactoglobulin protein isoforms containing two amino acid mutations.