Heteroprotein complex coacervation: A generic process

Viscoelastic and flow behaviour of β-lactoglobulin/lactoferrin coacervates: Influence of temperature and ionic strength

Heteroprotein complex coacervation has potential for a wide range of applications. However, the sensitivity of coacervates to slight changes in physico-chemical conditions may constitute a technological barrier for their development and deserves to be better understood. In this study, the rheological properties of β-lactoglobulin/lactoferrin (βLG/LF) heteroprotein complex coacervates were investigated with respect to narrow changes of temperature (5-40 °C) and ionic strength (0 to 10 mM added NaCl). The apparent viscosity of βLG/LF coacervates prepared at 20 °C showed a high sensitivity to temperature, decreasing progressively at elevated temperatures. Frequency sweep experiments demonstrated that coacervates behave as a viscoelastic liquid throughout the investigated frequency range at T>10°C. Time-temperature superposition principle revealed that the interaction involved in the coacervation process were temperature-independent. The calculated activation energy was approximately 85 kJ/mol. The addition of NaCl (up to 10mM) prior to coacervation, resulted in an increase of the viscosity but did not show a clear trend in the evolution of viscoelastic moduli. These new insights allow a better understanding of the interactions involved in concentrated protein coacervates enabling better control over their potential uses.

Novel high-protein dairy product based on fresh white cheese and whey protein isolate

The aim of the study was to obtain hard dairy product similar to the rennet hard cheese starting from the fresh white cheese (low and full-fat). This was accomplished with adding a powdered whey protein isolate to the fresh white cheese and heating. Fresh white cheese was produced from full or skim milk and ground with the whey protein isolate powder. The obtained mixture was heated at different temperatures. The increased heating temperature resulted in a more compact product characterized by higher hardness and elasticity. The full-fat product had lower hardness and elasticity in comparison with the low-fat product. The product approved by the organoleptic analysis panel was obtained by heating the mixed fat white cheese and the powdered whey protein isolate at 80°C for 30 min. The most significant achievement was to obtain in ∼1 h a product similar to that produced in ∼2 yr that is the hard rennet cheese. It contained ∼39% wt/wt of protein and can be an interesting offer for dairy industry.

Strong electrostatic attraction drives milk heteroprotein complex coacervation

Coacervates of oppositely charged milk proteins are used in functional food development, mainly to encapsulate bioactives. To uncover the driving forces behind coacervates formation, we study the association of lactoferrin and β-lactoglobulin at amino-acid level detail, using molecular simulations. Our findings show that inter-protein electrostatic interactions dominate and are, surprisingly, equally divided between an isotropic part, due to monopole-monopole attraction of the oppositely charged proteins, and an anisotropic part due to uneven surface charge distributions. In good agreement with recent experimental association constants, the calculated protein-protein interaction free energy is strongly dependent on pH and salt concentration. In addition to thermodynamics, we also investigate amino acid contacts in microstates of trimeric and pentameric protein complexes, and identify interaction hot-spots that drive heteroprotein complex coacervation process.

Single-molecule diffusivity quantification in Xenopus egg extracts elucidates physicochemical properties of the cytoplasm

The living cell creates a unique internal molecular environment that is challenging to characterize. By combining single-molecule displacement/diffusivity mapping (SMdM) with physiologically active extracts prepared from Xenopus laevis eggs, we sought to elucidate molecular properties of the cytoplasm. Quantification of the diffusion coefficients of 15 diverse proteins in extract showed that, compared to in water, negatively charged proteins diffused ~50% slower, while diffusion of positively charged proteins was reduced by ~80 to 90%. Adding increasing concentrations of salt progressively alleviated the suppressed diffusion observed for positively charged proteins, signifying electrostatic interactions within a predominately negatively charged macromolecular environment. To investigate the contribution of RNA, an abundant, negatively charged component of cytoplasm, extracts were treated with ribonuclease, which resulted in low diffusivity domains indicative of aggregation, likely due to the liberation of positively charged RNA-binding proteins such as ribosomal proteins, since this effect could be mimicked by adding positively charged polypeptides. Interestingly, in extracts prepared under typical conditions that inhibit actin polymerization, negatively charged proteins of different sizes showed similar diffusivity suppression consistent with our separately measured 2.22-fold higher viscosity of extract over water. Restoring or enhancing actin polymerization progressively suppressed the diffusion of larger proteins, recapitulating behaviors observed in cells. Together, these results indicate that molecular interactions in the crowded cell are defined by an overwhelmingly negatively charged macromolecular environment containing cytoskeletal networks.

Delivery and controlled release abilities of chitosan/carboxymethylcellulose micropolyelectrolyte complexes (PECs) toward niacinamide (vitamin B3)

The administration of bioactive compounds presents challenges due to the numerous physiological barriers in the gastrointestinal tract. To deal with one of these challenges, chitosan (CHS)/carboxymethylcellulose (CMC) micropolyelectrolyte complexes (micro-PECs) were developed without the use of crosslinking agents to carry niacinamide, a model hydrophilic bioactive agent. A Box-Behnken design was used to study the effects of processing time (X1 = 60, 120 or 180 min), pH (X2 = 3, 4 or 5) and niacinamide concentration (X3 = 0.02, 0.04 and 0.06, g·L-1) on the encapsulation efficiency (Y1) and loading capacity (Y2) of niacinamide by CMC/CHS micro-PECs. The encapsulation efficiency (Y1) varied from 0.86 % to 80.78 %, whereas the loading capacity (Y2) varied between 0.03 % and 3.89 %. The digestibility of CMC/CHS micro-PECs containing niacinamide was evaluated in vitro via a static gastrointestinal model. Empirical models (Zero Order, First Order, Higuchi and Korsemeyer-Peppas) were fitted to the niacinamide release kinetics data. The zero-order model exhibited the best fit across all points (gastric and enteric digestion), with low zero-order constants (K0) ~ 0.002-0.003, indicating a regular and subdued release rate in all cases. These results highlight the applicability of CMC/CHS micro-PECs as an efficient, novel oral delivery system, surpassing conventional approaches by offering a sustained release and high encapsulation efficiency, without needing any additional chemical crosslinking agent for their obtention.

Lactoferrin-Based Heteroprotein Systems, From Their Formation Mechanism, Properties, To Applications

Lactoferrin (LF) is an important iron-binding glycoprotein found in milk and mucosal secretions. The alkaline lactoferrin can interact with some acidic proteins to form heteroprotein systems with multifunctional properties and a wide range of applications. Lactoferrin can interact with animal and plant proteins mainly through the electrostatic forces, dipolar attraction, and hydrophobic interactions. In this review, the types of heteroprotein complexes formed by the complex coacervation of lactoferrin with other proteins are introduced, including the preparation, structure, and applications. The factors affecting the formation of heteroprotein complexes are described, such as pH, ionic strength, mixing ratio, total protein concentration, and temperature. The issues and challenges in the formation of heteroprotein complexes are also discussed.

Hazelnut Protein and Sodium Alginate Complex Coacervates: An Effective Tool for the Encapsulation of the Hydrophobic Polyphenol Quercetin

For valorization purposes of hazelnut byproducts, complex coacervation of hazelnut protein isolate (HPI) with sodium alginate (NaAlg) was investigated by turbidimetric analysis and zeta potential determination as a function of pH and protein/alginate mixing ratio. HPI-NaAlg complex coacervates were used as an encapsulating material of quercetin (QE) at different concentrations. The optimal pH and mixing ratio resulting in the highest turbidity and neutral charge were 3.5 and 6:1, respectively. The coacervation yield was 74.9% in empty capsules and up to 90.0% in the presence of QE. Under optimal conditions, HPI-NaAlg complex coacervates achieved an encapsulation efficiency higher than 99% in all coacervate/QE formulations. Fourier transform infrared spectroscopy (FTIR) results revealed the occurrence of electrostatic interactions between different functional groups within the ternary complex in addition to hydrogen and hydrophobic interactions between QE and HPI. HPI-NaAlg complex coacervates can serve as an alternative matrix for the microencapsulation of bioactive ingredients with low water solubility in food formulations, which adds an additional valorization of hazelnut byproducts.

Single-molecule diffusivity quantification in Xenopus egg extracts elucidates physicochemical properties of the cytoplasm

The living cell creates a unique internal molecular environment that is challenging to characterize. By combining single-molecule displacement/diffusivity mapping (SM d M) with physiologically active extracts prepared from Xenopus laevis eggs, we sought to elucidate molecular properties of the cytoplasm. Quantification of the diffusion coefficients of 15 diverse proteins in extract showed that, compared to in water, negatively charged proteins diffused ∼50% slower, while diffusion of positively charged proteins was reduced by ∼80-90%. Adding increasing concentrations of salt progressively alleviated the suppressed diffusion observed for positively charged proteins, signifying electrostatic interactions within a predominately negatively charged macromolecular environment. To investigate the contribution of RNA, an abundant, negatively charged component of cytoplasm, extracts were treated with ribonuclease, which resulted in low diffusivity domains indicative of aggregation, likely due to the liberation of positively charged RNA-binding proteins such as ribosomal proteins, since this effect could be mimicked by adding positively charged polypeptides. Interestingly, negatively charged proteins of different sizes showed similar diffusivity suppression in extract, which are typically prepared under conditions that inhibit actin polymerization. Restoring or enhancing actin polymerization progressively suppressed the diffusion of larger proteins, recapitulating behaviors observed in cells. Together, these results indicate that molecular interactions in the crowded cell are defined by an overwhelmingly negatively charged macromolecular environment containing cytoskeletal networks.

Significance Statement

The complex intracellular molecular environment is notably challenging to elucidate and recapitulate. Xenopus egg extracts provide a native yet manipulatable cytoplasm model. Through single-molecule microscopy, here we decipher the cytoplasmic environment and molecular interactions by examining the diffusion patterns of diverse proteins in Xenopus egg extracts with strategic manipulations. These experiments reveal an overwhelmingly negatively charged macromolecular environment with crosslinked meshworks, offering new insight into the inner workings of the cell.

Single-Molecule Diffusivity Quantification Unveils Ubiquitous Net Charge-Driven Protein-Protein Interaction

Recent microscopy and nuclear magnetic resonance (NMR) studies have noticed substantial suppression of intracellular diffusion for positively charged proteins, suggesting an overlooked role of electrostatic attraction in nonspecific protein interactions in a predominantly negatively charged intracellular environment. Utilizing single-molecule detection and statistics, here, we quantify in aqueous solutions how protein diffusion, in the limit of low diffuser concentration to avoid aggregate/coacervate formation, is modulated by differently charged interactor proteins over wide concentration ranges. We thus report substantially suppressed diffusion when oppositely charged interactors are added at parts per million levels, yet unvaried diffusivities when same-charge interactors are added beyond 1%. The electrostatic attraction-driven suppression of diffusion is sensitive to the protein net charge states, as probed by varying the solution pH and ionic strength or chemically modifying the proteins and is robust across different diffuser-interactor pairs. By converting the measured diffusivities to diffuser diameters, we further show that in the limit of excess interactors, a positively charged diffuser molecule effectively drags along just one monolayer of negatively charged interactors, where further interactions stop. We thus unveil ubiquitous, net charge-driven protein-protein interactions and shed new light on the mechanism of charge-based diffusion suppression in living cells.

Magnesium ions regulated ovalbumin-lysozyme heteroprotein complex: Aggregation kinetics, thermodynamics and morphologic structure

This study aims to investigate the mechanism of magnesium ions regulated ovalbumin-lysozyme (OVA-LYS) heteroprotein aggregation behavior via aggregation kinetics model, exploring the relationship between differential aggregation behavior and protein molecular structure, intermolecular interactions and thermal stability. Results showed that the aggregation rate (kapp) and maximum absorbance (Amax) of the OVA-LYS heteroprotein complex were located between OVA and LYS. Meanwhile, the thermal denaturation temperature (Td) and denaturation enthalpy (ΔH) were between the values of OVA and LYS as well. Compared with OVA, the thermal stability of the OVA-LYS heteroprotein complex increased owing to the electrostatic interactions between OVA and LYS, resulting in slower aggregation rate and lower aggregation degree. Molecular dynamics simulations revealed the molecular conformational changes during OVA-LYS binary protein binding and the stability of the complex conformation. Moreover, MgCl2 weakened the OVA-LYS interactions through Debye shielding while increasing thermal stability, allowing the two proteins to aggregate into amorphous precipitates rather than spherical coacervates. Overall, this study provides information to further understand the regulation mechanism of proteins differential aggregation behavior by ions.

Coacervate Droplets for Synthetic Cells

The design and construction of synthetic cells - human-made microcompartments that mimic features of living cells - have experienced a real boom in the past decade. While many efforts have been geared toward assembling membrane-bounded compartments, coacervate droplets produced by liquid-liquid phase separation have emerged as an alternative membrane-free compartmentalization paradigm. Here, the dual role of coacervate droplets in synthetic cell research is discussed: encapsulated within membrane-enclosed compartments, coacervates act as surrogates of membraneless organelles ubiquitously found in living cells; alternatively, they can be viewed as crowded cytosol-like chassis for constructing integrated synthetic cells. After introducing key concepts of coacervation and illustrating the chemical diversity of coacervate systems, their physicochemical properties and resulting bioinspired functions are emphasized. Moving from suspensions of free floating coacervates, the two nascent roles of these droplets in synthetic cell research are highlighted: organelle-like modules and cytosol-like templates. Building the discussion on recent studies from the literature, the potential of coacervate droplets to assemble integrated synthetic cells capable of multiple life-inspired functions is showcased. Future challenges that are still to be tackled in the field are finally discussed.

Ionic Strength Dependence of the Complex Coacervation between Lactoferrin and β-Lactoglobulin

Heteroprotein complex coacervation is an assembly formed by oppositely charged proteins in aqueous solution that leads to liquid-liquid phase separation. The ability of lactoferrin and β-lactoglobulin to form complex coacervates at pH 5.5 under optimal protein stoichiometry has been studied in a previous work. The goal of the current study is to determine the influence of ionic strength on the complex coacervation between these two proteins using direct mixing and desalting protocols. The initial interaction between lactoferrin and β-lactoglobulin and subsequent coacervation process were highly sensitive to the ionic strength. No microscopic phase separation was observed beyond a salt concentration of 20 mM. The coacervate yield decreased drastically with increasing added NaCl from 0 to 60 mM. The charge-screening effect induced by increasing the ionic strength is attributed to a decrease of interaction between the two oppositely charged proteins throughout a decrease in Debye length. Interestingly, as shown by isothermal titration calorimetry, a small concentration of NaCl around 2.5 mM promoted the binding energy between the two proteins. These results shed new light on the electrostatically driven mechanism governing the complex coacervation in heteroprotein systems.

Coacervates: recent developments as nanostructure delivery platforms for therapeutic biomolecules

Coacervation is a liquid-liquid phase separation that can occur in solutions of macromolecules through self-assembly or electrostatic interactions. Recently, coacervates composed of biocompatible macromolecules have been actively investigated as nanostructure platforms to encapsulate and deliver biomolecules such as proteins, RNAs, and DNAs. One particular advantage of coacervates is that they are derived from aqueous solutions, unlike other nanoparticle delivery systems that often require organic solvents. In addition, coacervates achieve high loading while maintaining the viability of the cargo material. Here, we review recent developments in the applications of coacervates and their limitations in the delivery of therapeutic biomolecules. Important factors for coacervation include molecular structures of the polyelectrolytes, mixing ratio, the concentration of polyelectrolytes, and reaction conditions such as ionic strength, pH, and temperature. Various compositions of coacervates have been shown to deliver biomolecules in vitro and in vivo with encouraging activities. However, major hurdles remain for the systemic route of administration other than topical or local delivery. The scale-up of manufacturing methods suitable for preclinical and clinical evaluations remains to be addressed. We conclude with a few research directions to overcome current challenges, which may lead to successful translation into the clinic.

Formation of lysozyme-caseinate heteroprotein complexes for encapsulation of lysozyme by spray-drying: Effect of mass ratio and temperature

The formation of heteroprotein complexes obtained by the interactions between sodium caseinate (CAS) and lysozyme (LYS) at pH 7 was investigated by using turbidimetric analysis, particle size distribution, and zeta potential at different CAS/LYS ratios. Moreover, isothermal titration calorimetry (ITC) was used to determine the type and magnitude of the energies involved in the CAS/LYS complexation process and evaluated the thermodynamic behavior of their complexation. Results revealed that the structure of CAS/LYS complexes drastically changed when CAS/LYS ratio increased to 1.0 and the structuring stages were characterized by exothermic signals and were controlled by favorable enthalpy changes due to electrostatic interactions between both proteins. In addition, the interaction between two proteins was temperature-dependent and mainly entropy-driven, which was verified by molecular dynamics (MD) simulations, and the hydrophobic interactions and hydrogen bonding were shown to play an important role in CAS/LYS interactions. Furthermore, CAS/LYS complexes showed minimum LYS enzymatic activity at CAS/LYS ratio 1.0. Though spray-drying of CAS/LYS complexes with ratio 1.0, the LYS activity in reconstituted solution was recovered >80 % of initial activity after calcium chloride addition. The present study provides useful information about CAS/LYS complexation and binding processes, which could facilitate their application in antimicrobial edible food packaging.

Spatiotemporal modulations in heterotypic condensates of prion and α-synuclein control phase transitions and amyloid conversion

Biomolecular condensation via liquid-liquid phase separation of proteins and nucleic acids is associated with a range of critical cellular functions and neurodegenerative diseases. Here, we demonstrate that complex coacervation of the prion protein and α-synuclein within narrow stoichiometry results in the formation of highly dynamic, reversible, thermo-responsive liquid droplets via domain-specific electrostatic interactions between the positively-charged intrinsically disordered N-terminal segment of prion and the acidic C-terminal tail of α-synuclein. The addition of RNA to these coacervates yields multiphasic, vesicle-like, hollow condensates. Picosecond time-resolved measurements revealed the presence of transient electrostatic nanoclusters that are stable on the nanosecond timescale and can undergo breaking-and-making of interactions on slower timescales giving rise to a liquid-like behavior in the mesoscopic regime. The liquid-to-solid transition drives a rapid conversion of complex coacervates into heterotypic amyloids. Our results suggest that synergistic prion-α-synuclein interactions within condensates provide mechanistic underpinnings of their physiological role and overlapping neuropathological features.

Heteroprotein Complex Coacervate Based on β-Conglycinin and Lysozyme: Dynamic Protein Exchange, Thermodynamic Mechanism, and Lysozyme Activity

Heteroprotein complex coacervate (HPCC) is a liquid-like protein concentrate produced by liquid-liquid phase separation. We revealed the protein dynamic exchange and thermodynamic mechanism of β-conglycinin/lysozyme coacervate, and clarified the effect of HPCC on protein structure and activity. β-conglycinin and lysozyme assembled into coacervate at pH 5.75-6.5 and assembled into amorphous precipitates at higher pH. As the pH dropped from 8 to 6, the number of binding sites of the complex decreased in half, and the desolvation degree corresponding to the entropy gain was greatly reduced, conducing to the formation of coacervates rather than precipitates. The coacervates achieved the unique dynamic exchange by exchanging proteins with the diluted phase, making the uniform distribution of proteins in coacervates. The lysozyme activity was completely retained in β-conglycinin/lysozyme coacervates. These results proved that β-conglycinin-based heteroprotein complex coacervate is a feasible method to encapsulate and enrich active proteins in a purely aqueous environment.

Structural and foaming properties of whey and soy protein isolates in mixed systems before and after heat treatment

The partial replacement of proteins from animal sources by plant proteins in formulated food products has been proposed as useful to improve sustainability aspects of the products without dramatically changing their techno-functional properties. Although several research groups have published on the gelling properties of mixed systems containing whey and soy protein isolates (WPI and SPI), their foaming properties are much less described. In this context, the main objective of this paper was to evaluate the structural and foaming properties of samples containing different mass ratios of WPI:SPI (100:0, 75:25, 50:50, 25:75 and 0:100) before and after heat treatment. The samples were evaluated according to their solubility, foaming capacity (FC), foam microstructure and foam stability (FS). Before heat treatment, mixing SPI to WPI did not affect the solubility of whey proteins, but, after heat treatment, insoluble co-aggregates were formed. Similar FC was measured for all samples despite their WPI:SPI ratio and the applied heat treatment. The partial replacement of WPI by SPI changed the microstructure of the foams and had an antagonistic effect on the FS of the samples, due to the negative effect of insoluble soy protein aggregates and/or insoluble co-aggregates on the reinforcement of the air-water interfacial film.

Assembled milk protein nano-architectures as potential nanovehicles for nutraceuticals

Nanoencapsulation of hydrophobic nutraceuticals with food ingredients has become one of topical research subjects in food science and pharmaceutical fields. To fabricate food protein-based nano-architectures as nanovehicles is one of effective strategies or approaches to improve water solubility, stability, bioavailability and bioactivities of poorly soluble or hydrophobic nutraceuticals. Milk proteins or their components exhibit a great potential to assemble or co-assemble with other components into a variety of nano-architectures (e.g., nano-micelles, nanocomplexes, nanogels, or nanoparticles) as potential nanovehicles for encapsulation and delivery of nutraceuticals. This article provides a comprehensive review about the state-of-art knowledge in utilizing milk proteins to assemble or co-assemble into a variety of nano-architectures as promising encapsulation and delivery nano-systems for hydrophobic nutraceuticals. First, a brief summary about composition, structure and physicochemical properties of milk proteins, especially caseins (or casein micelles) and whey proteins, is presented. Then, the disassembly and reassembly behavior of caseins or whey proteins into nano-architectures is critically reviewed. For caseins, casein micelles can be dissociated and further re-associated into novel micelles, through pH- or high hydrostatic pressure-mediated disassembly and reassembly strategy, or can be directly formed from caseinates through a reassembly process. In contrast, the assembly of whey protein into nano-architectures usually needs a structural unfolding and subsequent aggregation process, which can be induced by heating, enzymatic hydrolysis, high hydrostatic pressure and ethanol treatments. Third, the co-assembly of milk proteins with other components into nano-architectures is also summarized. Last, the potential and effectiveness of assembled milk protein nano-architectures, including reassembled casein micelles, thermally induced whey protein nano-aggregates, α-lactalbumin nanotubes or nanospheres, co-assembled milk protein-polysaccharide nanocomplexes or nanoparticles, as nanovehicles for nutraceuticals (especially those hydrophobic) are comprehensively reviewed. Due to the fact that milk proteins are an important part of diets for human nutrition and health, the review is of crucial importance not only for the development of novel milk protein-based functional foods enriched with hydrophobic nutraceuticals, but also for providing the newest knowledge in the utilization of food protein assembly behavior in the nanoencapsulation of nutraceuticals.

Energetically Preferred Bilayered Coacervation of Oppositely Charged ZrHP Nanoplatelets

A platform is introduced for bilayered coacervation of oppositely charged nanoplatelets (NPLs) at the oil-water interface. To this end, we synthesized two types of zirconium hydrogen phosphate (ZrHP) NPLs, cationically charged NPLs (CNPLs), and anionically charged NPLs (ANPLs) by conducting surface-initiated atom transfer radical polymerization. Taking advantage of the platelet geometry and controlled wettability, we demonstrated that ANPLs and CNPLs coacervate themselves to form a bilayered NPL membrane at the interface, which was directly confirmed by confocal laser scanning microscopy. Via theoretical consideration using the hit-and-miss Monte Carlo method, we determined that electrostatic attraction-driven coacervation of ANPLs and CNPLs at the interface shows a minimum attachment energy of ∼ -10⁶ k_BT, which is comparable to the cases where NPLs charged with the same type of ions are attached. Finally, this unique and novel interfacial coacervation behavior allowed us to develop a pH-responsive smart Pickering emulsion system.

Review on fat replacement using protein-based microparticulated powders or microgels: A textural perspective

Background

Due to the growing rise in obesity and food-linked diseases, the replacement of calorie-dense fat has been a key focus of food industries in the last few decades with proteins being identified as promising fat replacers (FRs).

Scope and approach

This review aims to provide an overview of animal and plant protein-based FR studies that have been performed in the last 5 years. Protein isolates/concentrates, their microparticulated forms and protein microgels in model and real foods have been examined. Special emphasis has been given on the characterisation techniques that have been used to compare the full fat (FF) and low fat (LF) versions of the foods using FRs.

Key findings and conclusions

Microparticulated whey protein (MWP) has been the preferred choice FR with some success in replacing fat in model foods and dairy applications. Plant proteins on the other hand have attracted limited research attention as FRs, but show success similar to that of animal proteins. Key characterisation techniques used to compare full fat with low fat products containing FRs have been apparent viscosity, texture profile analysis, microscopy, particle size and sensory properties with oral tribology being a relatively recent undertaking. Coupling tribology with adsorption techniques (muco-adhesion) can be effective to bridge the instrumental-sensory property gap and might accelerate the development cycle of designing low/no fat products. From a formulation viewpoint, sub-micron sized microgels that show shear-thinning behaviour and have boundary lubrication properties offer promises with respect to exploiting their fat replacement potential in the future.

Advancement of food-derived mixed protein systems: Interactions, aggregations, and functional properties

Recently, interests in binary protein systems have been developed considerably ascribed to the sustainability, environment-friendly, rich in nutrition, low cost, and tunable mechanical properties of these systems. However, the molecular coalition is challenged by the complex mechanisms of interaction, aggregation, gelation, and emulsifying of the mixed system in which another protein is introduced. To overcome these fundamental difficulties and better modulate the structural and functional properties of binary systems, efforts have been steered to gain basic information regarding the underlying dynamics, theories, and physicochemical characteristics of mixed systems. Therefore, the present review provides an overview of the current studies on the behaviors of proteins in such systems and highlights shortcomings and future challenges when applied in scientific fields.

Resolving the Mechanisms of Soy Glycinin Self-Coacervation and Hollow-Condensate Formation

Self-coacervation of animal-derived proteins has been extensively investigated while that of plant proteins remains largely unexplored. Here, we study the process of soy glycinin self-coacervation and transformation into hollow condensates. The protein hexameric structure composed of hydrophilic and hydrophobic polypeptides is crucial for coacervation. The process is driven by charge screening of the intrinsically disordered region of acidic polypeptides, allowing for weak hydrophobic interactions between exposed hydrophobic polypeptides. We find that the coacervate surface exhibits order, which stabilizes the coacervate shape during hollow-condensate formation. The latter process occurs via nucleation and growth of protein-poor phase in the coacervate interior, during which another ordered layer at the inner surface is formed. Aging enhances the stability of both coacervates and hollow condensates. Understanding plant protein coacervation holds promises for fabricating novel functional materials.

Towards Green Nanoscience: From extraction to nanoformulation

The use of nanotechnology has revolutionized many biotechnological sectors, from bioengineering to medicine, passing through food and cosmetic fields. However, their clinic and industrial application has been into the spotlight due to their safety risk and related side effects. As a result, Green Nanoscience/Nanotechnology emerged as a strategy to prevent any associated nanotoxicity, via implementation of sustainable processes across the whole lifecycle of nanoformulation. Notwithstanding its success across inorganic nanoparticles, the green concept for organic nanoparticle elaboration is still at its infancy. This, coupled with the organic nanoparticles being the most commonly used in biomedicine, highlights the need to implement specific green principles for their elaboration. In this review, we will discuss the possible green routes for the proper design of organic nanoparticles under the umbrella of Green Nanoscience: from the extraction of nanomaterials and active compounds to their final nanoformulation.

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

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

Protein Encapsulation Using Complex Coacervates: What Nature Has to Teach Us

Protein encapsulation is a growing area of interest, particularly in the fields of food science and medicine. The sequestration of protein cargoes is achieved using a variety of methods, each with benefits and drawbacks. One of the most significant challenges associated with protein encapsulation is achieving high loading while maintaining protein viability. This difficulty is exacerbated because many encapsulant systems require the use of organic solvents. By contrast, nature has optimized strategies to compartmentalize and protect proteins inside the cell-a purely aqueous environment. Although the mechanisms whereby aspects of the cytosol is able to stabilize proteins are unknown, the crowded nature of many newly discovered, liquid phase separated "membraneless organelles" that achieve protein compartmentalization suggests that the material environment surrounding the protein may be critical in determining stability. Here, encapsulation strategies based on liquid-liquid phase separation, and complex coacervation in particular, which has many of the key features of the cytoplasm as a material, are reviewed. The literature on protein encapsulation via coacervation is also reviewed and the parameters relevant to creating protein-containing coacervate formulations are discussed. Additionally, potential opportunities associated with the creation of tailored materials to better facilitate protein encapsulation and stabilization are highlighted.

Simple Calculation of Phase Diagrams for Liquid-Liquid Phase Separation in Solutions of Two Macromolecular Solute Species

A simple method is described for the calculation of two- and three-dimensional phase diagrams describing stability and coexistence curves or surfaces separating one- and two-phase regions in composition/temperature space of a solution containing solute species 1 and 2. The calculation requires a quantitative description of the intermolecular potentials of mean force acting between like (1–1 and 2–2) and unlike (1–2) species. Example calculations are carried out for solutions of species interacting via spherically symmetric square-well potentials as first-order models for protein–protein interaction. When the interaction between species 1 and 2 is more repulsive than those acting between like species, the two-phase region is characterized by an equilibrium between a phase enriched in 1 and depleted in 2 and a phase enriched in 2 and depleted in 1. When the interaction between species 1 and 2 is more attractive than those acting between like species, the two-phase region is characterized by an equilibrium between a phase enriched in both species and a phase depleted in both species. The latter example provides a first-order description of coacervate formation without postulating specific interactions between the two solute species.

Determination of Protein-Protein Interactions in a Mixture of Two Monoclonal Antibodies

The coformulation of monoclonal antibody (mAb) mixtures provides an attractive route to achieving therapeutic efficacy where the targeting of multiple epitopes is necessary. Controlling and predicting the behavior of such mixtures requires elucidating the molecular basis for the self- and cross-protein-protein interactions and how they depend on solution variables. While self-interactions are now beginning to be well understood, systematic studies of cross-interactions between mAbs in solution do not exist. Here, we have used static light scattering to measure the set of self- and cross-osmotic second virial coefficients in a solution containing a mixture of two mAbs, mAbA and mAbB, as a function of ionic strength and pH. mAbB exhibits strong association at a low ionic strength, which is attributed to an electrostatic attraction that is enhanced by the presence of a strong short-ranged attraction of nonelectrostatic origin. Under all solution conditions, the measured cross-interactions are intermediate self-interactions and follow similar patterns of behavior. There is a strong electrostatic attraction at higher pH values, reflecting the behavior of mAbB. Protein-protein interactions become more attractive with an increasing pH due to reducing the overall protein net charges, an effect that is attenuated with an increasing ionic strength due to the screening of electrostatic interactions. Under moderate ionic strength conditions, the reduced cross-virial coefficient, which reflects only the energetic contribution to protein-protein interactions, is given by a geometric average of the corresponding self-coefficients. We show the relationship can be rationalized using a patchy sphere model, where the interaction energy between sites i and j is given by the arithmetic mean of the i-i and j-j interactions. The geometric mean does not necessarily apply to all mAb mixtures and is expected to break down at a lower ionic strength due to the nonadditivity of electrostatic interactions.

Use of molecular interactions and mesoscopic scale transitions to modulate protein-polysaccharide structures

Mixed protein-polysaccharide structures have found widespread applications in various fields, such as in foods, pharmaceuticals or personal care products. A better understanding and a more precise control over the molecular interactions between the two types of macromolecules leading to an engineering of nanoscale and colloidal building blocks have fueled the design of novel structures with improved functional properties. However, these building blocks often do not constitute the final matrix. Rather, further process operations are used to transform the initially formed structural entities into bulk matrices. Systematic knowledge on the relation between molecular structure design and subsequent mesoscopic scale transitions induced by processing is scarce. This article aims at establishing a connection between these two approaches. Therefore, it reviews not only studies on the underlying molecular interaction phenomena leading to either a segregative or associative phase behavior and nanoscale or colloidal structures, but also looks at the less systematically studied approach of using macroscopic processing operations such as shearing, heating, crosslinking, and concentrating/drying to transform the initially generated structures into bulk matrices. Thereby, a more comprehensive look is taken at the relationship between different influencing factors, namely solvent conditions (i.e. pH, ionic strength), biopolymer characteristics (i.e. type, charge density, mixing ratio, biopolymer concentration), and processing parameters (i.e. temperature, mechanical stresses, pressure) to generate bulk protein-polysaccharide matrices with different morphological features. The need for a combinatorial approach is then demonstrated by reviewing in detail current mixed protein-polysaccharide applications that increasingly make use of this. In the process, open scientific questions that will need to be addressed in the future are highlighted.