Polyelectrolyte–protein complexes

Extreme dependence of dynamics on concentration in highly crowded polyelectrolyte solutions

Charge-carrying species, such as polyelectrolytes, are vital to natural and synthetic processes that rely on their dynamic behavior. Through single-particle tracking techniques, the diffusivity of individual polyelectrolyte chains and overall system viscosity are determined for concentrated polylysine solutions. These studies show scaling dependences of D ~ c-6.1 and η ~ c7.2, much stronger than theoretical predictions, drawing the applicability of power law fits into question. Similar trends are observed in concentrated solutions prepared at various pH and counterion conditions. These hindered system dynamics appear universal to polyelectrolyte systems and are attributed to the large effective excluded volumes of polyelectrolyte chains inducing glassy dynamics. The framework of the Vrentas-Duda free-volume theory is used to compare polyelectrolyte and neutral systems. Supported by this theory, excluding counterion mass from total polymer mass results in all environmental conditions collapsing onto a common trendline. These results are applicable to crowded biological systems, such as intracellular environments where protein mobility is strongly inhibited.

Quantitative turbidimetric characterization of stabilized complex coacervate dispersions

Stabilizing complex coacervate microdroplets is desirable due to their various applications, such as bioreactors, drug delivery vehicles, and encapsulants. Here, we present quantitative characterization of complex coacervate dispersion stability inferred by turbidimetry measurements. The stability of the dispersions is shown to be modulated by the concentrations of comb polyelectrolyte (cPE) stabilizers and salt. We demonstrate cPEs as effective stabilizers for complex coacervate dispersions independent of the chemistry or length of the constituent polyelectrolytes, salts, or preparation routes. By monitoring the temporal evolution of dispersion turbidity, we show that cPEs suppress microdroplet coalescence with minimal change in microdroplet sizes over 48 hours, even at salt concentrations up to 300 mM. The number density and average microdroplet size are shown to be controlled by varying the cPE and salt concentrations. Lastly, turbidity maps, akin to binodal phase maps, depict an expansion of the turbid two-phase region and an increase in the salt resistance of the coacervates upon the introduction of cPEs. The coacervate salt resistance is shown to increase by >3×, and this increase is maintained for up to 15 days, demonstrating that cPEs impart higher salt resistance over extended durations.

Membrane-Free Lateral Flow Assay with the Active Control of Fluid Transport for Ultrasensitive Cardiac Biomarker Detection

Membrane-based lateral flow immunoassays (LFAs) have been employed as early point-of-care (POC) testing tools in clinical settings. However, the varying membrane properties, uncontrollable sample transport in LFAs, visual readout, and required large sample volumes have been major limiting factors in realizing needed sensitivity and desirable precise quantification. Addressing these challenges, we designed a membrane-free system in which the desirable three-dimensional (3D) structure of the detection zone is imitated and used a small pump for fluid flow and fluorescence as readout, all the while maintaining a one-step assay protocol. A hydrogel-like protein-polyelectrolyte complex (PPC) within a polyelectrolyte multilayer (PEM) was developed as the test line by complexing polystreptavidin (pSA) with poly(diallyldimethylammonium chloride) (PDDA), which in turn was layered with poly(acrylic acid) (PAA) resulting in a superior 3D streptavidin-rich test line. Since the remainder of the microchannel remains material-free, good flow control is achieved, and with the total volume of 20 μL, 7.5-fold smaller sample volumes can be used in comparison to conventional LFAs. High sensitivity with desirable reproducibility and a 20 min total assay time were achieved for the detection of NT-proBNP in plasma with a dynamic range of 60-9000 pg·mL-1 and a limit of detection of 56 pg·mL-1 using probe antibody-modified fluorescence nanoparticles. While instrument-free visual detection is no longer possible, the developed lateral flow channel platform has the potential to dramatically expand the LFA applicability, as it overcomes the limitations of membrane-based immunoassays, ultimately improving the accuracy and reducing the sample volume so that finger-prick analyses can easily be done in a one-step assay for analytes present at very low concentrations.

Molecular Dynamics Simulations of Polyelectrolyte Complexes

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

Polymersomes for Therapeutic Protein and Peptide Delivery: Towards Better Loading Properties

Therapeutics based on proteins and peptides have profoundly transformed the landscape of treatment for diseases, from diabetes mellitus to cancers, yet the short half-life and low bioavailability of therapeutic proteins and peptides hinder their wide applications. To break through this bottleneck, biomolecules-loaded polymersomes with strong adjustability and versatility have attracted more and more attentions recently. Loading proteins or peptides into polymersomes is the first but extremely important step towards developing high-quality formulation products. However, increasing protein and peptide loading content is quite challenging due to the inherent nature of self-assembled vesicle formation mechanism and physiochemical characteristics of biomacromolecules. This review highlights the potential of polymersomes as the next-generation therapeutic proteins and peptides carrier and emphatically introduces novel approaches and recent progress to achieve satisfactory encapsulation capability of polymersomes for proteins and peptides. On the one hand, with the help of intermolecular interactions, such as electrostatic, lipid-protein, and hydrophobic interactions, the drug loading could be significantly improved. On the other hand, loading improvement could be attained through innovation of preparation methods, ranging from modified traditional film hydration techniques to the novel phase-guided assembly method.

Modulation of the electrostatic potential around α-lactalbumin using oligoelectrolyte chains, pH and salt concentration

In this study, we conducted a comprehensive computational investigation of the interaction between α-lactalbumin, a small globular protein, and strong anionic oligoelectrolyte chains with a polymerization degree from 2 to 9. Both the protein and oligoelectrolyte chains are represented using coarse-grained models, and their properties were calculated by the Monte Carlo method under constant pH conditions. We were able to estimate the effects of this interaction on the electrostatic potential around the protein. At acidic pH, the protein had a net positive charge; therefore, the electrostatic potential around it was also positive. To neutralize or reverse this electrostatic potential, oligoelectrolyte chains with a minimum size of six monomers were necessary. Simultaneously, low salt concentrations were required as elevated salt levels led to a significant attenuation of the electrostatic interactions and the corresponding electrostatic potential.

An insight into the effect of interaction with protein on antibacterial activity of chitosan derivatives

Antimicrobial activity of chitosan in protein-rich media is of a particular interest for various protein-based drug delivery and other systems. For the first time, bacteriostatic activity of chitosan derivatives in the presence of caseinate sodium (CAS) was studied and discussed. Complexation of chitosan derivatives soluble in acidic (CH and RCH) or alkalescent (RCH) media with CAS was confirmed by fluorescent spectroscopy, turbodimetry, light scattering data and measurement of electrical potentials of CAS/chitosan derivative complexes. An addition of CH and RCH caused a static quenching of CAS. Binding constants Kb determined for CH/CAS and RCH/CAS complexes at pH 6.0 were equal to 29.8 × 106 M-1 and 8.9 × 106 M-1, respectively. Kb value of RCH/CAS complex at pH 7.4 was equal to 1.1 × 105'M-1. The poisoned food method was used for counting the number and the direct measurement of the size of bacterial colonies on the surfaces of turbid agar media containing CAS/chitosan derivative complexex. Complete suppression of E. coli cells growth and restriction of S. aureus cells growth were observed on the surface of acidic media. A high concentration of CAS reduced the activity. The activity of RCH in alkalescent media is low or absent. These results can be promising for preparation of microbiologically stable protein-based drug delivery systems.

Polyelectrolyte Brushes with Protein-Like Nanocolloids

Electrostatic interaction of ampholytic nanocolloidal particles (NPs), which mimic globular proteins, with polyelectrolyte brushes is analyzed within mean-field Poisson-Boltzmann approximation. In accordance with experimental findings, the theory predicts that an electrostatic driving force for the particle uptake by the brush may emerge when the net charge of the particle in the buffer and the charge of the brush are of the same sign. The origin of this driving force is change in the ionization state of weak cationic and anionic groups on the NP surface provoked by interaction with the brush. In experimental systems, the ionic interactions are complemented by excluded-volume, hydrophobic, and other types of interactions that all together control NP uptake by or expulsion from the brush. Here, we focus on the NP-brush ionic interactions. It is demonstrated that deviation between the buffer pH and the NP isoelectric point, considered usually as the key control parameter, does not uniquely determine the insertion free energy patterns. The latter depends also on the proportion of cationic and anionic groups in the NPs and their specific ionization constants as well as on salt concentration in the buffer. The analysis of the free energy landscape proves that a local minimum in the free energy inside the brush appears, provided the NP charge reversal occurs upon insertion into the brush. This minimum corresponds either to a thermodynamically stable or to a metastable state, depending on the pH offset from the IEP and salt concentration, and is separated from the bulk of the solution by a free energy barrier. The latter, being fairly independent of salt concentration in height, may strongly impede the NP absorption kinetically even when it is thermodynamically favorable. Hence, change reversal is a necessary but insufficient condition for the uptake of the NPs by similarly charged polyelectrolyte brushes.

Reversible protein complexes as a promising avenue for the development of high concentration formulations of biologics

High concentration formulations have become an important pre-requisite in the development of biological drugs, particularly in the case of subcutaneous administration where limited injection volume negatively affects the administered dose. In this study, we propose to develop high concentration formulations of biologics using a reversible protein-polyelectrolyte complex (RPC) approach. First, the versatility of RPC was assessed using different complexing agents and formats of therapeutic proteins, to define the optimal conditions for complexation and dissociation of the complex. The stability of the protein was investigated before and after complexation, as well as upon a 4-week storage period at various temperatures. Subsequently, two approaches were selected to develop high concentration RPC formulations: first, using up-concentrated RPC suspensions in aqueous buffers, and second, by generating spray-dried RPC and further resuspension in non-aqueous solvents. Results showed that the RPC concept is applicable to a wide range of therapeutic protein formats and the complexation-dissociation process did not affect the stability of the proteins. High concentration formulations up to 200 mg/mL could be achieved by up-concentrating RPC suspensions in aqueous buffers and RPC suspensions in non-aqueous solvents were concentrated up to 250 mg/mL. Although optimization is needed, our data suggests that RPC may be a promising avenue to achieve high concentration formulations of biologics for subcutaneous administration.

Effects of Control Factors on Protein-Polyelectrolyte Complex Coacervation

Protein-polyelectrolyte complex coacervation is of particular interest for mimicking intracellular phase separation and organization. Yet, the challenge arises from regulating the coacervation due to the globular structure and anisotropic distributed charges of protein. Herein, we fully investigate the different control factors and reveal their effects on protein-polyelectrolyte coacervation. We prepared mixtures of BSA (bovine serum albumin) with different cationic polymers, which include linear and branched polyelectrolytes covering different spacer and charge groups, chain lengths, and polymer structures. With BSA-PDMAEMA [poly(N,N-dimethylaminomethyl methacrylate)] as the main investigated pair, we find that the moderate pH and ionic strength are essential for the adequate electrostatic interaction and formation of coacervate droplets. For most BSA-polymer mixtures, excess polyelectrolytes are required to achieve the full complexation, as evidenced by the deviated optimal charge mixing ratios from the charge stoichiometry. Polymers with longer chains or primary amine groups and a branched structure endow a strong electrostatic interaction with BSA and cause a bigger charge ratio deviation associated with the formation of solid-like coacervate complexes. Nevertheless, both the liquid- and solid-like coacervates hardly interrupt the BSA structure and activity, indicating the safe encapsulation of proteins by the coacervation with polyelectrolytes. Our study validates the crucial control of the diverse factors in regulating protein-polyelectrolyte coacervation, and the revealed principles shall be instructive for establishing other protein-based coacervations and boosting their potential applications.

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.

pH-Dependent Adsorption of Human Serum Albumin Protein on a Polystyrene-Block-Poly(acrylic acid)-Coated PVDF Membrane

This study reports the investigation of human serum albumin (HSA) adsorption on a poy-styrene-block-poly(acrylic acid) (PS-b-PAA)-coated PVDF membrane, which is a potential smart material for biomedical applications. First, copolymer coating on the membrane surface was successfully performed, due to the hydrophobic interaction of the PS anchoring group with the PVDF membrane. This was confirmed by Fourier transform infrared spectroscopy (FTIR) characterization of the membrane. Then, HSA adsorption onto the coated membrane was assessed and was proved to be strongly dependent on the pH of the protein solution. Indeed, both FTIR mapping and mass balance calculation using UV-visible spectroscopy displayed a greater HSA adsorption on the membrane at pH 5, even though it still took place at higher pH, but to a lower extent. Afterwards, an ionic strength influence study evinced the role of electrostatic interactions between HSA and the PAA layer on HSA adsorption. Dead-end filtration of HSA through the coated membrane confirmed the pH dependence of HSA adsorption on the coated membrane.

Reverse Engineering of a Pathogenic Antibody Reveals the Molecular Mechanism of Vaccine-Induced Immune Thrombotic Thrombocytopenia

The massive COVID-19 vaccine roll-out campaign illuminated a range of rare side effects, the most dangerous of which─vaccine-induced immune thrombotic thrombocytopenia (VITT)─is caused by adenoviral (Ad)-vectored vaccines. VITT occurrence had been linked to the production of pathogenic antibodies that recognize an endogenous chemokine, platelet factor 4 (PF4). Mass spectrometry (MS)-based evaluation of the ensemble of anti-PF4 antibodies obtained from a VITT patient's blood indicates that the major component is a monoclonal antibody. Structural characterization of this antibody reveals several unusual characteristics, such as the presence of an N-glycan in the Fab segment and high density of acidic amino acid residues in the complementarity-determining regions. A recombinant version of this antibody (RVT1) was generated by transient expression in mammalian cells based on the newly determined sequence. It captures the key properties of VITT antibodies such as their ability to activate platelets in a PF4 concentration-dependent fashion. Homology modeling of the Fab segment reveals a well-defined polyanionic paratope, and the docking studies indicate that the polycationic segment of PF4 readily accommodates two Fab segments, cross-linking the antibodies to yield polymerized immune complexes. Their existence was verified with native MS by detecting assemblies as large as (RVT1)3(PF4)2, pointing out at FcγRIIa-mediated platelet activation as the molecular mechanism underlying VITT clinical manifestations. In addition to the high PF4 affinity, RVT1 readily binds other polycationic targets, indicating a polyreactive nature of this antibody. This surprising promiscuity not only sheds light on VITT etiology but also opens up a range of opportunities to manage this pathology.

Monitoring Protein Complexation with Polyphosphazene Polyelectrolyte Using Automated Dynamic Light Scattering Titration and Asymmetric Flow Field Flow Fractionation and Protein Recognition Immunoassay

Polyphosphazenes represent a class of intrinsically flexible polyelectrolytes with potent immunoadjuvant activity, which is enabled through non-covalent self-assembly with antigenic proteins by charge complexation. The formation of supramolecular complexes between polyphosphazene adjuvant, poly[di(carboxylatophenoxy)phosphazene] (PCPP), and a model vaccine antigen, hen egg lysozyme, was studied under physiological conditions using automated dynamic light scattering titration, asymmetric flow field flow fractionation (AF4), enzyme-linked immunosorbent assay (ELISA), and fluorescent quenching methods. Three regimes of self-assembly were observed covering complexation of PCPP with lysozyme in the nano-scale range, multi-chain complexes, and larger aggregates with complexes characterized by a maximum loading of over six hundred protein molecules per PCPP chain and dissociation constant in the micromolar range (Kd = 7 × 10-6 mol/L). The antigenicity of PCPP bound lysozyme, when compared to equivalent lysozyme solutions, was largely retained for all complexes, but observed a dramatic reduction for heavily aggregated systems. Routes to control the complexation regimes with elevated NaCl or KCl salt concentrations indicate ion-specific effects, such that more smaller-size complexes are present at higher NaCl, counterintuitive with respect to PCPP solubility arguments. While the order of mixing shows a prominent effect at lower stoichiometries of mixing, higher NaCl salt reduces the effect all together.

Driving forces of the complex formation between highly charged disordered proteins

Highly disordered complexes between oppositely charged intrinsically disordered proteins present a new paradigm of biomolecular interactions. Here, we investigate the driving forces of such interactions for the example of the highly positively charged linker histone H1 and its highly negatively charged chaperone, prothymosin α (ProTα). Temperature-dependent single-molecule Förster resonance energy transfer (FRET) experiments and isothermal titration calorimetry reveal ProTα-H1 binding to be enthalpically unfavorable, and salt-dependent affinity measurements suggest counterion release entropy to be an important thermodynamic driving force. Using single-molecule FRET, we also identify ternary complexes between ProTα and H1 in addition to the heterodimer at equilibrium and show how they contribute to the thermodynamics observed in ensemble experiments. Finally, we explain the observed thermodynamics quantitatively with a mean-field polyelectrolyte theory that treats counterion release explicitly. ProTα-H1 complex formation resembles the interactions between synthetic polyelectrolytes, and the underlying principles are likely to be of broad relevance for interactions between charged biomolecules in general.

Effect of Soft Preheating of Bovine Serum Albumin on the Complexation with Oligochitosan: Structure and Conformation of BSA in the Complex

Phase analysis, spectroscopic, and light scattering methods are applied to investigate the peculiarities of the interaction of oligochitosan (OCHI) with native and preheated bovine serum albumin (BSA) as well as the conformational and structural changes of BSA in BSA/OCHI complex. As shown, untreated BSA binds with OCHI mainly forming soluble electrostatic nanocomplexes, with the binding causing an increase in BSA helicity without a change in the local tertiary structure and thermal stability of BSA. In contrast, soft preheating at 56 °C enhances the complexation of BSA with OCHI and slightly destabilizes the secondary and local tertiary structures of BSA within the complex particles. Preheating at 64 °C (below the irreversible stage of BSA thermodenaturation) leads to further enhancement in the complexation and formation of insoluble complexes stabilized by both Coulomb forces and hydrophobic interactions. The finding can be promising for the preparation of biodegradable BSA/chitosan-based drug delivery systems.

Protein-polysaccharide nanoconjugates: Potential tools for delivery of plant-derived nutraceuticals

Protein-polysaccharide nanoconjugates are covalently interactive networks that are currently the subject of intense research owing to their emerging applications in the food nanotechnology field. Due to their biocompatibility and biodegradability properties, they have played a significant role as wall materials for the formation of various nanostructures to encapsulate nutraceuticals. The food-grade protein-polysaccharide nanoconjugates would be employed to enhance the delivery and stability of nutraceuticals for their real use in the food industry. The most common edible polysaccharides (cellulose, chitosan, pectin, starch, carrageenan, fucoidan, mannan, glucomannan, and arabic gum) and proteins (silk fibroin, collagen, gelatin, soy protein, corn zein, and wheat gluten) have been used as potential building blocks in nano-encapsulation systems because of their excellent physicochemical properties. This article broadens the discussion of food-grade proteins and polysaccharides as nano-encapsulation biomaterials and their fabrication methods, along with a review of the applications of protein-polysaccharide nanoconjugates in the delivery of plant-derived nutraceuticals.

Efficient binding paradigm of protein and polysaccharide: Preparation of isolated soy protein-chitosan quaternary ammonium salt complex system and exploration of its emulsification potential

Soy protein isolate (SPI) has good emulsifying ability, but is greatly affected by the environment. The addition of polysaccharides either increases or decreases the stability of SPI. We report and prepared for the first time SPI/HACC complexes with different polysaccharide contents (SPI/HACC ratios are 1:1, 2:1 and 5:1). The binding properties, microstructure and emulsifying properties of the SPI/HACC complexes were determined and analyzed. The results showed that the interaction them is mainly through hydrogen bonding, electrostatic interaction, hydrophobic interaction and steric hindrance effect. The combination of SPI and HACC overcomes their respective limitations and the microstructure is more flat and smooth. It was also found that the emulsifying ability and concentration of SPI showed a certain correlation and the addition of HACC significantly improved the emulsifying ability and storage stability of SPI. This study shows that the prepared SPI/HACC complex has great potential for application in the food industry.

Complexation of oligochitosan with sodium caseinate in alkalescent and weakly acidic media

Сomplexation of oligochitosan (OCHI) having the degree of acetylation (DA 26 %) with sodium caseinate (SC) at pH 5.8 and 7.2 is described and compared with the complexation of OCHI (DA 2 %) at pH 5.8. In the alkalescent medium, the complexation of OCHI (DA 26 %) is weaker and dualistic depending on SC concentration in the system. In the diluted alkalescent system, the formation of only soluble complexes is observed at OCHI/SC ratio ≤0.9. In the semi diluted one, the complexation results in the formation of insoluble complexes those composition changes symbatically with the OCHI/SC ratio in the system. At pH 5.8, OCHI/SC ratio in insoluble complexes remains the same regardless of OCHI/SC ratio in the solution. At pH 5.8, the electrostatic complexation weakens with an increase in DA and is completely suppressed at a high ionic strength. These results can be promising for construction of biodegradable protein/chitosan drug delivery systems.

Effects of Chitosan Molecular Weight and Degree of Deacetylation on Chitosan-Cellulose Nanocrystal Complexes and Their Formation

This study was conducted to determine the effects of chitosan molecular weight and degree of deacetylation (DD) on chitosan-cellulose nanocrystal (CNC) polyelectrolyte-macroion complexes (PMCs) and their formation. Chitosan samples with three different molecular weights (81, 3 · 10³, 6 · 10³ kDa) and four different DDs (77, 80, 85, 89%) were used. The effects on PMC formation were determined by turbidimetric titration. An effect of the molecular weight of chitosan was not observed in turbidimetric titrations. Turbidity levels were higher for CNCs with lower sulfate group density and larger hydrodynamic diameter than for CNCs with higher sulfate group density and smaller hydrodynamic diameter. Conversely, turbidity levels were higher for chitosans with higher DD (higher charge density) than for chitosans with lower DD (lower charge density). PMC particles from chitosans with different molecular weights were characterized by scanning electron microscopy, laser Doppler electrophoresis, and dynamic light scattering. PMCs from high-molecular-weight chitosan were more spherical and those from medium-molecular-weight chitosan had a slightly larger hydrodynamic diameter than PMCs from the respective other two chitosans. The molecular weight of the chitosan was concluded to have no effect on the formation of chitosan-CNC PMC particles and only a minor effect on the shape and size of the particles. The higher turbidity levels for CNCs with lower sulfate group density and larger hydrodynamic diameter and for chitosans with higher DD were attributed to a larger number of CNCs being required for charge compensation.

Polyelectrolyte Influence on Beta-Hairpin Peptide Stability: A Simulation Study

Assemblies of proteins and charged macromolecules (polyelectrolytes) find important applications as pharmaceutical formulations, biocatalysts, and cell-contacting substrates. A key question is how the polymer component influences the structure and function of the protein. The present paper addresses the influence of charged polymers on the thermal stability of two model beta-hairpin-forming peptides through an all-atom, replica exchange molecular dynamics simulation. The (negatively charged) peptides consist of the terminal 16 amino acids of the B1 domain of Protein G (GB1) and a variant with three of the GB1 residues substituted with tryptophan (Tryptophan Zipper 4, or TZ4). A (cationic) lysine polymer is seen to thermally stabilize TZ4 and destabilize GB1, while a (also cationic) chitosan polymer slightly stabilizes GB1 but has essentially no effect on TZ4. Free energy profiles reveal folded and unfolded conformations to be separated by kinetic barriers generally acting in the direction of the thermodynamically favored state. Through application of an Ising-like statistical mechanical model, a mechanism is proposed based on competition between (indirect) entropic stabilization of folded versus unfolded states and (direct) competition for hydrogen-bonding and hydrophobic interactions. These findings have important implications to the design of polyelectrolyte-based materials for biomedical and biotechnological applications.

Biomolecular Complexation on the "Wrong Side": A Case Study of the Influence of Salts and Sugars on the Interactions between Bovine Serum Albumin and Sodium Polystyrene Sulfonate

In the protein purification, drug delivery, food industry, and biotechnological applications involving protein-polyelectrolyte complexation, proper selection of co-solutes and solution conditions plays a crucial role. The onset of (bio)macromolecular complexation occurs even on the so-called "wrong side" of the protein isoionic point where both the protein and the polyelectrolyte are net like-charged. To gain mechanistic insights into the modulatory role of salts (NaCl, NaBr, and NaI) and sugars (sucrose and sucralose) in protein-polyelectrolyte complexation under such conditions, interaction between bovine serum albumin (BSA) and sodium polystyrene sulfonate (NaPSS) at pH = 8.0 was studied by a combination of isothermal titration calorimetry, fluorescence spectroscopy, circular dichroism, and thermodynamic modeling. The BSA-NaPSS complexation proceeds by two binding processes (first, formation of intrapolymer complexes and then formation of interpolymer complexes), both driven by favorable electrostatic interactions between the negatively charged sulfonic groups (-SO₃^-) of NaPSS and positively charged patches on the BSA surface. Two such positive patches were identified, each responsible for one of the two binding processes. The presence of salts screened both short-range attractive and long-range repulsive electrostatic interactions between both macromolecules, resulting in a nonmonotonic dependence of the binding affinity on the total ionic strength for both binding processes. In addition, distinct anion-specific effects were observed (NaCl < NaBr < NaI). The effect of sugars was less pronounced: sucrose had no effect on the complexation, but its chlorinated analogue, sucralose, promoted it slightly due to the screening of long-range repulsive electrostatic interactions between BSA and NaPSS. Although short-range non-electrostatic interactions are frequently mentioned in the literature in relation to BSA or NaPSS, we found that the main driving force of complexation on the "wrong side" are electrostatic interactions.

Polyelectrolytes for Enzyme Immobilization and the Regulation of Their Properties

In this review, we considered aspects related to the application of polyelectrolytes, primarily synthetic polyanions and polycations, to immobilize enzymes and regulate their properties. We mainly focused on the description of works in which polyelectrolytes were used to create complex and unusual systems (self-regulated enzyme-polyelectrolyte complexes, artificial chaperones, polyelectrolyte brushes, layer-by-layer immobilization and others). These works represent the field of "smart polymers", whilst the trivial use of charged polymers as carriers for adsorption or covalent immobilization of proteins is beyond the scope of this short review. In addition, we have included a section on the molecular modeling of interactions between proteins and polyelectrolytes, as modeling the binding of proteins with a strictly defined, and already known, spatial structure, to disordered polymeric molecules has its own unique characteristics.

Responsive Hyaluronic Acid–Ethylacrylamide Microgels Fabricated Using Microfluidics Technique

Volume changes of responsive microgels can probe interactions between polyelectrolytes and species of opposite charges such as peptides and proteins. We have investigated a microfluidics method to synthesize highly responsive, covalently crosslinked, hyaluronic acid microgels for such purposes. Sodium hyaluronate (HA), pre-modified with ethylacrylamide functionalities, was crosslinked in aqueous droplets created with a microfluidic technique. We varied the microgel properties by changing the degree of modification and concentration of HA in the reaction mixture. The degree of modification was determined by 1H NMR. Light microscopy was used to investigate the responsiveness of the microgels to osmotic stress in aqueous saline solutions by simultaneously monitoring individual microgel species in hydrodynamic traps. The permeability of the microgels to FITC-dextrans of molecular weights between 4 and 250 kDa was investigated using confocal laser scanning microscopy. The results show that the microgels were spherical with diameters between 100 and 500 µm and the responsivity tunable by changing the degree of modification and the HA concentration. Microgels were fully permeable to all investigated FITC-dextran probes. The partitioning to the microgel from an aqueous solution decreased with the increasing molecular weight of the probe, which is in qualitative agreement with theories of homogeneous gel networks.

Preparation and Characterization of Rutin–Loaded Zein–Carboxymethyl Starch Nanoparticles

In this work, rutin (RT)–loaded zein–carboxymethyl starch (CMS) nanoparticles were successfully prepared by the antisolvent precipitation method. The effect of CMS on composite nanoparticles at different concentrations was studied. When the ratio of zein–RT–CMS was 10:1:30, the encapsulation efficiency (EE) was the highest, reaching 73.5%. At this ratio, the size of the composite nanoparticles was 196.47 nm, and the PDI was 0.13, showing excellent dispersibility. The results of fluorescence spectroscopy, FTIR, XRD, and CD showed that electrostatic interaction, hydrogen bonding, and hydrophobic interaction were the main driving forces for the formation of nanoparticles. It can be seen from the FE–SEM images that the zein–RT–CMS nanoparticles were spherical. With the increase in the CMS concentration, the particles gradually embedded in the cross–linked network of CMS (10:1:50). After RT was loaded on zein–CMS nanoparticles, the thermal stability and pH stability of RT were improved. The results showed that zein–CMS was an excellent encapsulation material for bioactive substances.

Engineering Bio-Adhesives Based on Protein–Polysaccharide Phase Separation

Glue-type bio-adhesives are in high demand for many applications, including hemostasis, wound closure, and integration of bioelectronic devices, due to their injectable ability and in situ adhesion. However, most glue-type bio-adhesives cannot be used for short-term tissue adhesion due to their weak instant cohesion. Here, we show a novel glue-type bio-adhesive based on the phase separation of proteins and polysaccharides by functionalizing polysaccharides with dopa. The bio-adhesive exhibits increased adhesion performance and enhanced phase separation behaviors. Because of the cohesion from phase separation and adhesion from dopa, the bio-adhesive shows excellent instant and long-term adhesion performance for both organic and inorganic substrates. The long-term adhesion strength of the bio-glue on wet tissues reached 1.48 MPa (shear strength), while the interfacial toughness reached ~880 J m−2. Due to the unique phase separation behaviors, the bio-glue can even work normally in aqueous environments. At last, the feasibility of this glue-type bio-adhesive in the adhesion of various visceral tissues in vitro was demonstrated to have excellent biocompatibility. Given the convenience of application, biocompatibility, and robust bio-adhesion, we anticipate the bio-glue may find broad biomedical and clinical applications.

Encapsulation of curcumin in soluble soybean polysaccharide-coated gliadin nanoparticles: interaction, stability, antioxidant capacity, and bioaccessibility

BACKGROUND

Gliadin nanoparticles are used as a delivery system for active substances because of their amphiphilicity and bioavailability. However, they are susceptible to destabilization by external agents. In this study, gliadin nanoparticles stabilized by soluble soybean polysaccharide (SSPS) were prepared by antisolvent precipitation. Formed stable complex nanoparticles were applied to protect and deliver curcumin (Cur).

RESULTS

Gliadin/SSPS nanoparticles with the smallest particle size (196.66 nm, polydispersity index < 0.2) were fabricated when the mass ratio of gliadin to SSPS was 1:1 at pH 5.0. SSPS-stabilized gliadin nanoparticles had excellent stability at pH 3.0-8.0, 0.02-0.1 mol L-1 NaCl and at 90 °C heat. Gliadin/SSPS nanoparticles were used to encapsulate the Cur. The encapsulation efficiency of the Cur-loaded gliadin/SSPS nanoparticles was 84.59%. Fourier transform infrared spectroscopy and fluorescence spectrophotometry showed that the main forces were hydrogen bonds, electrostatic interactions and hydrophobic interactions between gliadin and SSPS. The X-ray diffraction patterns exhibited that the crystalline form of Cur converted to an amorphous substance. The retention rates of Cur-loaded gliadin/SSPS nanoparticles reached 79.03%, 73.43% and 87.92% after ultraviolet irradiation for 4 h, heating at 90 °C and storage at 25 °C for 15 days, respectively. Additionally, simulated digestion demonstrated that the bioavailability of gliadin/SSPS-Cur nanoparticles was four times higher than that of free Cur.

CONCLUSION

This study showed that SSPS improved the stability of gliadin nanoparticles. Gliadin/SSPS nanoparticles have the function of loading and delivering Cur. © 2022 Society of Chemical Industry.

Evaluation of properties and structural transitions of Poly (2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylonitrile) / β-Galactosidase complex coacervates: effects of pH and aging

The coacervates of the Poly (2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylonitrile) / β-Galactosidase complex are characterized using several techniques (Turbidimetry, dynamic and static light scattering (DLS / SLS), optical microscopy, image dynamic light scattering (IDLS), and ultra-small angles light scattering (USALS)). Turbidity and SLS were used to accurately determine the critical pHs of complex formation (pH_c, pH_ϕ1, pH_opt, and pH_ϕ2), DLS was used to probe the microscopic structure of coacervate droplets rich in proteins and polyelectrolytes in liquid-liquid phase separation, and IDLS and USALS have been introduced to better understand, during aging, the topology of the network formed of materials based on fractals in the dense phase. Observations of the architecture, the spatial inhomogeneity, and the size distributions of liquid complex coacervate droplets and fractal solid precipitates, were performed by optical microscopy. The pair-distance distribution function, P(r), presented in this review, is a new methodology of calculus for determining with precision the radius of gyration R_g of droplets coacervates. These techniques show that aging improves the stability of swollen condensates, the growth of the coacervate droplets is due to the attractive electrostatic interactions within the complex and does not undergo Ostwald ripening, except for the case of pH_opt and having fractal dimensions D_f generated by diffusion-limited cluster aggregation (DLCA).

Covalent Labeling-Mass Spectrometry Provides a Molecular Understanding of Noncovalent Polymer-Protein Complexation

The delivery of functional proteins to the intracellular space offers tremendous advantages for the development of new therapeutics but is limited by the passage of these large polar biomacromolecules through the cell membrane. Noncovalent polymer-protein binding that is driven by strong carrier-cargo interactions, including electrostatics and hydrophobicity, has previously been explored in the context of delivery of functional proteins. Appropriately designed polymer-based carriers can take advantage of the heterogeneous surface of protein cargoes, where multiple types of physical binding interactions with polymers can occur. Traditional methods of assessing polymer-protein binding, including dynamic light scattering, circular dichroism spectroscopy, and fluorescence-based assays, are useful in the study of new polymer-based carriers but face a number of limitations. We implement for the first time the method of covalent labeling-mass spectrometry (CL-MS) to probe intermolecular surface interactions within noncovalent polymer-protein complexes. We demonstrate the utility of CL-MS for establishing binding of an amphiphilic block copolymer to negatively charged and hydrophobic surface patches of a model protein, superfolder green fluorescent protein (sfGFP), using diethylpyrocarbonate as a pseudo-specific labeling reagent. In addition, we utilize this method to explore differences at the intermolecular surface as the ratio of polymer to protein increases, particularly in the context of defining effective protein delivery regimes. By promoting an understanding of the intermolecular interactions in polymer-protein binding and identifying sites where polymers bind to protein surfaces, noncovalent polymer carriers can be more effectively designed for protein delivery applications.

The non-covalent interactions between whey protein and various food functional ingredients

In daily diet, Whey protein (WP) is often coexisted with various Food functional ingredients (FFI) such as proteins, polyphenols, polysaccharides and vitamins, which inevitably affect or interact with each other. Generally speaking, they may be interact by two different mechanisms: non-covalent and covalent interactions, of which the former is more common. We reviewed the non-covalent interactions between WP and various FFI, explained the effect of each WP-FFI interaction, and provided possible applications of WP-FFI complex in the food industry. The biological activity, physical and chemical stability of FFI, and the structure and functionalities of WP were enhanced through the non-covalent interactions. The development of non-covalent interactions between WP and FFI provides opportunities for the design of new ingredients and biopolymer complex, which can be applied in different fields. Future research will further focus on the influence of external or environmental factors in the food system and processing methods on interactions.

Layer-by-Layer Nanoarchitectonics Using Protein-Polyelectrolyte Complexes toward a Generalizable Tool for Protein Surface Immobilization

Layer-by-layer (LbL) self-assembly is an attractive method for the immobilization of macromolecules at interfaces. Integrating proteins in LbL thin films is however challenging due to their polyampholyte nature. Recently, we developed a method to integrate lysozyme into multilayers using protein-polyelectrolytes complexes (PPCs). In this work, we extended this method to a wide range of protein-polyelectrolyte combinations. We demonstrated the robustness and versatility of PPCs as building blocks. LL-37, insulin, lysozyme, and glucose oxidase were complexed with alginate, poly(styrenesulfonate), heparin, and poly(allylamine hydrochloride). The resulting PPCs were then LbL self-assembled with chitosan, PAH, and heparin. We demonstrated that multilayers built with PPCs are thicker compared to the LbL self-assembly of bare protein molecules. This is attributed to the higher mass of protein in the multilayers and/or the more hydrated state of the assemblies. PPCs enabled the self-assembly of proteins that could otherwise not be LbL assembled with a PE or with another protein. Furthermore, the results also show that LbL with PPCs enabled the construction of multilayers combining different proteins, highlighting the formation of multifunctional films. Importantly, we show that the adsorption behavior and thus the multilayer growth strongly depend on the nature of the protein and polyelectrolyte used. In this work, we elaborated a rationale to help and guide the use of PPCs for protein LbL assembly. It will therefore be beneficial to the many scientific communities willing to modify interfaces with hard-to-immobilize proteins and peptides.

Heparin-Assisted Amyloidogenesis Uncovered through Molecular Dynamics Simulations

Glycosaminoglycans (GAGs), in particular, heparan sulfate and heparin, are found colocalized with Aβ amyloid. They have been shown to enhance fibril formation, suggesting a possible pathological connection. We have investigated heparin's assembly of the KLVFFA peptide fragment using molecular dynamics simulation, to gain a molecular-level mechanistic understanding of how GAGs enhance fibril formation. The simulations reveal an exquisite process wherein heparin accelerates peptide assembly by first "gathering" the peptide molecules and then assembling them. Heparin does not act as a mere template but is tightly coupled to the peptides, yielding a composite protofilament structure. The strong intermolecular interactions suggest composite formation to be a general feature of heparin's interaction with peptides. Heparin's chain flexibility is found to be essential to its fibril promotion activity, and the need for optimal heparin chain length and concentration has been rationalized. These insights yield design rules (flexibility; chain-length) and protocol guidance (heparin:peptide molar ratio) for developing effective heparin mimetics and other functional GAGs.

Complex Coacervation and Precipitation Between Soluble Pea Proteins and Apple Pectin

Complex formation (leading to either coacervation or precipitation) offers a tool to generate plant-based novel food structures and textures. This study investigated the formation of complexes between soluble pea proteins and apple pectin upon varying the protein-to-pectin ratio (r = 2:1 to 10:1), pH (3–7), and temperature (25 and 85 °C) with a total biopolymer concentration set to 1% (w/w). The results showed that predominantly soluble biopolymer complexes were formed at pH 5, and at low ratio (r = 2:1), whereas lowering the pH to more acidic condition, and to higher ratios (r = 4:1–10:1) induced the formation of more insoluble biopolymer complexes. In general, the mean particle sizes of the biopolymer complexes ranged between approximately 20 and 100 μm. Upon heating to 85 °C, the amount of insoluble biopolymer complexes increased at pH 3–5 at all ratios, except at r = 2:1. In addition, the complex sizes became somewhat larger at r = 2:1 to 6:1 upon heat treatment, whereas only trivial size changes were observed at higher ratios (r = 8:1 to 10:1). Overall, electrostatic and hydrophobic interactions played a major role in the complex formation between the soluble pea proteins and apple pectin. These findings are important for designing solely plant-based food structures.

Preparation of trypsin-based nanoparticles, colloidal properties and ability to bind bioactive compounds

Nanoparticles (NPs) based on the proteolytic enzyme trypsin (TRY) were prepared by a biocompatible methodology. TRY co-assembled with the anionic polysaccharide chondroitin sulfate (CS) in complexes with well-defined distributions of radii in the range of 100-200 nm by electrostatic complexation at acidic conditions. At pH 7 the complexes were unstable and lost their monomodal size distribution which is potentially related to TRY's weak positive net surface charge and a large negative charge patch that forms at neutral pH. Thermal treatment at conditions which were not expected to interfere with TRY's proteolytic activity was used to stabilize the complexes into NPs that resisted disintegration at pH 7 taking advantage of the ability of the TRY globules to thermally aggregate. The secondary conformation of TRY within the NPs was found fairly unperturbed even after thermal treatment which is crucial for its physiological function. The CS-TRY NPs could bind and encapsulate the bioactive substances curcumin (CUR) and β-carotene (β-C) owing to TRY's hydrophobic domains. The CS-TRY NPs may be considered as a platform for the immobilized active enzyme and multifunctional NPs for hydrophobic bioactive compounds.

Modulating Role of Co-Solutes in Complexation between Bovine Serum Albumin and Sodium Polystyrene Sulfonate

The action of three types of co-solutes: (i) salts (NaCl, NaBr, NaI), (ii) polymer (polyethylene glycol; PEG-400, PEG-3000, PEG-20000), and (iii) sugars (sucrose, sucralose) on the complexation between bovine serum albumin (BSA) and sodium polystyrene sulfonate (NaPSS) was studied. Three critical pH parameters were extracted from the pH dependence of the solution’s turbidity: pHc corresponding to the formation of the soluble complexes, pHΦ corresponding to the formation of the insoluble complexes, and pHopt corresponding to the charge neutralization of the complexes. In the presence of salts, the formation of soluble and insoluble complexes as well as the charge neutralization of complexes was hindered, which is a consequence of the electrostatic screening of attractive interactions between BSA and NaPSS. Distinct anion-specific trends were observed in which the stabilizing effect of the salt increased in the order: NaCl < NaBr < NaI. The presence of PEG, regardless of its molecular weight, showed no measurable effect on the formation of soluble complexes. PEG-400 and PEG-3000 showed no effect on the formation of insoluble complexes, but PEG-20000 in high concentrations promoted their formation due to the molecular crowding effect. The presence of sugar molecules had little effect on BSA-NaPSS complexation. Sucralose showed a minor stabilizing effect with respect to the onset of complex formation, which was due to its propensity to the protein surface. This was confirmed by the fluorescence quenching assay (Stern-Volmer relationship) and all-atom MD simulations. This study highlights that when evaluating the modulatory effect of co-solutes on protein-polyelectrolyte interactions, (co-solute)-protein interactions and their subsequent impact on protein aggregation must also be considered.

Complex pH-Dependent Interactions between Weak Polyelectrolyte Block Copolymer Micelles and Molecular Fluorophores

Amphiphilic block copolymers with weak polyelectrolyte blocks can assemble stimulus-responsive nanostructures and interfaces. Applications of these materials in drug delivery, biomimetics, and sensing largely rely on the well-understood swelling of polyelectrolyte chains upon deprotonation, often induced by changes in pH or ionic strength. This deprotonation can also tune interfacial interactions between the polyelectrolyte blocks and surrounding solution, an effect which is less studied than morphological swelling of polyelectrolytes but can be just as critical for intended function. Here, we investigate whether the pH-driven morphological response of polyelectrolyte-bearing nanostructures also affects the interactions of these nanostructures with molecules in solution, using micelles of a short-chain polybutadiene-block-poly(acrylic acid) (pBd-pAA) as a model system. We introduce a Förster resonance energy transfer (FRET) approach to probe interactions between micelles and fluorescent molecular solutes as a function of solution pH. As expected, the pAA corona of these pBd-pAA micelles increases in thickness monotonically as a function of pH. However, FRET efficiency, which provides a metric of the spatial proximity of fluorescently labeled micelles and freely diffusing fluorophores, exhibits complex nonmonotonic behavior as a function of pH, indicating that the average separation of micelles and acceptor fluorophores is not strictly correlated with micelle swelling. Dialysis experiments quantify the affinity of fluorophores for micelles as a function of pH, confirming that changes in FRET are driven almost entirely by the pH-dependent affinity of the pAA block for the investigated molecular fluorophores, not simply by a shape change of the pAA corona. This study provides key insights into the interfacial interactions between weak-polyelectrolyte-bearing nanostructures and molecular solutes, of importance for the development of their stimulus-responsive applications.

Interpolyelectrolyte Complexes as an Emerging Technology for Pharmaceutical Delivery of Polypeptides

Polyelectrolyte complexes and the derivatives thereof comprise some of the most promising vehicles for the encapsulation and delivery of macromolecular therapeutics. In particular, protein therapeutics, which present a host of special considerations, can often be effectively packaged and delivered using interpolyelectrolyte complexes. While the technologies are still in the developmental phase, there are numerous examples of complexes where control is exerted over spacial and temporal delivery of a model protein cargo or candidate protein therapeutic agent. Here we provide a historical and practical background to promote a deeper understanding of interpolyelectrolyte complexes and the derivative technologies. Additionally, we review the physical principles underlying the association of polyelectrolyte complexes and the application of those principles to novel strategies and technologies driving interpolyelectrolyte complexation. Then, the application of polyelectrolyte complex technology to protein therapeutics is discussed in detail including discussions of several types of protein cargo with a special emphasis on Brain-Derived Neurotrophic Factor. Finally, we focus on the use of stealth polymers in block ionomer complexes, specifically PEG; its benefits, flaws, and possible alternatives. Comprehensive understanding of the field may promote the continued development of derivative technologies for the delivery of particularly intransigent protein therapeutics, much as has been accomplished for small molecule drugs. We also aim to link current advances to the historical developments which inaugurated the field. With consideration to the field, industrial and academic researchers can utilize the discussed technologies and continue to elucidate novel modalities for a myriad of therapeutic and commercial applications.

pH-Dependent complexation and polyelectrolyte chain conformation of polyzwitterion-polycation coacervates in salted water

The phase behavior and chain conformational structure of biphasic polyzwitterion-polyelectrolyte coacervates in salted aqueous solution are investigated with a model weak cationic polyelectrolyte, poly(2-vinylpyridine) (P2VP), whose charge fraction can be effectively tuned by pH. It is observed that increasing the pH leads to the increase of the yielding volume fraction and the water content of dense coacervates formed between net neutral polybetaine and cationic P2VP in contrast to the decrease of critical salt concentration for the onset of coacervation, where the P2VP charge fraction is reduced correspondingly. Surprisingly, a single-molecule fluorescence spectroscopic study suggests that P2VP chains upon coacervation seem to adopt a swollen or an even more expanded conformational structure at higher pH. As the hydrophobicity of P2VP chains is accompanied by a reduced charge fraction by increasing the pH, a strong pH-dependent phase and conformational behaviors suggest the shift of entropic and enthalpic contribution to the underlying thermodynamic energy landscape and chain structural dynamics of polyelectrolyte coacervation involving weak polyelectrolytes in aqueous solution.