Continuous Synthesis of Spherical Polyelectrolyte Brushes by Photo-Emulsion Polymerization in a Microreactor

Nanosized spherical polyelectrolyte brushes (SPBs) are ideal candidates for the preparation of nanometal catalysts, protein separation, and medical diagnostics. Until now, SPBs have been synthesized by photo-emulsion polymerization in a batch reactor, which remains challenging to scale up. This paper reports a successful continuous preparation of SPBs by photo-emulsion polymerization in a self-made microreactor. The effects of residence time, monomer concentration, and feed ratios on the conversion of monomers and SPB structures are systematically investigated by dynamic lighting scattering and transmission electron microscopy. Poly(acrylic acid) (PAA) SPBs obtained in a microreactor exhibiting a narrow size distribution with a short reaction time are very effective in inhibiting the calcium carbonate scale and are comparable to those produced in a batch reactor. This work confirms the feasibility of continuous preparation and scaled-up production of SPBs.

Photosensitive Spherical Polymer Brushes: Light-Triggered Process of Particle Repulsion

We report on a light-triggered process at which repulsive interactions between microparticles with a polyelectrolyte (PE) brush coating can be remotely controlled. The spherical polyelectrolyte brushes are loaded with photosensitive azobenzene containing surfactant which can undergo reversible photo-isomerization from trans to cis state. The surfactant hydrophilicity is altered by illumination with light of an appropriate wavelength, at which a dynamic exchange of the more surface-active trans isomer in comparison to the more water soluble cis isomer with the PE brush generates a concentration gradient of the cis isomers near a solid surface where the particle is sedimented. In this way, each spherical brush produces its local lateral diffusioosmotic flow pointing outside in a radial direction resulting in mutual long-range repulsive interactions. We demonstrate that a PE layer has a higher tendency to absorb surfactant in comparison to plain silica particles, yielding a larger flow strength. This correlation holds true up to a critical intensity, where the dynamic exchange is adsorption limited with respect to trans isomers and especially pronounced for the PE-coated particles.

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.

Interaction of Heparin with Proteins: Hydration Effects

We present a thermodynamic investigation of the interaction of heparin with lysozyme in the presence of potassium glutamate (KGlu). The binding constant K_b is measured by isothermal titration calorimetry (ITC) in a temperature range from 288 to 310 K for concentrations of KGlu between 25 and 175 mM. The free energy of binding ΔG_b derived from K_b is strongly decreasing with increasing concentration of KGlu, whereas the dependence of ΔG_b on temperature T is found to be small. The decrease of ΔG_b can be explained in terms of counterion release: Binding of lysozyme to the strong polyelectrolyte heparin liberates approximately three of the condensed counterions of heparin, thus increasing the entropy of the system. The dependence of ΔG_b on T, on the other hand, is traced back to a change of hydration of the protein and the polyelectrolyte upon complex formation. This dependence is quantitatively described by the parameter Δw that depends on T and vanishes at a characteristic temperature T₀. A comparison of the complex formation in the presence of KGlu with the one in the presence of NaCl demonstrates that the parameters related to hydration are changed considerably. The characteristic temperature T₀ in the presence of KGlu solutions is considerably smaller than that in the presence of NaCl solutions. The change of specific heat Δc_p is found to become more negative with increasing salt concentration: This finding agrees with the model-free analysis by the generalized van't Hoff equation. The entire analysis reveals a small but important change of the free energy of binding by hydration. It shows that these ion-specific Hofmeister effects can be modeled quantitatively in terms of a characteristic temperature T₀ and a parameter describing the dependence of Δc_p on salt concentration.

Dynamic Polymer Amphiphiles for Efficient Intracellular and In Vivo Protein Delivery

Intracellular delivery of proteins is receiving considerable attention in biotherapeutics for various diseases by replacing dysfunctional proteins. Successful intracellular protein delivery highly relies on the development of efficient and safe polymeric carriers, which remains a grand challenge due to the lack of strong binding sites on proteins and their distinct molecular sizes and polarities. In this work, a strategy is proposed for efficient intracellular protein delivery by using dynamic polymer supra-amphiphiles, which are prepared by grafting boronated polylysine with a series of lipidated catechols via dynamic covalent catechol-boronate ester bonds. The prepared supra-amphiphiles can coassemble with proteins to form stable nanoparticles in water and also enable the release of bound proteins in cells due to their dynamic features, thereby strongly promoting the intracellular delivery process. The lead supra-amphiphiles screened in the library demonstrate high efficiency in the delivery of various proteins including bovine serum albumin, β-galactosidase, α-chymotrypsin, saporin, R-phycoerythrin, ovalbumin, catalase, and superoxide dismutase, and show great potency in delivering superoxide dismutase to treat ulcerative colitis in vivo. This work provides new opportunities for rational design and facile construction of robust intracellular protein delivery materials by the integration of polymer chemistry and supramolecular engineering strategies.

Interaction of Linear Polyelectrolytes with Proteins: Role of Specific Charge-Charge Interaction and Ionic Strength

We present a thermodynamic study of the interaction of synthetic, linear polyelectrolytes with bovine serum albumin (BSA). All polyelectrolytes are based on poly(allyl glycidyl ether) which has been modified by polymer-analogous reaction with anionic (-SO₃Na), cationic (-NH₃Cl or -NHMe₂Cl) or zwitterionic groups (-NMe₂(CH₂)₃SO₃). While the anionic polymer shows a very weak interaction, the zwitterionic polymer exhibits no interaction with BSA (pI = 4.7) under the applied pH = 7.4, ionic strength (I = 23–80 mM) and temperature conditions (T = 20–37 °C). A strong binding, however, was observed for the polycations bearing primary amino or tertiary dimethyl amino groups, which could be analysed in detail by isothermal titration calorimetry (ITC). The analysis was done using an expression which describes the free energy of binding, ΔG_b, as the function of the two decisive variables, temperature, T, and salt concentration, c_s. The underlying model splits ΔG_b into a term related to counterion release and a term related to water release. While the number of released counter ions is similar for both systems, the release of bound water is more important for the primary amine compared to the tertiary N,N-dimethyl amine presenting polymer. This finding is further traced back to a closer contact of the polymers’ protonated primary amino groups in the complex with oppositely charged moieties of BSA as compared to the bulkier protonated tertiary amine groups. We thus present an investigation that quantifies both driving forces for electrostatic binding, namely counterion release and change of hydration, which contribute to a deeper understanding with direct impact on future advancements in the biomedical field.

Interaction of Polyelectrolytes with Proteins: Quantifying the Role of Water

A theoretical model is presented for the free energy ΔGb of complex formation between a highly charged polyelectrolyte and a protein. The model introduced here comprises both the effect of released counterions and the uptake or release of water molecules during complex formation. The resulting expression for ΔGb is hence capable of describing the dependence of ΔGb on temperature as well as on the concentration of salt in the system: An increase of the salt concentration in the solution increases the activity of the ions and counterion release becomes less effective for binding. On the other hand, an increased salt concentration leads to the decrease of the activity of water in bulk. Hence, release of water molecules during complex formation will be more advantageous and lead to an increase of the magnitude of ΔGb and the binding constant. It is furthermore demonstrated that the release or uptake of water molecules is the origin of the marked enthalpy-entropy cancellation observed during complex formation of polyelectrolytes with proteins. The comparison with experimental data on complex formation between a synthetic (sulfated dendritic polyglycerol) and natural polyelectrolytes (DNA; heparin) with proteins shows full agreement with theory.

Interaction of Proteins with a Planar Poly(acrylic acid) Brush: Analysis by Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D)

We describe the preparation of a poly(acrylic acid) (PAA) brush, polymerized by atom transfer radical polymerization (ATRP) of tert-butyl acrylate (tBA) and subsequent acid hydrolysis, on the flat gold surfaces of quartz-crystal microbalance (QCM) crystals. The PAA brushes were characterized by Fourier transform infrared (FT-IR) spectroscopy, ellipsometry and water contact angle analysis. The interaction of the PAA brushes with human serum albumin (HSA) was studied for a range of ionic strengths and pH conditions by quartz-crystal microbalance with dissipation monitoring (QCM-D). The quantitative analysis showed a strong adsorption of protein molecules onto the PAA brush. By increasing the ionic strength, we were able to release a fraction of the initially bound HSA molecules. This finding highlights the importance of counterions in the polyelectrolyte-mediated protein adsorption/desorption. A comparison with recent calorimetric studies related to the binding of HSA to polyelectrolytes allowed us to fully analyze the QCM data based on the results of the thermodynamic analysis of the binding process.

Selective Protein Separation Based on Charge Anisotropy by Spherical Polyelectrolyte Brushes

Protein purification is of vital importance in the food industry, drug discovery, and other related fields. Among many separation methods, polyelectrolyte (PE)-based phase separation was developed and recognized as a low-cost purification technique. In this work, spherical polyelectrolyte brushes (SPBs) with a high specific surface area were utilized to study the protein accessibility and selective protein binding on highly charged nanoparticles (NPs) as well as the selective phase separation of proteins. The correlation between charge anisotropy, protein binding, and phase separation was investigated on various protein systems including those proteins with similar isoelectric points (pI) such as bovine serum albumin (BSA) and β-lactoglobulin (BLG), proteins with similar molecular weights such as BSA and hemoglobin (HB), and even protein variants (BLG-A and -B) with a tiny difference of amino acids. The nonspecific electrostatic interaction studied by turbidimetric titrations and isothermal calorimetry titration (ITC) indicates a specific binding between proteins and SPBs arising from the charge anisotropy of proteins. An optimized output based on selective protein binding on SPBs could be correlated for efficient protein separation through tuning external conditions including pH and ionic strength. These findings, therefore, proved that phase separation based on selective protein adsorption by SPBs was an efficient alternative for protein separation compared with the traditional practice.