Protection of Enzymes Against Heat Inactivation by Enzyme-Polymer Conjugates

Along with the quick advancements in enzyme technology, inactivation has emerged as the key barrier for enzymes to be fully utilized as biocatalysts. Here, a novel strategy is presented for the preservation of the enzymatic activity even after heat treatment by grafting enzymes onto the thermal responsive block copolymer via an activated ester-amine reaction. A new water-soluble activated ester monomer, acrylic polyethylene glycol (PEG) functionalized 3-fluoro-4-hydroxybenzoate is synthesized. This activated ester monomer and 2-methoxyethoxyethyl methacrylate (MEEMA) as copolymer monomers are first used to synthesize water-soluble polymers bearing activated ester for post-polymerization modification with amines. Two model enzymes containing amine residues, urease, and papain, are grafted onto the resulting thermal responsive polymers to obtain PMEEMA-co-Enzyme, respectively. The obtained particles of polymer-enzyme conjugates flocculate above the low critical solution temperature (LCST) and redissolve when cooled below that temperature. The activity of the conjugated enzymes has been studied after high temperatures treatment and compared to that of free enzymes. The enzymatic activity assays show that the thermosensitive polymer can act as a stabilizer under high-temperature conditions after multipoint grafting with the enzyme, thus protecting the enzyme from thermal inactivation.

A Comparative Study of "Grafting to" and "Grafting from" Conjugation Methods for the Preparation of Antibody-Temperature-Responsive Polymer Conjugates

Early diagnosis of infectious diseases is still challenging particularly in a nonlaboratory environment or limited resources areas. Thus, sensitive, inexpensive, and easily handled diagnostic approaches are required. The lateral flow immunoassay (LFIA) is commonly used in the screening of infectious diseases despite its poor sensitivity, especially with low pathogenic loads (early stages of infection). This article introduces a novel polymeric material that might help in the enrichment and concentration of pathogens to overcome the LFIA misdiagnosis. To achieve this, we evaluated the efficiency of introducing poly(N-isopropylacrylamide) (PNIPAAm) into immunoglobulin G (IgG) as a model antibody using two different conjugation methods: grafting to (GT) and grafting from (GF). The IgG-PNIPAAm conjugates were characterized using SDS-PAGE, DLS, and temperature-responsive phase transition behavior. SDS-PAGE analysis revealed that the GF method was more efficient in introducing the polymer than the GT method, with calculated polymer introduction ratios of 61% and 34%, respectively. The GF method proved to be less susceptible to steric hindrance and more efficient in introducing high-molecular-weight polymers into proteins. These results are consistent with previous studies comparing the GT and GF methods in similar systems. This study represents an important step toward understanding how the choice of polymer incorporation method affects the properties of IgG-PNIPAAm conjugates. The synthesized polymer allowed binding and enrichment of mouse IgG that was used as a model antigen with a clear LFIA band. On the basis of our findings, this system might help in improving the sensitivity of simple diagnostics.

Polymer Functionalization by RAFT Interchange

Here, we report a simple approach for end group functionalization of linear polymers and graft copolymers via an interchange process of reversible addition-fragmentation chain transfer (RAFT) polymerization chain transfer agents (CTAs). The high functional group tolerance of the RAFT process allows a library of functionalities to be introduced. Moreover, this approach allows multiple functional groups to be installed simultaneously. Furthermore, as an alternative to end group analysis, we report the utility of the supernatant of the reaction mixture to determine the degree of functionalization.

Phase transition and potential biomedical applications of thermoresponsive compositions based on polysaccharides, proteins and DNA: A review

Smart thermoresponsive polymers have long attracted attention as materials of a great potential for biomedical applications, mainly for drug delivery, tissue engineering and wound dressing, with a special interest to injectable hydrogels. Poly-N-isopropylacrylamide (PNIPAM) is the most important synthetic thermoresponsive polymer due to its physiologically relevant transition temperature. However, the use of unmodified PNIPAM encounters such problems as low biodegradability, low drug loading capacity, slow response to thermal stimuli, and insufficient mechanical robustness. The use of natural polysaccharides and proteins in combinations with PNIPAM, in the form of grafted copolymers, IPNs, microgels and physical mixtures, is aimed at overcoming these drawbacks and creating dual-functional materials with both synthetic and natural polymers' properties. When developing such compositions, special attention should be paid to preserving their key property, thermoresponsiveness. Addition of hydrophobic and hydrophilic fragments to PNIPAM is known to affect its transition temperature. This review covers various classes of natural polymers - polysaccharides, fibrous and non-fibrous proteins, DNA - used in combination with PNIPAM for the prospective biomedical purposes, with a focus on their phase transition temperatures and its relation to the natural polymer's structure.

Temperature Responsive Polymer Conjugate Prepared by "Grafting from" Proteins toward the Adsorption and Removal of Uremic Toxin

In this study, temperature-responsive polymer-protein conjugate was synthesized using a “grafting from” concept by introducing a chain transfer agent (CTA) into bovine serum albumin (BSA). The BSA-CTA was used as a starting point for poly(N-isopropylacrylamide) (PNIPAAm) through reversible addition-fragmentation chain transfer polymerization. The research investigations suggest that the thermally responsive behavior of PNIPAAm was controlled by the monomer ratio to CTA, as well as the amount of CTA introduced to BSA. The study further synthesized the human serum albumin (HSA)-PNIPAAm conjugate, taking the advantage that HSA can specifically adsorb indoxyl sulfate (IS) as a uremic toxin. The HSA-PNIPAAm conjugate could capture IS and decreased the concentration by about 40% by thermal precipitation. It was also revealed that the protein activity was not impaired by the conjugation with PNIPAAm. The proposed strategy is promising in not only removal of uremic toxins but also enrichment of biomarkers for early diagnostic applications.

Synergistic Interplay of Covalent and Non-Covalent Interactions in Reactive Polymer Nanoassembly Facilitates Intracellular Delivery of Antibodies

The primary impediments in developing large antibodies as drugs against intracellular targets involve their low transfection efficiency and suitable reversible encapsulation strategies for intracellular delivery with retention of biological activity. To address this, we outline an electrostatics-enhanced covalent self-assembly strategy to generate polymer-protein/antibody nanoassemblies. Through structure-activity studies, we down-select the best performing self-immolative pentafluorophenyl containing activated carbonate polymer for bioconjugation. With the help of an electrostatics-aided covalent self-assembly approach, we demonstrate efficient encapsulation of medium to large proteins (HRP, 44 kDa and β-gal, 465 kDa) and antibodies (ca. 150 kDa). The designed polymeric nanoassemblies are shown to successfully traffic functional antibodies (anti-NPC and anti-pAkt) to cytosol to elicit their bioactivity towards binding intracellular protein epitopes and inducing apoptosis.

Polymer Modification of Lipases, Substrate Interactions, and Potential Inhibition

An industrially important enzyme, Candida antarctica lipase B (CalB), was modified with a range of functional polymers including hydrophilic, hydrophobic, anionic, and cationic character using a "grafting to" approach. We determined the impact of polymer chain length on CalB activity by synthesizing biohybrids of CalB with each polymer at three different chain lengths, using reversible addition-fragmentation chain transfer (RAFT) polymerization. The activity of CalB in both aqueous and aqueous-organic media mixtures was significantly enhanced for acrylamide (Am) and N,N-dimethyl acrylamide (DMAm) conjugates, with activity remaining approximately constant in 25 and 50% ethanol solvent systems. Interestingly, the activity of N,N-dimethylaminopropyl-acrylamide (DMAPA) conjugates increased gradually with increasing organic solvent content in the system. Contrary to other literature reports, our study showed significantly diminished activity for hydrophobic polymer-protein conjugates. Functional thermal stability assays also displayed a considerable enhancement of retained activity of Am, DMAm, and DMAPA conjugates compared to the native CalB enzyme. Thus, this study provides an insight into possible advances in lipase production, which can lead to new improved lipase bioconjugates with increased activity and stability.

Thermoresponsive Poly(ß-hydroxyl amine)s: Synthesis of a New Stimuli Responsive Amphiphilic Homopolymer Family through Amine-Epoxy 'Click' Polymerization

A new synthesis of amphiphilic homopolymers is described. In this synthesis, commercially available and inexpensive primary amines and di-epoxide molecules are utilized as AA- and BB-types of monomers in an amine-epoxy ‘click’ polymerization process. This process can be carried out in water and at room temperature. It does not require a catalyst or inert conditions and forms no byproducts. Therefore, the polymer synthesis can be carried out in open-air and bench-top conditions and a post-synthesis purification step is not required. The modularity of the synthesis, on the other hand, allows for facile structural modulation and tuning of the thermally triggered aggregation process in the temperature range of 7 to 91 °C. Finally, the underlying principles can be translated from linear architectures to polymer networks (hydrogels).

SANS partial structure factor analysis for determining protein-polymer interactions in semidilute solution

The interaction between proteins and polymers in solution contributes to numerous important technological processes, including protein crystallization, biofouling, and the self-assembly of protein-polymer bioconjugates. To quantify these interactions, three different polymers-PNIPAM, POEGA, and PDMAPS-were each blended with a model protein mCherry and studied using contrast variation small angle neutron scattering (SANS). This technique allows for the decomposition of the SANS scattering intensity into partial structure factors corresponding to interactions between two polymer chains, interactions between two proteins, and interactions between a polymer chain and a protein, even for concentrations above the overlap concentration. Examining correlations between each component offers insight into the interactions within the system. In particular, mCherry-PNIPAM interactions are consistent with a depletion interaction, and mCherry-POEGA interactions suggest a considerable region of polymer enrichment close to the protein surface, indicative of attractive forces between the two. Interactions between mCherry and PDMAPS are more complex, with possible contributions from both depletion forces and electrostatic forces.

Combinatorial approaches in post-polymerization modification for rational development of therapeutic delivery systems

The combinatorial polymer library approach has been proven to be effective for the optimization of therapeutic delivery systems. The library of polymers with chemical diversity has been synthesized by (i) polymerization of functionalized monomers or (ii) post-polymerization modification of reactive polymers. Most scientists have followed the first approach so far, and the second method has emerged as a versatile approach for combinatorial biomaterials discovery. This review focuses on the second approach, especially discussing the post-modifications that employ reactive polymers as templates for combinatorial synthesis of a library of functional polymers with distinct structural diversity or a combination of different functionalities. In this way, the functional polymers have a consistent chain length and distribution, which allows for systematic optimization of therapeutic delivery polymers for the efficient delivery of genes, small-molecule drugs, and protein therapeutics. In this review, the modification of representative reactive polymers for the delivery of different therapeutic payloads are summarized. The recent advances in rational design and optimization of therapeutic delivery systems based on reactive polymers are highlighted. This review ends with a summary of the current achievements and the prospect on future directions in applying the approach of post-polymerization modification of polymers to accelerate the development of therapeutic delivery systems.

STATEMENT OF SIGNIFICANCE

A strategy to rationally design and systematically optimize polymers for the efficient delivery of specific therapeutics is highly needed. The combinatorial polymer library approach could be an effective way to this end. The post-polymerization modification of reactive polymer precursors is applicable for the combinatorial synthesis of a library of functional polymers with distinct structural diversity across a consistent degree of polymerization. This allows for parallel comparison and systematic evaluation/optimization of functional polymers for efficient therapeutic delivery. This review summarizes the key elements of this combinatorial polymer synthesis approach realized by post-polymerization modification of reactive polymer precursors towards the development and identification of optimal polymers for the efficient delivery of therapeutic agents.

Fabrication of Polymer-Protein Hybrids

Rapid developments in organic chemistry and polymer chemistry promote the synthesis of polymer-protein hybrids with different structures and biofunctionalities. In this feature article, recent progress achieved in the synthesis of polymer-protein conjugates, protein-nanoparticle core-shell structures, and polymer-protein nanogels/hydrogels is briefly reviewed. The polymer-protein conjugates can be synthesized by the "grafting-to" or the "grafting-from" approach. In this article, different coupling reactions and polymerization methods used in the synthesis of bioconjugates are reviewed. Protein molecules can be immobilized on the surfaces of nanoparticles by covalent or noncovalent linkages. The specific interactions and chemical reactions employed in the synthesis of core-shell structures are discussed. Finally, a general introduction to the synthesis of environmentally responsive polymer-protein nanogels/hydrogels by chemical cross-linking reactions or molecular recognition is provided.

Labeling of native proteins with fluorescent RAFT polymer probes: application to the detection of a cell surface protein using flow cytometry

Fluorescent RAFT polymer probes with an activated ester reactive end-group can be advantageously used to label native proteins.

Grafting-from lipase: utilization of a common amino acid residue as a new grafting site

A previously overlooked amino acid residue was utilized to grow polymers from proteins.

Templated Self-Assembly of a Covalent Polymer Network for Intracellular Protein Delivery and Traceless Release

Trafficking proteins inside cells is an emerging field with potential utility in basic cell biology and biological therapeutics. A robust and sustainable delivery strategy demands not only good protection of the cargo but also reversibility in conjugation and activity. We report a protein-templated polymer self-assembly strategy for forming a sheath around the proteins and then tracelessly releasing them in the cytosol. The versatility of the approach, demonstrated here, suggests that the strategy is compatible with a wide array of biologics.

Grafting-From Proteins Using Metal-Free PET-RAFT Polymerizations under Mild Visible-Light Irradiation

We report a new strategy toward polymer-protein conjugates using a grafting-from method that employs photoinduced electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization. Initial screening of reaction conditions showed rapid polymerization of acrylamides under high dilution in water using eosin Y as a photocatalyst in the presence of a tertiary amine. A lysozyme-modified chain transfer agent allowed the same conditions to be utilized for grafting-from polymerizations, and we further demonstrated the broad scope of this technique by polymerizing acrylic and styrenic monomers. Finally, retention of the RAFT end group was suggested by successful chain extension with N-isopropylacrylamide from the polymer-protein conjugates to form block copolymer-protein conjugates. This strategy should expand the capabilities of grafting-from proteins with RAFT polymerization under mild conditions to afford diverse functional materials.

Grafting challenging monomers from proteins using aqueous ICAR ATRP under bio-relevant conditions

Aqueous ICAR ATRP was applied to graft well defined acrylamide, N,N-dimethylacrylamide and N-vinylimidazole homo and block copolymers from a model protein initiator (bovine serum albumin (BSA)) under bio-relevant conditions.

The synthesis and self-assembly of bioconjugates composed of thermally-responsive polymer chains and pendant lysozyme molecules

Thermal-responsive polymer chains with pendant lysozyme molecules were prepared via a “grafting to” approach. The bioconjugates were able to self-assemble into mesoglobules at a temperature above their cloud point.

Efficient DNA-Polymer Coupling in Organic Solvents: A Survey of Amide Coupling, Thiol-Ene and Tetrazine-Norbornene Chemistries Applied to Conjugation of Poly(N-Isopropylacrylamide)

A range of chemistries were explored for the efficient covalent conjugation of DNA to poly(N-isopropylacrylamide) (poly(NIPAM)) in organic solvents. Amide coupling and thiol–ene Michael addition were found to be ineffective for the synthesis of the desired products. However, the inverse electron-demand Diels–Alder (DA_inv) reaction between tetrazine (Tz) and norbornene (Nb) was found to give DNA–polymer conjugates in good yields (up to 40%) in organic solvents (N,N-dimethylformamide, N,N-dimethylacetamide and N-methyl-2-pyrrolidone), and without the need for a catalyst. Methods for the synthesis of Tz-and Nb- functionalised DNA were developed, along with a post-polymerisation functionalisation strategy for the production of Tz-functionalised polymers.

A Stimuli-Responsive, Binary Reagent System for Rapid Isolation of Protein Biomarkers

Magnetic microbeads exhibit rapid separation characteristics and are widely employed for biomolecule and cell isolations in research laboratories, clinical diagnostics assays, and cell therapy manufacturing. However, micrometer particle diameters compromise biomarker recognition, which leads to long incubation times and significant reagent demands. Here, a stimuli-responsive binary reagent system is presented that combines the nanoscale benefits of efficient biomarker recognition and the microscale benefits of rapid magnetic separation. This system comprises magnetic nanoparticles and polymer-antibody (Ab) conjugates that transition from hydrophilic nanoscale reagents to microscale aggregates in response to temperature stimuli. The binary reagent system was benchmarked against Ab-labeled Dynabeads in terms of biomarker isolation kinetics, assay speed, and reagent needs. Surface plasmon resonance (SPR) measurements showed that polymer conjugation did not significantly alter the Ab's binding affinity or kinetics. ELISA analysis showed that the unconjugated Ab, polymer-Ab conjugates, and Ab-labeled Dynabeads exhibited similar equilibrium dissociation constants (Kd), ∼2 nM. However, the binary reagent system isolated HIV p24 antigen from spiked serum specimens (150 pg/mL) much more quickly than Dynabeads, which resulted in shorter binding times by tens of minutes, or about 30-50% shorter overall assay times. The binary reagent system showed improved performance because the Ab molecules were not conjugated to large, solid microparticle surfaces. This stimuli-responsive binary reagent system illustrates the potential advantages of nanoscale reagents in molecule and cell isolations for both research and clinical applications.

Surfactant Behavior of Amphiphilic Polymer-Tethered Nanoparticles

In recent years, an emerging research area has been the surfactant behavior of polymer-tethered nanoparticles. In this feature article, we have provided a general introduction to the synthesis, self-assembly, and interfacial activity of polymer-tethered inorganic nanoparticles, polymer-tethered organic nanoparticles, and polymer-tethered natural nanoparticles. In addition, applications of the polymer-tethered nanoparticles in colloidal and materials science are briefly reviewed. All research demonstrates that amphiphilic polymer-tethered nanoparticles exhibit surfactant behavior and can be used as elemental building blocks for the fabrication of advanced structures by the self-assembly approach. The polymer-tethered nanoparticles provide new opportunities to engineer materials and biomaterials possessing specific functionality and physical properties.

The microbial transglutaminase immobilization on carboxylated poly(N-isopropylacrylamide) for thermo-responsivity

Microbial transglutaminase (mTG) is widely utilized in the PEGylation of pharmaceutical proteins. mTG immobilization can be achieved via covalent bonding on solid supports. However, the catalytic efficiency of mTG immobilized on solid supports was significantly reduced by mass transfer limitation. To overcome this limitation, mTG was covalently immobilized on the thermo-responsive carboxylated poly(N-isopropylacrylamide) (pNIPAM). The pNIPAM-mTG conjugate exhibited reversibly solubility in aqueous solution with a low critical solution temperature (LCST) at 39°C, i.e., it was insoluble above 39°C and soluble below 39°C. The pH dependence of the pNIPAM-mTG conjugate was similar with that of the native mTG. Upon conjugation to pNIPAM, the optimal temperature of mTG shifted down from 50-55°C to 40-45°C, and the thermal stability of the conjugate was elevated. The easy separation of the pNIPAM-mTG conjugate with its substrate and the catalytic efficiency of the pNIPAM-mTG conjugate were demonstrated by employing the pNIPAM-mTG conjugate to cross-link bovine serum albumin (BSA) and catalyze PEGylation of therapeutic protein, cytochrome c (Cyt C), respectively. The thermo-responsive mTG is suitable to modify proteins in food processing and biomedical engineering.

In situ fabrication of PHEMA–BSA core–corona biohybrid particles

Poly(2-hydroxyethyl methacrylate)–bovine serum albumin core–corona particles were prepared using in situ activators generated by electron transfer for atom transfer radical polymerizations of HEMA initiated by a BSA macroinitiator.

Design of Self-Assembling Protein-Polymer Conjugates

Protein-polymer conjugates are of particular interest for nanobiotechnology applications because of the various and complementary roles that each component may play in composite hybrid-materials. This chapter focuses on the design principles and applications of self-assembling protein-polymer conjugate materials. We address the general design methodology, from both synthetic and genetic perspective, conjugation strategies, protein vs. polymer driven self-assembly and finally, emerging applications for conjugate materials. By marrying proteins and polymers into conjugated bio-hybrid materials, materials scientists, chemists, and biologists alike, have at their fingertips a vast toolkit for material design. These inherently hierarchical structures give rise to useful patterning, mechanical and transport properties that may help realize new, more efficient materials for energy generation, catalysis, nanorobots, etc.

Biomedical applications of polymers derived by reversible addition - fragmentation chain-transfer (RAFT)

RAFT- mediated polymerization, providing control over polymer length and architecture as well as facilitating post polymerization modification of end groups, has been applied to virtually every facet of biomedical materials research. RAFT polymers have seen particularly extensive use in drug delivery research. Facile generation of functional and telechelic polymers permits straightforward conjugation to many therapeutic compounds while synthesis of amphiphilic block copolymers via RAFT allows for the generation of self-assembled structures capable of carrying therapeutic payloads. With the large and growing body of literature employing RAFT polymers as drug delivery aids and vehicles, concern over the potential toxicity of RAFT derived polymers has been raised. While literature exploring this complication is relatively limited, the emerging consensus may be summed up in three parts: toxicity of polymers generated with dithiobenzoate RAFT agents is observed at high concentrations but not with polymers generated with trithiocarbonate RAFT agents; even for polymers generated with dithiobenzoate RAFT agents, most reported applications call for concentrations well below the toxicity threshold; and RAFT end-groups may be easily removed via any of a variety of techniques that leave the polymer with no intrinsic toxicity attributable to the mechanism of polymerization. The low toxicity of RAFT-derived polymers and the ability to remove end groups via straightforward and scalable processes make RAFT technology a valuable tool for practically any application in which a polymer of defined molecular weight and architecture is desired.

Smart hybrid materials by conjugation of responsive polymers to biomacromolecules

The chemical structure and function of biomacromolecules has evolved to fill many essential roles in biological systems. More specifically, proteins, peptides, nucleic acids and polysaccharides serve as vital structural components, and mediate chemical transformations and energy/information storage processes required to sustain life. In many cases, the properties and applications of biological macromolecules can be further expanded by attaching synthetic macromolecules. The modification of biomacromolecules by attaching a polymer that changes its properties in response to environmental variations, thus affecting the properties of the biomacromolecule, has led to the emergence of a new family of polymeric biomaterials. Here, we summarize techniques for conjugating responsive polymers to biomacromolecules and highlight applications of these bioconjugates reported so far. In doing so, we aim to show how advances in synthetic tools could lead to rapid expansion in the variety and uses of responsive bioconjugates.

Polymerization-induced thermal self-assembly (PITSA)

Polymerization-induced self-assembly (PISA) is a versatile technique to achieve a wide range of polymeric nanoparticle morphologies. Most previous examples of self-assembled soft nanoparticle synthesis by PISA rely on a growing solvophobic polymer block that leads to changes in nanoparticle architecture during polymerization in a selective solvent. However, synthesis of block copolymers with a growing stimuli-responsive block to form various nanoparticle shapes has yet to be reported. This new concept using thermoresponsive polymers is termed polymerization-induced thermal self-assembly (PITSA). A reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide from a hydrophilic chain transfer agent composed of N,N-dimethylacrylamide and acrylic acid was carried out in water above the known lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAm). After reaching a certain chain length, the growing PNIPAm self-assembled, as induced by the LCST, into block copolymer aggregates within which dispersion polymerization continued. To characterize the nanoparticles at ambient temperatures without their dissolution, the particles were crosslinked immediately following polymerization at elevated temperatures via the reaction of the acid groups with a diamine in the presence of a carbodiimide. Size exclusion chromatography was used to evaluate the unimer molecular weight distributions and reaction kinetics. Dynamic light scattering and transmission electron microscopy provided insight into the size and morphologies of the nanoparticles. The resulting block copolymers formed polymeric nanoparticles with a range of morphologies (e.g., micelles, worms, and vesicles), which were a function of the PNIPAm block length.

Polymer-protein conjugation via a ‘grafting to’ approach – a comparative study of the performance of protein-reactive RAFT chain transfer agents

The performances of various protein-reactive RAFT CTAs to afford polymer-protein conjugation via a grafting-to approach were compared.

Transiently responsive protein–polymer conjugates via a ‘grafting-from’ RAFT approach for intracellular co-delivery of proteins and immune-modulators

‘Grafting-from’ RAFT polymerization is used to synthesize protein–polymer conjugates that change from the soluble to the aggregated state in response to temperature, but become fully soluble by acid triggered hydrolysis.