Cysteine-Reactive Polymers Synthesized by Atom Transfer Radical Polymerization for Conjugation to Proteins

Oxygen-Driven Atom Transfer Radical Polymerization

In traditional atom transfer radical polymerization (ATRP), oxygen must be meticulously eliminated due to its propensity to quench radical species and halt the polymerization process. Additionally, oxygen oxidizes the lower-valent Cu catalyst, compromising its ability to activate alkyl halides and propagate polymerization. In this study, we present an oxygen-driven ATRP utilizing alkylborane compounds, a method that not only circumvents the need for stringent oxygen removal but also exploits oxygen as an essential cofactor to promote polymerization. This approach exhibits broad compatibility in organic or aqueous media, yielding well-defined polymers with low dispersity (Đ as low as 1.11) and molecular weights closely aligned with theoretical values. Triethylborane (Et3B) and its air-stable triethylborane-amine complex (Et3B-DMAP) facilitate controlled polymerization under open-to-air conditions, demonstrating efficiency across a wide range of monomers. Moreover, the technique enables the successful synthesis of protein-polymer conjugates and supports surface modifications of nanoparticles and silicon wafers under aerobic conditions. This oxygen-driven ATRP represents a robust and versatile platform for precision polymerization with far-reaching implications in materials science, biomedicine, and advanced surface engineering.

Hydrogels from Protein-Polymer Conjugates: A Pathway to Next-Generation Biomaterials

Hybrid hydrogels from protein-polymer conjugates are biomaterials formed via the chemical bonding of a protein molecule with a polymer molecule. Protein-polymer conjugates offer a variety of biological properties by combining the mechanical strength of polymers and the bioactive functionality of proteins. These properties allow these conjugates to be used as biocompatible components in biomedical applications. Protein-polymer conjugation is a vital bioengineering strategy in many fields, such as drug delivery, tissue engineering, and cancer therapy. Protein-polymer conjugations aim to create materials with new and unique properties by combining the properties of different molecular components. There are various ways of creating protein-polymer conjugates. PEGylation is one of the most common conjugation techniques where a protein is conjugated with Polyethylene Glycol. However, some limitations of PEGylation (like polydispersity and low biodegradability) have prompted researchers to devise novel synthesis techniques like PEGylation, where synthetic polypeptides are used as the polymer component. This review will illustrate the properties of protein-polymer conjugates, their synthesis methods, and their various biomedical applications.

Multifunctional Temperature-Sensitive Lipid-Protein-Polymer Conjugates: Tailored Drug Delivery and Bioimaging

In this study, we introduce a protein-polymer bioconjugate comprising bovine serum albumin (BSA) and a lipid-based thermoresponsive block copolymer. These amphiphilic BSA-polymer conjugates can autonomously be organized into vesicular compartments for codelivery of glucose oxidase (GOx) and doxorubicin (DOX), demonstrating high drug loading content and remarkable antitumor activity via synergistic cancer therapy combining chemo-starvation strategies. Through the incorporation of a hydrophilic BSA block, the lower critical solution temperature (LCST) of the bioconjugates is tuned to around 40 °C, facilitating their targeted drug delivery to tumor cells. Consequently, these smart protein-polymer conjugates present greater promise compared to traditional drug delivery vehicles, particularly in the realm of anticancer therapy. Moreover, these bioconjugates displayed enhanced intracellular fluorescence intensity with increasing temperature, attributed to the clustering-triggered emission of the nonconventional chromophore moieties within poly(vinylcaprolactam) (PNVCL). The active aggregation-induced emission (AIE) characteristic and excellent biocompatibility suggest an opportunity to further apply these bioconjugates for biosensing and cellular imaging.

Thiol-selective native grafting from polymerization for the generation of protein-polymer conjugates

Protein-polymer conjugates combine properties of biopolymers and synthetic polymers, such as specific bioactivity and increased stability, with great benefits for various applications from catalysis to biomedicine. Furthermore, polymer conjugation can mimic important posttranslational modifications of proteins such as glycosylation. There are typically two approaches to create protein-polymer conjugates: the protein is functionalized in advance with an initiator for a grafting-from method or a previously produced polymer is conjugated to the protein via a grafting-to method. In this study, we present a new approach that uses native proteins and allows for direct grafting-from using a thiol-induced, light-activated controlled radical polymerization (TIRP) that is initiated at thiols from specific cysteine residues of the protein. This straightforward method is employed to introduce polymers onto proteins and enzymes without any prior protein modifications, it works in aqueous buffer and maintains the protein's native structure and activity. The resulting protein-polymer conjugates exhibit high molar masses and low dispersities. We demonstrate the versatility of this approach by introducing different types of polymers such as hydrophilic poly(2-hydroxyethyl acrylate) (pHEAA), temperature-responsive poly(N-isopropylacrylamide) (pNIPAM) as well as glycopolymers mimicking the natural protein glycosylation and enabling selective interactions. We present successful combinations of the protein and polymer functions e.g., temperature-induced aggregation leading to an increase in enzyme activity and the introduction of artificial glycosylation inducing specific protein-protein cluster formation and giving straightforward access to glycosurfaces. Based on this straightforward, potentially scalable yet highly controlled synthesis of protein-polymer conjugates, various areas of applications are envisioned ranging from biomedicine to material sciences.

Fluorescence Sensing of Eclampsia Biomarkers via the Immunosorbent Atom Transfer Radical Polymerization Assay

Accurate and quantitative detection of pre-eclampsia markers is crucial in reducing pregnancy mortality rates. This study introduces a novel approach utilizing a fluorescent biosensor by the immunosorbent atom transfer radical polymerization (immuno-ATRP) assay to detect the pre-eclampsia protein marker CD81. The critical step used in this sensor is the novel signal amplification strategy of fluorescein polymerization mediated by ferritin-enhanced controlled radical polymerization, which combines with a traditional enzyme-linked immunosorbent assay (ELISA) to further reduce the detection limit of the CD81 protein concentration. The fluorescence intensity was linear versus logarithmic CD81 protein concentration from 0.1 to 10,000 pg mL-1, and the detection limit was 0.067 pg mL-1. Surprisingly, in 30% normal human serum (NHS), the sensor can also detect target protein over 0.1-10,000 pg mL-1, with 0.083 pg mL-1 for the detection limit. Moreover, the proposed biosensor is designed to be cost-effective, making it accessible, particularly in resource-limited settings where expensive detection techniques may not be available. The affordability of this method enables widespread screening and monitoring of preeclampsia, ultimately benefiting many pregnant women by improving their healthcare outcomes. In short, developing of a low-cost and susceptible direct detection method for preeclampsia protein markers, such as CD81, through the use of the immuno-ATRP assay, has significant implications for reducing pregnancy mortality. This method holds promise for early detection, precise treatment, and improved management of preeclampsia, thereby contributing to better maternal and fetal health.

Embedding Thiols into Choline Phosphate Polymer Zwitterions

The compositional scope of polymer zwitterions has grown significantly in recent years and now offers designer synthetic materials that are broadly applicable across numerous areas, including supracolloidal structures, electronic materials interfaces, and macromolecular therapeutics. Among recent developments in polymer zwitterion syntheses are those that allow insertion of reactive functionality directly into the zwitterionic moiety, yielding new monomer and polymer structures that hold potential for maximizing the impact of zwitterions on the macromolecular materials chemistry field. This manuscript describes the preparation of zwitterionic choline phosphate (CP) methacrylates containing either aromatic or aliphatic thiols embedded directly into the zwitterionic moiety. The polymerization of these functional CP methacrylates by reversible addition-fragmentation chain-transfer methodology yields polymeric zwitterionic thiols containing protected thiol functionality in the zwitterionic units. After polymerization, the protected thiols are liberated to yield thiol-rich polymer zwitterions which serve as precursors to subsequent reactions that produce polymer networks as well as polymer-protein bioconjugates.

Enzyme-Polymer Conjugates with Photocleavable Linkers for Control Over Protein Activity

Reversible conjugation of polymers to proteins is important for a variety of applications, for example to control protein activity. Light is often employed as an external trigger to allow for spatio and temporal control over release of a payload. In this report, we demonstrate preparation of photocleavable poly(polyethylene glycol) acrylate)-lysozyme (pPEGA-Lys) conjugates via ortho-nitrobenzyl linkages. The conjugates were made by both grafting-to and grafting-from in order to compare and contrast the two synthetic approaches. First, a lysine-reactive ortho-nitrobenzyl atom transfer radical polymerization (ATRP) initiator was synthesized. For the grafting-to strategy, the initiator was employed in the ATRP of PEGA, and the subsequent polymer was conjugated to the lysine residues of lysozyme. For the grafting-from strategy, lysozyme was modified first with the photocleavable initiator, and the purified macroinitiator was then subjected to polymerization conditions to synthesize the protein-polymer conjugate. The polymer was cleaved from the protein via UV light, and activity before and after polymer removal was evaluated, showing 83% recovery. This work provides evidence that reversing conjugation is successful for activity modulation for ortho-nitrobenzyl linked protein-polymer conjugates.

Synthesis and Self-Assembly of Hyperbranched Multiarm Copolymer Lysozyme Conjugates Based on Light-Induced Metal-Free Atrp

In recent years, the coupling of structurally and functionally controllable polymers with biologically active protein materials to obtain polymer-protein conjugates with excellent overall properties and good biocompatibility has been important research in the field of polymers. In this study, the hyperbranched polymer hP(DEGMA-co-OEGMA) was first prepared by combining self-condensation vinyl polymerization (SCVP) with photo-induced metal-free atom transfer radical polymerization (ATRP), with 2-(2-bromo-2-methylpropanoyloxy) ethyl methacrylate (BMA) as inimer, and Di (ethylene glycol) methyl ether methacrylate (DEGMA) and (oligoethylene glycol) methacrylate (OEGMA, M_n = 300) as the copolymer monomer. Then, hP(DEGMA-co-OEGMA) was used as a macroinitiator to continue the polymerization of a segment of pyridyl disulfide ethyl methacrylate (DSMA) monomer to obtain the hyperbranched multiarm copolymers hP(DEGMA-co-OEGMA)-star-PDSMA. Finally, the lysozyme with sulfhydryl groups was affixed to the hyperbranched multiarm copolymers by the exchange reaction between sulfhydryl groups and disulfide bonds to obtain the copolymer protein conjugates hP(DEGMA-co-OEGMA)-star-PLZ. Three hyperbranched multiarm copolymers with relatively close molecular weights but different degrees of branching were prepared, and all three conjugates could self-assemble to form nanoscale vesicle assemblies with narrow dispersion. The biological activity and secondary structure of lysozyme on the assemblies remained essentially unchanged.

Recent Advances in the Application of ATRP in the Synthesis of Drug Delivery Systems

Advances in atom transfer radical polymerization (ATRP) have enabled the precise design and preparation of nanostructured polymeric materials for a variety of biomedical applications. This paper briefly summarizes recent developments in the synthesis of bio-therapeutics for drug delivery based on linear and branched block copolymers and bioconjugates using ATRP, which have been tested in drug delivery systems (DDSs) over the past decade. An important trend is the rapid development of a number of smart DDSs that can release bioactive materials in response to certain external stimuli, either physical (e.g., light, ultrasound, or temperature) or chemical factors (e.g., changes in pH values and/or environmental redox potential). The use of ATRPs in the synthesis of polymeric bioconjugates containing drugs, proteins, and nucleic acids, as well as systems applied in combination therapies, has also received considerable attention.

A Review of Protein- and Peptide-Based Chemical Conjugates: Past, Present, and Future

Over the past few decades, the complexity of molecular entities being advanced for therapeutic purposes has continued to evolve. A main propellent fueling innovation is the perpetual mandate within the pharmaceutical industry to meet the needs of novel disease areas and/or delivery challenges. As new mechanisms of action are uncovered, and as our understanding of existing mechanisms grows, the properties that are required and/or leveraged to enable therapeutic development continue to expand. One rapidly evolving area of interest is that of chemically enhanced peptide and protein therapeutics. While a variety of conjugate molecules such as antibody-drug conjugates, peptide/protein-PEG conjugates, and protein conjugate vaccines are already well established, others, such as antibody-oligonucleotide conjugates and peptide/protein conjugates using non-PEG polymers, are newer to clinical development. This review will evaluate the current development landscape of protein-based chemical conjugates with special attention to considerations such as modulation of pharmacokinetics, safety/tolerability, and entry into difficult to access targets, as well as bioavailability. Furthermore, for the purpose of this review, the types of molecules discussed are divided into two categories: (1) therapeutics that are enhanced by protein or peptide bioconjugation, and (2) protein and peptide therapeutics that require chemical modifications. Overall, the breadth of novel peptide- or protein-based therapeutics moving through the pipeline each year supports a path forward for the pursuit of even more complex therapeutic strategies.

Ferritin-Enhanced Direct MicroRNA Detection via Controlled Radical Polymerization

Accurate quantitative detection of tracing nucleic acids remains a great challenge in cancer genetic testing. It is crucial to propose a low-cost and highly sensitive direct gene detection method for cancer prevention and treatment. Herein, this work reports an ultrasensitive biosensor via a ferritin-enhanced atom-transfer radical polymerization (Ft-ATRP) process. Intriguingly, microRNA-21, an early marker of lung cancer, can be detected without being transcribed in advance by an innovative signal amplification strategy using ferritin-mediated aggregation of hydrophilic nitroxide radical monomers as an electrochemical biosensor. The sensor uses peptide nucleic acid probes modified on a gold electrode to accurately bind the target lung cancer marker in the sample, and then ferritin, which is naturally present in human blood, induces Ft-ATRP on the electrode surface under mild conditions. Many of 4-methacryloyloxy-2,2,6,6-tetramethylpiperidine 1-oxyl free radical (MATMP) monomers with electrochemical signals are combined into polymeric chains to be modified on target assays. The limit of detection (LOD) of microRNA-21 is as low as 6.03 fM, and the detection concentration ranges from 0.01 to 100 pM (R² = 0.994). The RNA biosensor can realize great performance analysis of complicated samples in simple operation, in addition, the detection process used by the catalyst, polymers containing electrochemical signals, and the electrolyte solution all have good water solubility. The superior performance of the RNA biosensor demonstrates its potential to screen and identify lung cancer in target patients.

Mucus-Inspired Dynamic Hydrogels: Synthesis and Future Perspectives

Mucus hydrogels at biointerfaces are crucial for protecting against foreign pathogens and for the biological functions of the underlying cells. Since mucus can bind to and host both viruses and bacteria, establishing a synthetic model system that can emulate the properties and functions of native mucus and can be synthesized at large scale would revolutionize the mucus-related research that is essential for understanding the pathways of many infectious diseases. The synthesis of such biofunctional hydrogels in the laboratory is highly challenging, owing to their complex chemical compositions and the specific chemical interactions that occur throughout the gel network. In this perspective, we discuss the basic chemical structures and diverse physicochemical interactions responsible for the unique properties and functions of mucus hydrogels. We scrutinize the different approaches for preparing mucus-inspired hydrogels, with specific examples. We also discuss recent research and what it reveals about the challenges that must be addressed and the opportunities to be considered to achieve desirable de novo synthetic mucus hydrogels.

Rapid Oxygen-Tolerant Synthesis of Protein-Polymer Bioconjugates via Aqueous Copper-Mediated Polymerization

The synthesis of protein-polymer conjugates usually requires extensive and costly deoxygenation procedures, thus limiting their availability and potential applications. In this work, we report the ultrafast synthesis of polymer-protein bioconjugates in the absence of any external deoxygenation via an aqueous copper-mediated methodology. Within 10 min and in the absence of any external stimulus such as light (which may limit the monomer scope and/or disrupt the secondary structure of the protein), a range of hydrophobic and hydrophilic monomers could be successfully grafted from a BSA macroinitiator, yielding well-defined polymer-protein bioconjugates at quantitative yields. Our approach is compatible with a wide range of monomer classes such as (meth) acrylates, styrene, and acrylamides as well as multiple macroinitiators including BSA, BSA nanoparticles, and beta-galactosidase from Aspergillus oryzae. Notably, the synthesis of challenging protein-polymer-polymer triblock copolymers was also demonstrated, thus significantly expanding the scope of our strategy. Importantly, both lower and higher scale polymerizations (from 0.2 to 35 mL) were possible without compromising the overall efficiency and the final yields. This simple methodology paves the way for a plethora of applications in aqueous solutions without the need of external stimuli or tedious deoxygenation.

Rational Control of Protein-Protein Interactions with Protein-ATRP-Generated Protease-Sensitive Polymer Cages

Protease-protease interactions lie at the heart of the biological cascades that provide rapid molecular responses to living systems. Blood clotting cascades, apoptosis signaling networks, bacterial infection, and virus trafficking have all evolved to be activated and sustained by protease-protease interactions. Biomimetic strategies designed to target drugs to specific locations have generated proprotein drugs that can be activated by proteolytic cleavage to release native protein. We have previously demonstrated that the modification of enzymes with a custom-designed comb-shaped polymer nanoarmor can shield the enzyme surface and eliminate almost all protein-protein interactions. We now describe the synthesis and characterization of protease-sensitive comb-shaped nanoarmor cages using poly(ethylene glycol) [Sundy, J. S. Arthritis Rheum. 2008, 58(9), 2882-2891]methacrylate macromonomers where the PEG tines of the comb are connected to the backbone of the growing polymer chain by peptide linkers. Protease-induced cleavage of the tines of the comb releases a polymer-modified protein that can once again participate in protein-protein interactions. Atom transfer radical polymerization (ATRP) was used to copolymerize the macromonomer and carboxybetaine methacrylate from initiator-labeled chymotrypsin and trypsin enzymes, yielding proprotease conjugates that retained activity toward small peptide substrates but prevented activity against proteins. Native proteases triggered the release of the PEG side chains from the polymer backbone within 20 min, thereby increasing the activity of the conjugate toward larger protein substrates by 100%. Biomimetic cascade initiation of nanoarmored protease-sensitive protein-polymer conjugates may open the door to a new class of responsive targeted therapies.

Direct-Ink Write 3D Printing Multistimuli-Responsive Hydrogels and Post-Functionalization Via Disulfide Exchange

Herein, we describe a multi-stimuli-responsive hydrogel that can be 3D printed via a direct-ink write process to afford cross-linked hydrogel networks that can be post-functionalized with thiol-bearing molecules. Poly(alkyl glycidyl ether)s with methacrylate groups at their termini were synthesized and self-assembled into hydrogels with three key stimuli-responsive behaviors necessary for extrusion based 3D printing: a sol-gel temperature response, shear-thinning behavior, and the ability to be photochemically crosslinked. In addition, the chemically crosslinked hydrogels demonstrated a temperature dependent swelling consistent with an LCST behavior. Pyridyl disulfide urethane methacrylate (PDS-UM) monomers were introduced into the network as a thiol-reactive handle for post-functionalization of the hydrogel. The reactivities of these hydrogels were investigated at different temperatures (5, 25, 37 °C) and swelling statuses (as-cured versus preswollen) using glutathione as a reactive probe. To illustrate the versatility of the platform, a number of additional thiol-containing probes such as proteins, polymers, and small molecules were conjugated to the hydrogel network at different temperatures, pH's, and concentrations. In a final demonstration of the multi-stimuli-responsive hydrogel platform, a customized DIW 3D printer was used to fabricate a printed object that was subsequently conjugated with a fluorescent tag and displayed the ability to change in size with environmental temperature.

A Water-Soluble Organic Photocatalyst Discovered for Highly Efficient Additive-Free Visible-Light-Driven Grafting of Polymers from Proteins at Ambient and Aqueous Environments

Since the pioneering discovery of a protein bound to poly(ethylene glycol), the utility of protein-polymer conjugates (PPCs) is rapidly expanding to currently emerging applications. Photoinduced energy/electron-transfer reversible addition-fragmentation chain-transfer (PET-RAFT) polymerization is a very promising method to prepare structurally well-defined PPCs, as it eliminates high-cost and time-consuming deoxygenation processes due to its oxygen tolerance. However, the oxygen-tolerance behavior of PET-RAFT polymerization is not well-investigated in aqueous environments, and thereby the preparation of PPCs using PET-RAFT polymerization needs a substantial amount of sacrificial reducing agents or inert-gas purging processes. Herein a novel water-soluble and biocompatible organic photocatalyst (PC) is reported, which enables visible-light-driven additive-free "grafting-from" polymerizations of a protein in ambient and aqueous environments. Interestingly, the developed PC shows unconventional "oxygen-acceleration" behavior for a variety of acrylic and acrylamide monomers in aqueous conditions without any additives, which are apparently distinct from previously reported systems. With such a PC, "grafting-from" polymerizations are successfully performed from protein in ambient buffer conditions under green light-emitting diode (LED) irradiation, which result in various PPCs that have neutral, anionic, cationic, and zwitterionic polyacrylates, and polyacrylamides. It is believed that this PC will be widely employed for a variety of photocatalysis processes in aqueous environments, including the living cell system.

Double thermoresponsive graft copolymers with different chain ends: feasible precursors for covalently crosslinked hydrogels

The tailored synthesis of graft copolymers from acrylic and methacrylic monomers can be accomplished solely through photoiniferter reversible addition-fragmentation chain transfer (RAFT) polymerization. Samples with poly[oligo(ethylene glycol) methacrylate] (POEGMA) backbones synthesized under green light irradiation and poly(N-isopropylacrylamide) (PNIPAM) side chains growing under blue light irradiation are presented. As monitored by temperature-dependent dynamic light scattering (DLS) measurements and temperature-variable nuclear magnetic resonance (NMR) spectroscopy, the architecture of the graft copolymers allows unique two-step lower critical solution temperature (LCST) transitions in aqueous solutions. Meanwhile, different end-groups introduced by the corresponding RAFT agents affect the detailed thermoresponsive behavior remarkably. This RAFT strategy shows more advantages when the multiple trithiocarbonate groups are converted into thiol reactive pyridyl disulfide (PDS) groups via a facile post-polymerization modification. The PDS-terminated graft copolymer can then be regarded as a usable precursor for various applications, such as thermoresponsive hydrogels.

End-functionalized polymers by controlled/living radical polymerizations: synthesis and applications

This review focuses on end-functionalized polymers synthesized by controlled/living radical polymerizations and the applications in fields including bioconjugate formation, surface modification, topology construction, and self-assembly.

A comparison of RAFT and ATRP methods for controlled radical polymerization

Reversible addition-fragmentation chain-transfer (RAFT) polymerization and atom transfer radical polymerization (ATRP) are the two most common controlled radical polymerization methods. Both methods afford functional polymers with a predefined length, composition, dispersity and end group. Further, RAFT and ATRP tame radicals by reversibly converting active polymeric radicals into dormant chains. However, the mechanisms by which the ATRP and RAFT methods control chain growth are distinct, so each method presents unique opportunities and challenges, depending on the desired application. This Perspective compares RAFT and ATRP by identifying their mechanistic strengths and weaknesses, and their latest synthetic applications.

Bioresponsive nanomedicines based on dynamic covalent bonds

Trends in the development of modern medicine necessitate the efficient delivery of therapeutics to achieve the desired treatment outcomes through precise spatiotemporal accumulation of therapeutics at the disease site. Bioresponsive nanomedicine is a promising platform for this purpose. Dynamic covalent bonds (DCBs) have attracted much attention in studies of the fabrication of bioresponsive nanomedicines with an abundance of combinations of therapeutic modules and carrier function units. DCB-based nanomedicines could be designed to maintain biological friendly synthesis and site-specific release for optimal therapeutic effects, allowing the complex to retain an integrated structure before accumulating at the disease site, but disassembling into individual active components without compromising function in the targeted organs or tissues. In this review, we focus on responsive nanomedicines containing dynamic chemical bonds that can be cleaved by various specific stimuli, enabling achievement of targeted drug release for optimal therapy in various diseases.

Protein-Based Nanohydrogels for Bioactive Delivery

Hydrogels possess a unique three-dimensional, cross-linked network of polymers capable of absorbing large amounts of water and biological fluids without dissolving. Nanohydrogels (NGs) or nanogels are composed of diverse types of polymers of synthetic or natural origin. Their combination is bound by a chemical covalent bond or is physically cross-linked with non-covalent bonds like electrostatic interactions, hydrophobic interactions, and hydrogen bonding. Its remarkable ability to absorb water or other fluids is mainly attributed to hydrophilic groups like hydroxyl, amide, and sulphate, etc. Natural biomolecules such as protein- or peptide-based nanohydrogels are an important category of hydrogels which possess high biocompatibility and metabolic degradability. The preparation of protein nanohydrogels and the subsequent encapsulation process generally involve use of environment friendly solvents and can be fabricated using different proteins, such as fibroins, albumin, collagen, elastin, gelatin, and lipoprotein, etc. involving emulsion, electrospray, and desolvation methods to name a few. Nanohydrogels are excellent biomaterials with broad applications in the areas of regenerative medicine, tissue engineering, and drug delivery due to certain advantages like biodegradability, biocompatibility, tunable mechanical strength, molecular binding abilities, and customizable responses to certain stimuli like ionic concentration, pH, and temperature. The present review aims to provide an insightful analysis of protein/peptide nanohydrogels including their preparation, biophysiochemical aspects, and applications in diverse disciplines like in drug delivery, immunotherapy, intracellular delivery, nutraceutical delivery, cell adhesion, and wound dressing. Naturally occurring structural proteins that are being explored in protein nanohydrogels, along with their unique properties, are also discussed briefly. Further, the review also covers the advantages, limitations, overview of clinical potential, toxicity aspects, stability issues, and future perspectives of protein nanohydrogels.

Thiol-Disulfide Exchange Reaction Promoted Highly Efficient Cellular Uptake of Pyridyl Disulfide Appended Nonionic Polymers

The intracellular transport of molecules, macromolecules or materials is a key step in probing cellular structure and function, as well as regulating a plethora of physical and chemical events for treating disease. This communication reveals direct cellular uptake of pyridyl-disulfide (Py-Ds)-conjugated nonionic and biocompatible macromolecules with the aid of rapid exchange of the highly reactive Py-Ds groups with exofacial cell-surface thiols. Confocal microscopy and flow cytometry analysis confirmed highly efficient cellular uptake of Py-Ds-appended polymers (>50 % in 15 min) by avoiding lysosome as a consequence of thiol-disulfide exchange in the cell surface. In contrast, a control polymer lacking the Py-Ds group followed caveolae-mediated endocytosis. Other control polymers containing either the pyridine group (but not disulfide) or the disulfide group (but not pyridine) revealed significantly low cellular uptake, and thus essential role of the highly reactive Py-Ds group was established beyond doubt.

Pyridyl disulfide-based thiol–disulfide exchange reaction: shaping the design of redox-responsive polymeric materials

This review provides an overview of synthetic approaches utilized to incorporate the thiol-reactive pyridyl-disulfide motif into various polymeric materials, and briefly highlights its utilization to obtain functional materials.

Controlled polymerization for the development of bioconjugate polymers and materials

Conjugates of various biopolymers with synthetic polymers were preparedvialiving radical polymerization. The conjugates have precise structures and potential for novel biofunctional materials.

Thiolated polymers: Bioinspired polymers utilizing one of the most important bridging structures in nature

Thiolated polymers designated "thiomers" are obtained by covalent attachment of thiol functionalities on the polymeric backbone of polymers. In 1998 these polymers were first described as mucoadhesive and in situ gelling compounds forming disulfide bonds with cysteine-rich substructures of mucus glycoproteins and crosslinking through inter- and intrachain disulfide bond formation. In the following, it was shown that thiomers are able to form disulfides with keratins and membrane-associated proteins exhibiting also cysteine-rich substructures. Furthermore, permeation enhancing, enzyme inhibiting and efflux pump inhibiting properties were demonstrated. Because of these capabilities thiomers are promising tools for drug delivery guaranteeing a strongly prolonged residence time as well as sustained release on mucosal membranes. Apart from that, thiomers are used as drugs per se. In particular, for treatment of dry eye syndrome various thiolated polymers are in development and a first product has already reached the market. Within this review an overview about the thiomer-technology and its potential for different applications is provided discussing especially the outcome of studies in non-rodent animal models and that of numerous clinical trials. Moreover, an overview on product developments is given.

Grafting phenolics onto milk protein via conjugated polymerization for delivery of multiple functionalities: Synthesis and characterization

A synthetic scenario for functionalization of β-lactoglobulin (βLg) with polymeric units containing caffeic acid (βLg-polyCA) was developed; and all intermediates and final products were structurally confirmed using nuclear magnetic resonance spectroscopy, matrix assisted laser desorption ionization time-of-flight mass spectrometry, and physico-chemically characterized using differential scanning calorimetry and circular dichroism. The antioxidant properties and emulsion stability of βLg, βLg-CA conjugate and βLg-polyCA based systems containing high percentage of fish oil (50%) were evaluated; and βLg-polyCA presented the highest antioxidant and free radical-scavenging activity based on DPPH, ABTS and HS scavenging assays (92.4, 87.92 and 67.35% respectively). Thiobarbituric acid (TBARS) test demonstrated that compared to native βLg, βLg-polyCA afford up 4-5 fold of inhibition of oxidative rancidity and displayed drastic secondary structure changes. Compared to native βLg based emulsions, βLg-polyCA had larger oil droplet sizes, stronger negative zeta potentials (-69.9 mv), narrower size distributions (PDI: 0.22) and less creaming index.

Site-specific conjugation of antifreeze proteins onto polymer-stabilized nanoparticles

Antifreeze proteins (AFPs) have many potential applications, ranging from cryobiology to aerospace, if they can be incorporated into materials. Here, a range of engineered AFP mutants were prepared and site-specifically conjugated onto RAFT polymer-stabilized gold nanoparticles to generate new hybrid multivalent ice growth inhibitors. Only the SNAP-tagged AFPs lead to potent 'antifreeze' active nanomaterials with His-Tag capture resulting in no activity, showing the mode of conjugation is essential. This versatile strategy will enable the development of multivalent AFPs for translational and fundamental studies.

Physicochemical and functional properties of a protein isolate from maca (Lepidium meyenii) and the secondary structure and immunomodulatory activity of its major protein component

Maca protein isolate (MPI) was extracted from maca root, and its physicochemical and functional properties, and the secondary structure and immunomodulatory activity of its major protein component, MMP, were investigated. The MPI lacked essential amino acids compared with soybean protein isolate (SPI) and casein, but was rich in cysteine and proline. The MPI had rich free sulfhydryl (20.6 μmol g-1), and its surface hydrophobicity (H0, 812.4), oil absorption capacity (7.4 g g-1), foaming capacity (100%) and emulsifying activity (58.2 m2 g-1) were higher than that of SPI. However, the thermal stability (Td, 87.4 °C), foaming stability (75%) and emulsifying stability (26.3 min) of the MPI were weaker than that of the SPI. MMP was a pentamer with a molecular weight of 22 kDa and rich in β-sheets. MMP could significantly enhance the phagocytic capacity and promote the NO, TNF-α and IL-6 secretion of RAW 264.7 cells, involving toll-like receptor 4 and complement receptor 3 mainly.

Transition Metal Mediated Living Radical Polymerisation

Living radical polymerisation has witnessed an unprecedented interest from polymer and materials scientists. Traditionally, polymers tended to replace natural materials such as wood, cotton and glass, and were used primarily for their structural features and performance and cost advantages. New functional polymers are essential for the manufacture of cell phones, lap-top computers, new cosmetics, and many pharmaceuticals. It is important to be able to control how monomers are put together within the macromolecule for the design at the molecular level for specific applications. Living polymerisation allows for end group control, polymer chain length and relatively narrow polydispersity polymers. In nature, the ability to control monomer distribution and chain length is obvious with approximately 20 amino acids being the monomers for polymers as diverse as hair, insulin and haemoglobin. Living radical polymerisation solves many of the problems in the use of monomers that contain heteroatoms and functional groups. These tend to be reactive towards strong nucleophiles and electrophiles which are required in ionic polymerisation. Protecting group chemistry as used in small molecule organic synthesis is not practical in polymer synthesis. Thus radicals that are inert to most functional groups and in particular protic species seem to be the answer. The mechanism of the transition metal mediate systems is extremely complicated with a range of organometallic species present in the reaction mixture. Solvents and coordinating monomers drastically affect the ideal reaction conditions and it is impossible to predict the optimum conditions for each synthesis without certain experiments being carried out. Nevertheless, catalyst systems are available which are acceptable and work well enough to be able to make a plethora of different macromolecules for a diverse range of applications /properties.

Biodegradable Polymeric Architectures via Reversible Deactivation Radical Polymerizations

Reversible deactivation radical polymerizations (RDRPs) have proven to be the convenient tools for the preparation of polymeric architectures and nanostructured materials. When biodegradability is conferred to these materials, many biomedical applications can be envisioned. In this review, we discuss the synthesis and applications of biodegradable polymeric architectures using different RDRPs. These biodegradable polymeric structures can be designed as well-defined star-shaped, cross-linked or hyperbranched via smartly designing the chain transfer agents and/or post-polymerization modifications. These polymers can also be exploited to fabricate micelles, vesicles and capsules via either self-assembly or cross-linking methodologies. Nanogels and hydrogels can also be prepared via RDRPs and their applications in biomedical science are also discussed. In addition to the synthetic polymers, varied natural precursors such as cellulose and biomolecules can also be employed to prepare biodegradable polymeric architectures.

Glycan-Directed Grafting-from Polymerization of Immunoglobulin G: Site-Selectively Modified IgG-Polymer Conjugates with Preserved Biological Activity

Antibody and related antibody drugs for the treatment of malignancies have led to progress in targeted cancer therapy. Preparation of diverse antibody conjugates is critical for preclinical and clinical applications. However, precise control in tagging molecules at specific locations on antibodies is essential to preserve their native function. In this study, a synthetic boronic acid (BA)-tosyl initiator was used to trigger a glycan-directed modification of IgGs, and the obtained IgG macroinitiators allowed a growth of the poly N-isopropylacrylamide (PNIPAAm) chains specifically at Fc-domains. Therefore, the PNIPAAm chains are located away from the critical antigen-binding domains (Fab), which could reasonably prevent the loss of biological activity after the attachment of polymer chains. According to the proposed strategy, a site-selectively modified anticoncanavalin A (Con A) antibody-PNIPAAm conjugate showed 6-times higher efficiency in the binding of targeted Con A antigen to a randomly conjugated anti-Con A antibody-PNIPAAm conjugate. In this study, we developed the first chemical strategy for the site-specific preparation of IgG-polymer conjugates with conserved biological activity as well as intact glycan structures.

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.

Advanced Materials by Atom Transfer Radical Polymerization

Atom transfer radical polymerization (ATRP) has been successfully employed for the preparation of various advanced materials with controlled architecture. New catalysts with strongly enhanced activity permit more environmentally benign ATRP procedures using ppm levels of catalyst. Precise control over polymer composition, topology, and incorporation of site specific functionality enables synthesis of well-defined gradient, block, comb copolymers, polymers with (hyper)branched structures including stars, densely grafted molecular brushes or networks, as well as inorganic-organic hybrid materials and bioconjugates. Examples of specific applications of functional materials include thermoplastic elastomers, nanostructured carbons, surfactants, dispersants, functionalized surfaces, and biorelated materials.

Facile Fabrication of a Modular "Catch and Release" Hydrogel Interface: Harnessing Thiol-Disulfide Exchange for Reversible Protein Capture and Cell Attachment

Surfaces engineered to "specifically capture" and "release on demand" analytes ranging from biomolecules to cells find niche applications in areas such as diagnostics and detection. Utilization of a disulfide-based linker as a building block allows fabrication of a novel hydrogel-based platform that incorporates a "catch and release" attribute. Hydrogels incorporating pyridyl disulfide groups as thiol-reactive handles were prepared by photopolymerization in the presence of a poly(ethylene glycol) (PEG)-based cross-linker. A range of bulk and micropatterned hydrogels with varying amounts of the reactive group were prepared using PEG-based monomers with different chain lengths. Thiol-containing molecules were conjugated to these hydrogels through the thiol-disulfide exchange reaction under ambient conditions with high efficiencies, as determined by UV-vis spectroscopy. Facile conjugation of a thiol-containing fluorescent dye, namely 4,4-difluoro-1,3,5,7-tetramethyl-8-[(10-mercapto)]-4-bora-3 a,4 a-diaza- s-indacene, was demonstrated, followed by its effective cleavage in the presence of dithiothreitol (DTT), a thiol-containing disulfide-reducing agent. Conjugation of a biotin-containing ligand onto the hydrogels allowed specific binding of protein extravidin when exposed to a mixture of extravidin and bovine serum albumin. The bound protein could be released from the hydrogel by simple exposure to a DTT solution. Likewise, hydrogels modified with a cell-adhesive peptide unit containing the RGD sequence acted as favorable substrates for cellular attachment. Incubation of these cell-attached hydrogel surfaces in a DTT-containing solution leads to facile detachment of cells from the surfaces, while retaining a high level of cell viability. It can be envisioned that the benign nature of these hydrogels, their facile fabrication, and modular functionalization will make them attractive platforms for many applications.

Insights on the Structure, Molecular Weight and Activity of an Antibacterial Protein-Polymer Hybrid

Protein-polymer conjugates are attractive biomaterials which combine the functions of both proteins and polymers. The bioactivity of these hybrid materials, however, is often reduced upon conjugation. It is important to determine and monitor the protein structure and active site availability in order to optimize the polymer composition, attachment point, and abundance. The challenges in probing these insights are the large size and high complexity in the conjugates. Herein, we overcome the challenges by combining electron paramagnetic resonance (EPR) spectroscopy and atomic force microscopy (AFM) and characterize the structure of antibacterial hybrids formed by polyethylene glycol (PEG) and an antibacterial protein. We discovered that the primary reasons for activity loss were PEG blocking the substrate access pathway and/or altering protein surface charges. Our data indicated that the polymers tended to stay away from the protein surface and form a coiled conformation. The structural insights are meaningful for and applicable to the rational design of future hybrids.

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.

Maleimide end-functionalized poly(2-oxazoline)s by the functional initiator route: synthesis and (bio)conjugation

The synthesis of poly(2-ethyl-2-oxazoline)s with a maleimide group at the α chain end was carried out from new sulfonate ester initiators bearing a furan-protected maleimide group. The conditions of the polymerization were optimized for 50 °C using conventional heating (in contrast to microwave irradiation) to counteract the thermal lability of the cycloadduct introduced to protect the maleimide double bond. At this temperature, a tosylate variant was found to be unable to initiate the polymerization after several days. The controlled polymerization of 2-ethyl-2-oxazoline with a nosylate derivative was, however, successful as shown by kinetic experiments monitored by gas chromatography (GC) and size-exclusion chromatography (SEC). Poly(2-ethyl-oxazoline)s of various molar masses (4500 < M_n < 12 000 g mol⁻¹) with narrow dispersity (Đ < 1.2) were obtained. The stability of the protected maleimide functionality during the polymerization, its deprotection, and the reactivity of the deprotected end group by coupling with a model thiol molecule were proven by ¹H NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS). Finally, the conjugation of maleimide-functionalized poly(2-oxazoline) to a model protein (bovine serum albumin) was demonstrated by gel electrophoresis and MALDI-ToF mass spectrometry.

A new route for the synthesis of polyoxazolines with a maleimide end group is reported using a functional initiator.

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.

Combining Orthogonal Reactive Groups in Block Copolymers for Functional Nanoparticle Synthesis in a Single Step

We report on the synthesis of polysarcosine-block-poly(S-alkylsulfonyl)-l-cysteine block copolymers, which combine three orthogonal addressable groups enabling site-specific conversion of all reactive entities in a single step. The polymers are readily obtained by ring-opening polymerization (ROP) of corresponding α-amino acid N-carboxyanhydrides (NCAs) combining azide and amine chain ends, with a thiol-reactive S-alkylsulfonyl cysteine. Functional group interconversion of chain ends using strain-promoted azide-alkyne cycloaddition (SPAAC) and activated ester chemistry with NHS- and DBCO-containing fluorescent dyes could be readily performed without affecting the cross-linking reaction between thiols and S-alkylsulfonyl protective groups. Eventually, all three functionalities can be combined in the formation of multifunctional disulfide core cross-linked nanoparticles bearing spatially separated functionalities. The simultaneous attachment of dyes in core and corona during the formation of core-cross-linked nanostructures with controlled morphology is confirmed by fluorescence cross-correlation spectroscopy (FCCS).