Aqueous photo-RAFT polymerization under ambient conditions: synthesis of protein-polymer hybrids in open air

A photoinduced reversible addition-fragmentation chain-transfer (photo-RAFT) polymerization technique in the presence of sodium pyruvate (SP) and pyruvic acid derivatives was developed. Depending on the wavelength of light used, SP acted as a biocompatible photoinitiator or promoter for polymerization, allowing rapid open-to-air polymerization in aqueous media. Under UV irradiation (370 nm), SP decomposes to generate CO2 and radicals, initiating polymerization. Under blue (450 nm) or green (525 nm) irradiation, SP enhances the polymerization rate via interaction with the excited state RAFT agent. This method enabled the polymerization of a range of hydrophilic monomers in reaction volumes up to 250 mL, eliminating the need to remove radical inhibitors from the monomers. In addition, photo-RAFT polymerization using SP allowed for the facile synthesis of protein-polymer hybrids in short reaction times (<1 h), low organic content (≤16%), and without rigorous deoxygenation and the use of transition metal photocatalysts. Enzymatic studies of a model protein (chymotrypsin) showed that despite a significant loss of protein activity after conjugation with RAFT chain transfer agents, the grafting polymers from proteins resulted in a 3-4-fold recovery of protein activity.

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.

Self-assembly of protein-polymer conjugates for drug delivery

Protein-polymer conjugates are a class of molecules that combine the stability of polymers with the diversity, specificity, and functionality of biomolecules. These bioconjugates can result in hybrid materials that display properties not found in their individual components and can be particularly relevant for drug delivery applications. Engineering amphiphilicity into these bioconjugate materials can lead to phase separation and the assembly of high-order structures. The assembly, termed self-assembly, of these hierarchical structures entails multiple levels of organization: at each level, new properties emerge, which are, in turn, influenced by lower levels. Here, we provide a critical review of protein-polymer conjugate self-assembly and how these materials can be used for therapeutic applications and drug delivery. In addition, we discuss central bioconjugate design questions and propose future perspectives for the field of protein-polymer conjugate self-assembly.

Full Thermal Switching of Enzymes by Thermoresponsive Poly(2-oxazoline)-Based Enzyme Inhibitors

Controlling the activity of enzymes is an important feature for many processes in medicine, bioanalytics, and biotechnology. So far, it has not been possible to fully switch biocatalysts on and off by thermoresponsive enzyme inhibitors. Herein, we present poly(2-oxazoline)s with iminodiacetic acid end groups (POx-IDA) that are lower critical solution temperature (LCST) polymers and thus thermosensitive. They are capable of reversibly inhibiting the activity of horse radish peroxidase and laccase by more than 99 %. Increasing the temperature makes the POx-IDA precipitate, which leads to 100 % recovery of the enzyme activity. This switching cycle is fully reversible. The LCST of the POx-IDA can be tuned by varying the polymer composition to generate a wide range of switching windows.

Enlightening advances in polymer bioconjugate chemistry: light-based techniques for grafting to and from biomacromolecules

Photochemistry has revolutionized the field of polymer-biomacromolecule conjugation. Ligation reactions necessitate biologically benign conditions, and photons have a significant energy advantage over what is available thermally at ambient temperature, allowing for rapid and unique reactivity. Photochemical reactions also afford many degrees of control, specifically, spatio-temporal control, light source tunability, and increased oxygen tolerance. Light-initiated polymerizations, in particular photo-atom-transfer radical polymerization (photo-ATRP) and photoinduced electron/energy transfer reversible addition-fragmentation chain transfer polymerization (PET-RAFT), have been used for grafting from proteins, DNA, and cells. Additionally, the spatio-temporal control inherent to light-mediated chemistry has been utilized for grafting biomolecules to hydrogel networks for many applications, such as 3-D cell culture. While photopolymerization has clear advantages, there are factors that require careful consideration in order to obtain optimal control. These factors include the photocatalyst system, light intensity, and wavelength. This Perspective aims to discuss recent advances of photochemistry for polymer biomacromolecule conjugation and potential considerations while tailoring these systems.

Preparation of Biomolecule-Polymer Conjugates by Grafting-From Using ATRP, RAFT, or ROMP

Biomolecule-polymer conjugates are constructs that take advantage of the functional or otherwise beneficial traits inherent to biomolecules and combine them with synthetic polymers possessing specially tailored properties. The rapid development of novel biomolecule-polymer conjugates based on proteins, peptides, or nucleic acids has ushered in a variety of unique materials, which exhibit functional attributes including thermo-responsiveness, exceptional stability, and specialized specificity. Key to the synthesis of new biomolecule-polymer hybrids is the use of controlled polymerization techniques coupled with either grafting-from, grafting-to, or grafting-through methodology, each of which exhibit distinct advantages and/or disadvantages. In this review, we present recent progress in the development of biomolecule-polymer conjugates with a focus on works that have detailed the use of grafting-from methods employing ATRP, RAFT, or ROMP.

Seeing the Light: Advancing Materials Chemistry through Photopolymerization

The application of photochemistry to polymer and material science has led to the development of complex yet efficient systems for polymerization, polymer post-functionalization, and advanced materials production. Using light to activate chemical reaction pathways in these systems not only leads to exquisite control over reaction dynamics, but also allows complex synthetic protocols to be easily achieved. Compared to polymerization systems mediated by thermal, chemical, or electrochemical means, photoinduced polymerization systems can potentially offer more versatile methods for macromolecular synthesis. We highlight the utility of light as an energy source for mediating photopolymerization, and present some promising examples of systems which are advancing materials production through their exploitation of photochemistry.

Thermoresponsive cyclodextrins with benzenesulfonamide showing tunable inhibition for carbonic anhydrase

Monodisperse thermoresponsive cyclodextrins appended with benzenesulfonamides were demonstrated to reversibly regulate the enzymatic activity of carbonic anhydrase, which was found to be dependent on both scaffold effect and thermoresponsiveness.

A red-emissive antibody-AIEgen conjugate for turn-on and wash-free imaging of specific cancer cells

An antibody-AIEgen conjugate is designed and developed as a "turn-on" fluorescent probe for wash-free specific cancer cell imaging. The cetuximab-conjugated AIEgen shows red fluorescence only when it is internalized and accumulated in cancer cells with overexpressed epidermal growth factor receptor through endocytosis. The probe first lights up the lysosomes. After hydrolysis, its residue is accumulated in mitochondria, making them highly emissive with a long cell retention time. Compared with conventional "always-on" fluorescent probes, the antibody-AIEgen conjugate exhibits a very good image contrast during wash-free cancer cell imaging and less interference from normal cells. To the best of our knowledge, this is the first time "turn-on" antibody-AIEgen conjugates have been reported. This new strategy can be further extended to many proteins and water-soluble AIEgens, and many of their potential applications such as real-time tracking of cell dynamics and cancer theranostics will be explored. The present work is expected to inspire more marvellous research in the fields of AIE and cancer imaging.

Photoinitiated Polymerization-Induced Self-Assembly (Photo-PISA): New Insights and Opportunities

The polymerization-induced self-assembly (PISA) process is a useful synthetic tool for the efficient synthesis of polymeric nanoparticles of different morphologies. Recently, studies on visible light initiated PISA processes have offered a number of key research opportunities that are not readily accessible using traditional thermally initiated systems. For example, visible light mediated PISA (Photo-PISA) enables a high degree of control over the dispersion polymerization process by manipulation of the wavelength and intensity of incident light. In some cases, the final nanoparticle morphology of a single formulation can be modulated by simple manipulation of these externally controlled parameters. In addition, temporal (and in principle spatial) control over the Photo-PISA process can be achieved in most cases. Exploitation of the mild room temperature polymerizations conditions can enable the encapsulation of thermally sensitive therapeutics to occur without compromising the polymerization rate and their activities. Finally, the Photo-PISA process can enable further mechanistic insights into the morphological evolution of nanoparticle formation such as the effects of temperature on the self-assembly process. The purpose of this mini-review is therefore to examine some of these recent advances that have been made in Photo-PISA processes, particularly in light of the specific advantages that may exist in comparison with conventional thermally initiated systems.

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.

Nano-thin walled micro-compartments from transmembrane protein-polymer conjugates

The high interfacial activity of protein-polymer conjugates has inspired their use as stabilizers for Pickering emulsions, resulting in many interesting applications such as synthesis of templated micro-compartments and protocells or vehicles for drug and gene delivery. In this study we report, for the first time, the stabilization of Pickering emulsions with conjugates of a genetically modified transmembrane protein, ferric hydroxamate uptake protein component A (FhuA). The lysine residues of FhuA with open pore (FhuA ΔCVF^tev) were modified to attach an initiator and consequently controlled radical polymerization (CRP) carried out via the grafting-from technique. The resulting conjugates of FhuA ΔCVF^tev with poly(N-isopropylacrylamide) (PNIPAAm) and poly((2-dimethylamino)ethyl methacrylate) (PDMAEMA), the so-called building blocks based on transmembrane proteins (BBTP), have been shown to engender larger structures. The properties such as pH-responsivity, temperature-responsivity and interfacial activity of the BBTP were analyzed using UV-Vis spectrophotometry and pendant drop tensiometry. The BBTP were then utilized for the synthesis of highly stable Pickering emulsions, which could remain non-coalesced for well over a month. A new UV-crosslinkable monomer was synthesized and copolymerized with NIPAAm from the protein. The emulsion droplets, upon crosslinking of polymer chains, yielded micro-compartments. Fluorescence microscopy proved that these compartments are of micrometer scale, while cryo-scanning electron microscopy and scanning force microscopy analysis yielded a thickness in the range of 11.1 ± 0.6 to 38.0 ± 18.2 nm for the stabilizing layer of the conjugates. Such micro-compartments would prove to be beneficial in drug delivery applications, owing to the possibility of using the channel of the transmembrane protein as a gate and the smart polymer chains as trigger switches to tune the behavior of the capsules.

Light-induced synthesis of triazine N-oxide-based cross-linked polymers for effective photocatalytic degradation of methyl orange

The polymeric aromatic N-oxides (LCPO-1 and LCPO-2) as organic metal-free photocatalysts have shown great potential in the photodegradation of methyl orange (MO) in solution.

Grafting PNIPAAm from β-barrel shaped transmembrane nanopores

The research on protein-polymer conjugates by grafting from the surface of proteins has gained significant interest in the last decade. While there are many studies with globular proteins, membrane proteins have remained untouched to the best of our knowledge. In this study, we established the conjugate formation with a class of transmembrane proteins and grow polymer chains from the ferric hydroxamate uptake protein component A (FhuA; a β-barrel transmembrane protein of Escherichia coli). As the lysine residues of naturally occurring FhuA are distributed over the whole protein, FhuA was reengineered to have up to 11 lysines, distributed symmetrically in a rim on the membrane exposed side (outside) of the protein channel and exclusively above the hydrophobic region. Reengineering of FhuA ensures a polymer growth only on the outside of the β-barrel and prevents blockage of the channel as a result of the polymerization. A water-soluble initiator for controlled radical polymerization (CRP) was consecutively linked to the lysine residues of FhuA and N-isopropylacrylamide (NIPAAm) polymerized under copper-mediated CRP conditions. The conjugate formation was analyzed by using MALDI-ToF mass spectrometry, SDS-PAGE, circular dichroism spectroscopy, analytical ultracentrifugation, dynamic light scattering, transmission electron microscopy and size exclusion chromatography. Such conjugates combine the specific functions of the transmembrane proteins, like maintaining membrane potential gradients or translocation of substrates with the unique properties of synthetic polymers such as temperature and pH stimuli handles. FhuA-PNIPAAm conjugates will serve as functional nanosized building blocks for applications in targeted drug delivery, self-assembly systems, functional membranes and transmembrane protein gated nanoreactors.

Improvement of Site-Directed Protein-Polymer Conjugates: High Bioactivity and Stability Using a Soft Chain-Transfer Agent

Protein has been widely applied in biotechnology and biomedicine thanks to its unique properties of high catalytic activity, outstanding receptor-ligand specificity, and controllable sequence mutability. Owing to the easily induced structural variation and thus the inactivation of protein, there has been much effort to improve the structural stability and biological activity of proteins by the use of polymers to modify protein to construct protein-polymer conjugates. However, during the conjugation of polymer to protein active center, the great loss in the original biological activity of the protein is still a serious and so far unsolved question. Here, for the purpose of preparing site-directed and highly structurally stable protein-polymer conjugate, which would possess at least a substantially similar level of biological activity as the original unmodified protein, we proposed a new strategy by using a pyridine chain-transfer agent (CTA-Py) with a soft pyridine-terminated chain for visible-light-induced reversible addition-fragmentation chain transfer (RAFT) polymerization specifically on a number of sites close to the protein active center. The results showed that all the intermediate conjugates PPa-CTA-Py at different modification sites could retain full enzymatic activities (about 110-130% of the unmodified PPa). It was demonstrated by dynamic computer simulation that introducing of CTA-Py had little interference to the protein spatial structure as compared to the popular maleimide chain-transfer agent (CTA-Ma) with rigid maleimide-terminated. Moreover, intermediate conjugates PPa-CTA-Py is facile and ready for further light polymerization under mild conditions. Final PPa-PNIPAAm conjugate produced from CTA-Py exhibited excellent temperature responsiveness and maintained its enzymatic activity even at high temperature. These highly stable and responsive protein-polymer conjugates have great potential and could be widely used in various industrial, chemical, biological, and pharmaceutical applications.

One-Pot Synthesis of Block Copolymers by Orthogonal Ring-Opening Polymerization and PET-RAFT Polymerization at Ambient Temperature

Well-defined poly(ε-caprolactone)-b-poly(methyl acrylate) (PCL-b-PMA) block copolymers were synthesized at ambient temperature by one-pot combination of diphenyl phosphate (DPP)-catalyzed ring-opening polymerization (ROP) and photoinduced electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization. Full orthogonality of PET-RAFT polymerization and DPP-catalyzed ROP was confirmed by kinetic studies, which allowed facile synthesis of PCL-b-PMA block copolymers without a specific polymerization sequence. The resulting PCL-b-PMA block copolymers synthesized by either sequential or simultaneous ROP and PET-RAFT polymerization showed remarkably low molecular weight distributions (≤1.15), indicating that both ROP and PET-RAFT polymerizations proceeded in a controlled manner. In contrast to previous synthetic methods to prepare block copolymers, this facile one-pot method allows for rapid synthesis of block copolymers controlled via visible light.

Multifunctional nanoparticle-protein conjugates with controllable bioactivity and pH responsiveness

The modulation of protein activity is of significance for disease therapy, molecular diagnostics, and tissue engineering. Nanoparticles offer a new platform for the preparation of protein conjugates with improved protein properties. In the present work, Escherichia coli (E. coli) inorganic pyrophosphatase (PPase) and poly(methacrylic acid) (PMAA) were attached together to gold nanoparticles (AuNPs), forming AuNP-PPase-PMAA conjugates having controllable multi-biofunctionalities and responsiveness to pH. By treating with poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and regulating the pH, the bioactivity of the conjugate becomes "on/off"-switchable. In addition, by taking advantage of the ability of AuNPs to undergo reversible aggregation/dispersion, the conjugates can be recycled and reused multiple times; and due to the shielding effect of the PMAA, the conjugated enzyme has high resistance to protease digestion. This approach has considerable potential in areas such as controlled delivery and release of drugs, biosensing, and biocatalysis.

A novel fluorescence assay for inorganic pyrophosphatase based on modulated aggregation of graphene quantum dots

A simple and highly sensitive fluorometric method has been developed for inorganic pyrophosphatase (PPase) activity detection based on the disaggregation and aggregation of graphene quantum dots (GQDs).

The use of electrostatic association for rapid RAFT synthesis of histamine polyelectrolyte in aqueous solutions at and below 25 °C

Electrostatic association for rapid and quantitative synthesis of well-defined polyelectrolytes in dilute aqueous solutions at and below 25 °C.

Protein–polymer conjugates prepared via host–guest interactions: effects of the conjugation site, polymer type and molecular weight on protein activity

Protein–polymer conjugates are prepared via host–guest interactions and the effects of various parameters on protein activity are investigated.

One-step synthesis of glycoprotein mimics in vitro: improvement of protein activity, stability and application in CPP hydrolysis

Glycoprotein mimics produced in vitro by one-step conjugation of glycopolymer and pyrophosphatase have improved bioactivity and stability for potential biomedical applications.

Improvement in the Thermal Stability of Pyrophosphatase by Conjugation to Poly(N-isopropylacrylamide): Application to the Polymerase Chain Reaction

Polymerase chain reaction (PCR) is a powerful method for nucleic acid amplification. However, the PCR is inhibited in its yield due to its byproduct, pyrophosphate (PPi), a byproduct of the reaction; the yield is thereby limited. The conventional method for hydrolysis of PPi by pyrophosphatase (PPase) is not well adapted for operation at elevated temperatures over long times as required during the PCR. In this work, we reported a strategy to improve the PCR yield using a conjugate of the enzyme with the thermally responsive polymer poly(N-isopropylacrylamide) (PNIPAM). Pyrophosphatase (PPase) was conjugated to PNIPAM site-specifically near the active center. As compared to the free enzyme, the optimum temperature of the conjugate was shown to increase from 45 to 60 °C. For the conjugate, about 77% enzyme activity was retained after incubation at 60 °C for 3 h, representing a 6.8-fold increase as compared to the unconjugated enzyme. For the PCR using the conjugate, the yield was 1.5-fold greater than using the unconjugated enzyme. As well as improving the yield of the PCR (and possibly other biological reactions) at elevated temperature, polymer conjugation may also provide a strategy to improve the heat resistance of proteins more generally.

Temperature-Responsive Poly(N-isopropylacrylamide) Modified Gold Nanoparticle-Protein Conjugates for Bioactivity Modulation

It is important to effectively maintain and modulate the bioactivity of protein-nanoparticle conjugates for their further and intensive applications. The strategies of controlling protein activity via "tailor-made surfaces" still have some limitations, such as the difficulties in further modulation of the bioactivity and the proteolysis by some proteases. Thus, it is essential to establish a responsive protein-nanoparticle conjugate system to realize not only controllable modulations of protein activity in the conjugates by incorporating sensitivity to environmental cues but also high resistance to proteases. In the work reported here, Escherichia coli (E. coli) inorganic pyrophosphatase (PPase) and poly(N-isopropylacrylamide) (pNIPAM) were both fabricated onto gold nanoparticles (AuNPs), forming AuNP-PPase-pNIPAM conjugates. The bioactivity-modulating capability of the conjugates with changes in temperature was systematically investigated by varying the molecular weight of pNIPAM, the PPase/pNIPAM molar ratio on AuNP, and the orientation of the proteins. Under proper conditions, the activity of the conjugate at 45 °C was approximately 270% of that at 25 °C. In the presence of trypsin digestion, much less conjugate activity than protein activity was lost. These findings indicate that the fabrication of AuNP-protein-pNIPAM conjugates can both modulate protein activity on a large scale and show much higher resistance to protease digestion, exhibiting great potential in targeted delivery, controllable biocatalysis, and molecular/cellular recognition.

Modulating the activity of protein conjugated to gold nanoparticles by site-directed orientation and surface density of bound protein

The key property of protein-nanoparticle conjugates is the bioactivity of the protein. The ability to accurately modulate the activity of protein on the nanoparticles at the interfaces is important in many applications. In the work reported here, modulation of the activity of protein-gold nanoparticle (AuNP) conjugates by specifically orienting the protein and by varying the surface density of the protein was investigated. Different orientations were achieved by introducing cysteine (Cys) residues at specific sites for binding to gold. We chose Escherichia coli inorganic pyrophosphatase (PPase) as a model protein and used site-directed mutagenesis to generate two mutant types (MTs) with a single Cys residue on the surface: MT1 with Cys near the active center and MT2 with Cys far from the active center. The relative activities of AuNP conjugates with wild type (WT), MT1, and MT2 were found to be 44.8%, 68.8%, and 91.2% of native PPase in aqueous solution. Site-directed orientation with the binding site far from the active center thus allowed almost complete preservation of the protein activity. The relative activity of WT and MT2 conjugates did not change with the surface density of the protein, while that of MT1 increased significantly with increasing surface density. These results demonstrate that site-directed orientation and surface density can both modulate the activity of proteins conjugated to AuNP and that orientation has a greater effect than density. Furthermore, increasing the surface density of the specifically oriented protein MT2, while having no significant effect on the specific activity of the protein, still allowed increased protein loading on the AuNP and thus increased the total protein activity. This is of great importance in the study on the interface of protein and nanoparticle and the applications for enzyme immobilization, drug delivery, and biocatalysis.

Improving the protein activity and stability under acidic conditions via site-specific conjugation of a pH-responsive polyelectrolyte

Maintaining the protein activity and stability under acidic conditions is important in bioengineering and biomedical applications. Polyelectrolyte conjugation as a means of stabilizing proteins has received much recent attention. Retention of protein activity, and especially, improvement of protein stability by minimizing the number of polymer chains in the conjugate, as well as by choosing the optimal site for conjugation, is critical in practical applications. In this research, the cationic polyelectrolyte poly(2-(dimethylamino)ethyl methacrylate) (pDMAEMA) was conjugated to the inorganic pyrophosphatase (PPase) site specifically. Conjugation of pDMAEMA to the specific site N124 on the protein surface led to a significant increase in activity at acidic pH. At pH 4.0, the activity of the pDMAEMA-conjugated protein was increased 3-fold relative to the unconjugated one. Dynamic light scattering (DLS) measurements showed that the aggregation state of the protein depended on the polymer charge as the pH was varied. Protein aggregation at low pH was prevented by pDMAEMA conjugation, resulting in an increase in protein stability under acidic conditions. The conjugate retained 60% of its initial activity after 4 h at pH 4.0, whereas the unconjugated protein lost 40% of its initial activity within 15 min at this pH. These results suggest an approach for preserving the protein activity and stability at low pH based on site-specific polyelectrolyte conjugation to the protein surface, thereby providing a new strategy for expanding the use of proteins in an acidic environment.

Dextran-based thermo-responsive hemoglobin–polymer conjugates with oxygen-carrying capacity

Graft copolymer dextran-g-poly(NIPAAm) was synthesized via SET-LRP and covalently attached to bovine hemoglobin to form thermo-responsive protein–polymer conjugates as novel oxygen carriers.