Chemical Strategies for the Synthesis of Protein–Polymer Conjugates

Amphiphilic Molecular Brushes with Regular Polydimethylsiloxane Backbone and Poly-2-isopropyl-2-oxazoline Side Chains. 2. Self-Organization in Aqueous Solutions on Heating

The behavior of amphiphilic molecular brushes in aqueous solutions on heating was studied by light scattering and turbidimetry. The main chain of the graft copolymers was polydimethylsiloxane, and the side chains were thermosensitive poly-2-isopropyl-2-oxazoline. The studied samples differed in the length of the grafted chains (polymerization degrees were 14 and 30) and, accordingly, in the molar fraction of the hydrophobic backbone. The grafting density of both samples was 0.6. At low temperatures, macromolecules and aggregates, which formed due to the interaction of main chains, were observed in solutions. At moderate temperatures, heating solutions of the sample with short side chains led to aggregation due to dehydration of poly-2-isopropyl-2-oxazoline and the formation of intermolecular hydrogen bonds. In the case of the brush with long grafted chains, dehydration caused the formation of intramolecular hydrogen bonds and the compaction of molecules and aggregates. The lower critical solution temperature for solutions of the sample with long side chains was higher than LCST for the sample with short side chains. It was shown that the molar fraction of the hydrophobic component and the intramolecular density are the important factors determining the LCST behavior of amphiphilic molecular brushes in aqueous solutions.

Acoustic Patterning of Growth Factor for Three-Dimensional Tissue Engineering

Temporal and spatial presentations of biological cues are critical for tissue engineering. There is a great need in improving the incorporation of bioagent(s) (specifically growth factor(s) [GF(s)]) onto three-dimensional scaffolds. In this study, we developed a process to combine additive manufacturing (AM) technology with acoustic droplet ejection (ADE) technology to control GF distribution. More specifically, we implemented ADE to control the distribution of recombinant human bone morphogenetic protein-2 (rhBMP-2) onto polycaprolactone (PCL)-based tissue engineering constructs (TECs). Three substrates were used in this study: (1) succinimide-terminated PCL (PCL-N-hydroxysuccinimide [NHS]) as model material, (2) alkali-treated PCL (PCL-NaOH) as first control material, and (3) fibrin-coated PCL (PCL-Fibrin) as second control material. It was shown that our process enables a pattern of BMP-2 spots of ∼250 μm in diameter with ∼700 μm center-to-center spacing. An initial concentration of BMP-2 higher than 300 μg/L was required to retain a detectable amount of GF on the substrate after a wash with phosphate-buffered solution. However, to obtain detectable osteogenic differentiation of C2C12 cells, the initial concentration of BMP-2 higher than 750 μg/L was needed. The cells on PCL-NHS samples showed spatial alkaline phosphatase staining correlating with local patterns of BMP-2, although the intensity was lower than the controls (PCL-NaOH and PCL-Fibrin). Our results have demonstrated that the developed AM-ADE process holds great promise in creating TECs with highly controlled GF patterning. Impact statement The combined process of additive manufacturing with acoustic droplet ejection to control growth factor (GF) distribution across three-dimensional (3D) porous scaffolds that is presented in this study enables creating 3D tissue engineering constructs with highly controlled GF patterning. Such constructs enable temporal and spatial presentations of biological cues for enhancing cell migration and differentiation and eventually the formation of targeted tissues in vitro and in vivo.

Multifunctional monomer acts as co-initiator and crosslinker to provide autonomous strengthening with enhanced hydrolytic stability in dental adhesives

OBJECTIVE

The purpose of this study was to evaluate a new synthesized multifunctional monomer, aminosilane functionalized methacrylate (ASMA), containing polymerizable methacrylate, tertiary amine, and methoxysilane functionalities in dental adhesive formulations, and to investigate the polymerization kinetics, leachates, thermal and mechanical properties of copolymers.

METHODS

Adhesive contained HEMA/BisGMA (45/55, w/w) was used as a control, and mixtures based on HEMA/BisGMA/ASMA at the mass ratio of 45/(55-x)/x were used as experimental adhesive. Adhesives were characterized with regard to water miscibility, photo-polymerization behavior (Fourier transform infrared spectroscopy, FTIR), leached co-monomers (high performance liquid chromatography, HPLC), thermal properties (modulated differential scanning calorimeter, MDSC), and mechanical properties (dynamic mechanical analyzer, DMA). Stress relaxation times and the corresponding moduli, obtained from stress relaxation tests, are used in a simulated linear loading case.

RESULTS

As compared to the control, ASMA-containing adhesives showed higher water miscibility, lower viscosity, improved monomer-to-polymer conversion, significantly greater T_g and rubbery modulus. HPLC results indicated a substantial reduction of leached HEMA (up to 85wt%) and BisGMA (up to 55wt%) in ethanol. The simulation reveals that the ASMA-containing adhesive becomes substantially stiffer than the control.

SIGNIFICANCE

ASMA monomer plays multiple roles, i.e. it serves as both a co-initiator and crosslinker while also providing autonomous strengthening and enhanced hydrolytic stability in the adhesive formulations. This multifunctional monomer offers significant promise for improving the durability of the adhesive at the composite/tooth interface.

A Biocatalytically Active Membrane Obtained from Immobilization of 2-Deoxy-d-ribose-5-phosphate Aldolase on a Porous Support

Aldol reactions play an important role in organic synthesis, as they belong to the class of highly beneficial C-C-linking reactions. Aldol-type reactions can be efficiently and stereoselectively catalyzed by the enzyme 2-deoxy-d-ribose-5-phosphate aldolase (DERA) to gain key intermediates for pharmaceuticals such as atorvastatin. The immobilization of DERA would open the opportunity for a continuous operation mode which gives access to an efficient, large-scale production of respective organic intermediates. In this contribution, we synthesize and utilize DERA/polymer conjugates for the generation and fixation of a DERA bearing thin film on a polymeric membrane support. The conjugation strongly increases the tolerance of the enzyme toward the industrial relevant substrate acetaldehyde while UV-cross-linkable groups along the conjugated polymer chains provide the opportunity for covalent binding to the support. First, we provide a thorough characterization of the conjugates followed by immobilization tests on representative, nonporous cycloolefinic copolymer supports. Finally, immobilization on the target supports constituted of polyacrylonitrile (PAN) membranes is performed, and the resulting enzymatically active membranes are implemented in a simple membrane module setup for the first assessment of biocatalytic performance in the continuous operation mode using the combination hexanal/acetaldehyde as the substrate.

Peptide-Dendron Hybrids that Adopt Sequence-Encoded β-Sheet Conformations

Rational design rules for programming hierarchical organization and function through mutations of monomers in sequence-defined polymers can accelerate the development of novel polymeric and supramolecular materials. Our strategy for designing peptide-dendron hybrids that adopt predictable secondary and quaternary structures in bulk is based on patterning the sites at which dendrons are conjugated to short peptides. To validate this approach, we have designed and characterized a series of β-sheet-forming peptide-dendron hybrids. Spectroscopic studies of the hybrids in films reveal that the peptide portion of the hybrids adopts the intended secondary structure.

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.

Biocatalytically Active Thin Films via Self-Assembly of 2-Deoxy-d-ribose-5-phosphate Aldolase-Poly(N-isopropylacrylamide) Conjugates

2-Deoxy-d-ribose-5-phosphate aldolase (DERA) is a biocatalyst that is capable of converting acetaldehyde and a second aldehyde as acceptor into enantiomerically pure mono- and diyhydroxyaldehydes, which are important structural motifs in a number of pharmaceutically active compounds. However, substrate as well as product inhibition requires a more-sophisticated process design for the synthesis of these motifs. One way to do so is to the couple aldehyde conversion with transport processes, which, in turn, would require an immobilization of the enzyme within a thin film that can be deposited on a membrane support. Consequently, we developed a fabrication process for such films that is based on the formation of DERA-poly(N-isopropylacrylamide) conjugates that are subsequently allowed to self-assemble at an air-water interface to yield the respective film. In this contribution, we discuss the conjugation conditions, investigate the interfacial properties of the conjugates, and, finally, demonstrate a successful film formation under the preservation of enzymatic activity.

Nanoarmoring of Proteins by Conjugation to Block Copolymer Micelles

The creation of polymer nanoparticles with protein functionality is of great interest to many applications such as targeted drug or gene delivery, diagnostic imaging, cancer theranostics, delivery of protein therapeutics, sensing chemical and biomolecular analytes in complex environments, and design of protective clothing resembling a second skin. Many approaches to achieving this goal are being explored in the current literature. In this chapter, we describe a relatively simple and flexible approach of conjugating the protein to an amphiphilic block copolymer and creating polymer nanoparticles with protein functionality by taking advantage of the intrinsic self-assembly behavior of the amphiphilic block copolymer. The commercially available and biocompatible polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer is used as the polymer building block. For demonstrative purposes, bovine serum albumin was chosen as the protein. We determine the molecular weight of the protein-polymer conjugate and thereby the degree of conjugation using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry measurements. Retention of protein secondary structure in the conjugate was determined by circular dichroism spectroscopy, and the biological activity of the protein in the conjugated state has been evaluated by kinetic assay involving hydrolysis of an organophosphate compound. Dynamic light scattering and zeta potential measurements were used to characterize the size and charge of the protein-polymer conjugate micelle. Precise control of the size of the micelle and surface number density of the proteins on the micelle surface by coassembling with a second block copolymer have been demonstrated. These studies document a rational approach to armor the protein by conjugation with a block copolymer micelle, as a general approach.

Controlled Radical Polymerization as an Enabling Approach for the Next Generation of Protein-Polymer Conjugates

Protein-polymer conjugates are unique constructs that combine the chemical properties of a synthetic polymer chain with the biological properties of a biomacromolecule. This often leads to improved stabilities, solubilities, and in vivo half-lives of the resulting conjugates, and expands the range of applications for the proteins. However, early chemical methods for protein-polymer conjugation often required multiple polymer modifications, which were tedious and low yielding. To solve these issues, work in our laboratory has focused on the development of controlled radical polymerization (CRP) techniques to improve synthesis of protein-polymer conjugates. Initial efforts focused on the one-step syntheses of protein-reactive polymers through the use of functionalized initiators and chain transfer agents. A variety of functional groups such as maleimide and pyridyl disulfide could be installed with high end-group retention, which could then react with protein functional groups through mild and biocompatible chemistries. While this grafting to method represented a significant advance in conjugation technique, purification and steric hindrance between large biomacromolecules and polymer chains often led to low conjugation yields. Therefore, a grafting from approach was developed, wherein a polymer chain is grown from an initiating site on a functionalized protein. These conjugates have demonstrated improved homogeneity, characterization, and easier purification, while maintaining protein activity. Much of this early work utilizing CRP techniques focused on polymers made up of biocompatible but nonfunctional monomer units, often containing oligoethylene glycol meth(acrylate) or N-isopropylacrylamide. These branched polymers have significant advantages compared to the historically used linear poly(ethylene glycols) including decreased viscosities and thermally responsive behavior, respectively. Recently, we were motivated to use CRP techniques to develop polymers with rationally designed and functional biological properties for conjugate preparation. Specifically, two families of saccharide-inspired polymers were developed for stabilization and activation of therapeutic biomolecules. A series of polymers with trehalose side-chains and vinyl backbones were prepared and used to stabilize proteins against heat and lyophilization stress as both conjugates and additives. These materials, which combine properties of osmolytes with nonionic surfactants, have significant potential for in vivo therapeutic use. Additionally, polymers that mimic the structure of the naturally occurring polysaccharide heparin were prepared. These polymers contained negatively charged sulfonate groups and imparted stabilization to a heparin-binding growth factor after conjugation. A screen of other sulfonated polymers led to the development of a polymer with improved heparin mimesis, enhancing both stability and activity of the protein to which it was attached. Chemical improvements over the past decade have enabled the preparation of a diverse set of protein-polymer conjugates by controlled polymerization techniques. Now, the field should thoroughly explore and expand both the range of polymer structures and also the applications available to protein-polymer conjugates. As we move beyond medicine toward broader applications, increased collaboration and interdisciplinary work will result in the further development of this exciting field.

Synthesis, characterization, and electrospinning of novel polyaniline-peptide polymers

Aniline-peptide (FLDQV, FLDQVC, Dansyl-FLDQV, Dansyl-FLDQVC, and FLDQV-AMC) mixtures underwent oxidative chemical and electrochemical polymerization in excess of aniline. The products of the chemical polymerization were low molecular weight polymers containing more than 70% peptide. Electrochemically polymerized species polyaniline-FLDQV (PANI-FLDQV) consisted mainly of polyaniline units containing about 10% peptide. The solubility of the latter in 1,1,1,3,3,3-hexafluoro-2-propanol (HFP) was similar to the camphorsulfonic acid (CSA) doped emeraldine base (PANI-CSA) solubility, however the weight composition of the electrospun fibers produced from the two polymers was significantly different. 2D 1H-13C HSQC analyses were employed to analyze the binding between the aniline and peptide moieties. Binding of peptide to polyaniline is reflected by the appearance of extra cross-peaks which display line broadening between the free polyaniline and the free pentapeptide. Peptides may be chemically bonded to the polymer molecules, but they may also act as doping agents to the nitrogen atoms via hydrogen bonding.

Bioconjugated gold nanoparticles enhance cellular uptake: A proof of concept study for siRNA delivery in prostate cancer cells

The chemistry of gold nanoparticles (AuNPs) facilitates surface modifications and thus these bioengineered NPs have been investigated as a means of delivering a variety of therapeutic cargos to treat cancer. In this study we have developed AuNPs conjugated with targeting ligands to enhance cell-specific uptake in prostate cancer cells, with a purpose of providing efficient non-viral gene delivery systems in the treatment of prostate cancer. As a consequence, two novel AuNPs were synthesised namely AuNPs-PEG-Tf (negatively charged AuNPs with the transferrin targeting ligands) and AuNPs-PEI-FA (positively charged AuNPs with the folate-receptor targeting ligands). Both bioconjugated AuNPs demonstrated low cytotoxicity in prostate cancer cells. The attachment of the targeting ligand Tf to AuNPs successfully achieved receptor-mediated cellular uptake in PC-3 cells, a prostate cancer cell line highly expressing Tf receptors. The AuNPs-PEI-FA effectively complexed small interfering RNA (siRNA) through electrostatic interaction. At the cellular level the AuNPs-PEI-FA specifically delivered siRNA into LNCaP cells, a prostate cancer cell line overexpressing prostate specific membrane antigen (PSMA, exhibits a hydrolase enzymic activity with a folate substrate). Following endolysosomal escape the AuNPs-PEI-FA.siRNA formulation produced enhanced endogenous gene silencing compared to the non-targeted formulation. Our results suggest both formulations have potential as non-viral gene delivery vectors in the treatment of prostate cancer.

Silk protein-based hydrogels: Promising advanced materials for biomedical applications

Hydrogels are a class of advanced material forms that closely mimic properties of the soft biological tissues. Several polymers have been explored for preparing hydrogels with structural and functional features resembling that of the extracellular matrix. Favourable material properties, biocompatibility and easy processing of silk protein fibers into several forms make it a suitable material for biomedical applications. Hydrogels made from silk proteins have shown a potential in overcoming limitations of hydrogels prepared from conventional polymers. A great deal of effort has been made to control the properties and to integrate novel topographical and functional characteristics in the hydrogel composed from silk proteins. This review provides overview of the advances in silk protein-based hydrogels with a primary emphasis on hydrogels of fibroin. It describes the approaches used to fabricate fibroin hydrogels. Attempts to improve the existing properties or to incorporate new features in the hydrogels by making composites and by improving fibroin properties by genetic engineering approaches are also described. Applications of the fibroin hydrogels in the realms of tissue engineering and controlled release are reviewed and their future potentials are discussed.

STATEMENT OF SIGNIFICANCE

This review describes the potentiality of silk fibroin hydrogel. Silk Fibroin has been widely recognized as an interesting biomaterial. Due to its properties including high mechanical strength and excellent biocompatibility, it has gained wide attention. Several groups are exploring silk-based materials including films, hydrogels, nanofibers and nanoparticles for different biomedical applications. Although there is a good amount of literature available on general properties and applications of silk based biomaterials, there is an inadequacy of extensive review articles that specifically focus on silk based hydrogels. Silk-based hydrogels have a strong potential to be utilized in biomedical applications. Our work is an effort to highlight the research that has been done in the area of silk-based hydrogels. It aims to provide an overview of the advances that have been made and the future course available. It will provide an overview of the silk-based hydrogels as well as may direct the readers to the specific areas of application.

Synthesis of an End-to-End Protein-Glycopolymer Conjugate via Bio-Orthogonal Chemistry

We report the synthesis of an end-to-end protein-glycopolymer conjugate, namely, site-specific modification of recombinant thrombomodulin at the C-terminus with a chain-end-functionalized glycopolymer. Thrombomodulin (TM) is an endothelial membrane glycoprotein that acts as a major cofactor in the protein C anticoagulant pathway. To closely mimic the glycoprotein structural feature of native TM, we proposed a site-specific glyco-engineering of recombinant TM with a glycopolymer. Briefly, recombinant TM containing the epidermal growth factor (EGF)-like domains 4, 5, and 6 (rTM₄₅₆) and a C-terminal azidohomoalanine was modified with a dibenzylcyclooctyne (DBCO) chain-end-functionalized glycopolymer via copper-free click chemistry to afford the end-to-end TM-glycopolymer conjugate. The TM glycoconjugation was confirmed with SDS-PAGE, Western blot, and protein C activation assay, respectively. The reported site-specific end-to-end protein glycopolymer conjugation approach facilitates uniform glycoconjugate formation via biocompatible chemistry and in high efficiency providing a rational strategy for generating an rTM-based anticoagulant agent.

Effect of Ionic Strength and Surface Charge on Convective Deposition

Particle-particle and particle-substrate interactions play a crucial role in capillary driven convective self-assembly for continuous deposition of particles. This systematic study demonstrates the nontrivial effects of varying surface charge and ionic strength of monosized silica microspheres in water on the quality of the deposited monolayer. Increase in particle surface charge results a broader range of parameters that result in monolayer deposition which can be explained considering the particle-substrate electrostatic repulsion in solution. Resulting changes in the coating morphology and microstructure at different solution conditions were observed using confocal microscopy enabling correlation of order to disorder transitions with relative particle stability. These results, in part, may explain similar results seen by Muangnapoh et al., 2013 in vibration-assisted convective deposition.

Purification of anti-bromelain antibodies by affinity precipitation using pNIPAm-linked bromelain

Affinity precipitation has emerged as a very useful technique for the purification of proteins. Here it has been employed for the purification of anti-bromelain antibodies from rabbit serum. A system has been developed for reversibly binding and thermoprecipitating antibodies. Anti-bromelain antibodies were raised in rabbit by immunizing it with bromelain. Poly-N-isopropylacrylamide (pNIPAm)-bromelain conjugate was prepared and incubated with rabbit serum. After that the temperature was raised for thermal precipitation of the polymer. Antibodies were then eluted from the complex by incubating it with a small volume of buffer, pH 3.0. This method is very effective in concentrating the antibodies. Purity and specificity of the antibodies were checked by gel electrophoresis and enzyme-linked immunosorbent assay (ELISA), respectively. The study of the effect of pH and temperature on the binding of the antibodies to the conjugate showed that the optimum binding occurred at pH 8.0 and 25°C.The polymer enzyme conjugate was further used for another cycle.

Encapsidated atom-transfer radical polymerization in Qβ virus-like nanoparticles

Virus-like particles (VLPs) are unique macromolecular structures that hold great promise in biomedical and biomaterial applications. The interior of the 30 nm-diameter Qβ VLP was functionalized by a three-step process: (1) hydrolytic removal of endogenously packaged RNA, (2) covalent attachment of initiator molecules to unnatural amino acid residues located on the interior capsid surface, and (3) atom-transfer radical polymerization of tertiary amine-bearing methacrylate monomers. The resulting polymer-containing particles were moderately expanded in size; however, biotin-derivatized polymer strands were only very weakly accessible to avidin, suggesting that most of the polymer was confined within the protein shell. The polymer-containing particles were also found to exhibit physical and chemical properties characteristic of positively charged nanostructures, including the ability to easily enter mammalian cells and deliver functional small interfering RNA.

Self-assembled biodegradable protein-polymer vesicle as a tumor-targeted nanocarrier

Self-assembled nanostructures based on amphiphilic protein-polymer conjugates have shown great advantages in the field of nanomedicine such as inherent biocompatibility with biosystems because of their excellent performance. Herein, a novel biodegradable protein-polymer conjugate was prepared by covalently linking the tailor-made hydrophobic maleimide-functionalized poly(ε-caprolactone) (PCL) to hydrophilic bovine serum albumin (BSA) via the maleimide-sulfhydryl coupling reaction. This protein-based conjugate with a biodegradable polyester was reported for the first time, and the obtained biohybrid displayed well-defined structure, excellent biocompatibility and low cytotoxicity, and self-assembly behaviors similar to those of the traditional amphiphilic small molecules and block copolymers. The amphiphilic BSA-PCL conjugate can self-assemble into a nanosized vesicle with a negative surface charge. Furthermore, the self-assembled vesicle based on the BSA-PCL conjugate was functionalized via linking targeting ligand cetuximab to its surface to enhance cell uptake, and the doxorubicin (DOX)-encapsulated cetuximab-functionalized vesicle exhibited enhanced antitumor activity compared with that of free DOX in vitro. These results indicate that the biodegradable protein-polymer conjugate based on BSA and PCL had great potential as a drug delivery vehicle for cancer therapy.