Facile synthesis of multiamino vinyl poly(amino acid)s for promising bioapplications

Cationic amino-acid functionalized polymethacrylamide vectors for siRNA transfection based on modification of poly(2-isopropenyl-2-oxazoline)

Poly(2-isopropenyl-2-oxazoline) (PiPOx) is a functional polymer showing great potential for the development of smart biomaterials. The straightforward synthesis and post-polymerization functionalization of PiPOx offers many opportunities for tailoring the properties of the polymer towards biomaterials. In this study we report for the first time PiPOx-based cationic charged polymethacrylamides with amino acid side chains that can complex siRNA and promote transfection in vitro. Therefore, PiPOx was fully modified via ring opening addition reactions with the carboxylic acid groups of a series of N-Boc-L-amino acids and their reaction kinetics were investigated. Based on the determined kinetic constants, another series of PiPOx-based copolymers with balanced hydrophilic/hydrophobic content of N-Boc-L-amino acids were obtained via one-pot modification reaction with two different N-Boc-L-amino acids. The N-Boc protected homopolymers and related copolymers were deprotected to obtain (co)polymers with the targeted side chain cationic charged units. The (co)polymers' structures were fully investigated via FT-IR and 1H NMR spectroscopy, size exclusion chromatography (SEC), and TGA-DSC-MS analysis. The polarimetry measurements revealed that the homopolymers retain their chiroptical properties after post-modification, and a sign inversion is noticed from (L) N-Boc-protected analogues to (D) for the TFA cationic charged homopolymers. Generally, cationically charged homopolymers with hydrophilic amino acids on the side chain showed efficient complexation of siRNA, but poor transfection while cationic copolymers having both tryptophan and valine or proline side chains revealed moderate siRNA binding, high transfection efficiency (> 90% of the cells) and potent gene silencing with IC50 values down to 5.5 nM. Particularly, these cationic copolymers showed higher gene silencing potency as compared to the commercial JetPRIME® reference, without reducing cell viability in the concentration range used for transfection, making this a very interesting system for in vitro siRNA transfection.

A Guideline for the Synthesis of Amino-Acid-Functionalized Monomers and Their Polymerizations

Amino acids have emerged as a sustainable source for the design of functional polymers. Besides their wide availability, especially their high degree of biocompatibility makes them appealing for a broad range of applications in the biomedical research field. In addition to these favorable characteristics, the versatility of reactive functional groups in amino acids (i.e., carboxylic acids, amines, thiols, and hydroxyl groups) makes them suitable starting materials for various polymerization approaches, which include step- and chain-growth reactions. This review aims to provide an overview of strategies to incorporate amino acids into polymers. To this end, it focuses on the preparation of polymerizable monomers from amino acids, which yield main chain or side chain-functionalized polymers. Furthermore, postpolymerization modification approaches for polymer side chain functionalization are discussed. Amino acids are presented as a versatile platform for the development of polymers with tailored properties.

Phenylalanine-Tethered pH-Responsive Poly(2-Hydroxyethyl Methacrylate)

A series of pH-responsive random copolymers comprised of 2-hydroxyethyl methacrylate (HEMA) and tert-butyl carbamate (Boc)-protected phenylalanine methacryloyloxyethyl ester (Boc-Phe-EMA) were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization in N,N'-dimethylformamide (DMF) at 70 °C. The synthesized copolymers were comprehensively characterized using a combination of techniques, including ¹ H NMR, FT-IR spectroscopy and size exclusion chromatography (SEC). Reactivity of each monomers towards controlled radical polymerization was evaluated by determining the reactivity ratios by virtue of extended Kelen-Tüdös method at high conversions revealed the higher reactivity of non-modified HEMA (r_HEMA =1.03) in contrast to Boc-Phe-EMA (r_Boc-Phe-EMA =0.48). Furthermore, the expulsion of the Boc-groups resulted copolymers with ionizable pendant primary ammonium and hydroxyl groups. To understand the glass transition behaviours of homo- and co-polymers, differential scanning calorimetric (DSC) measurements were carried out. The effect of HEMA content on the pH-sensitivity of the copolymers in aqueous medium was investigated through turbidity measurements. Finally, the counteranion exchange from trifluoroacetate to chloride provided copolymers with enhanced water solubility and unaltered phase transition pH.

Bilayer-Templated Two-Dimensional RAFT Polymerization for Directed Assembly of Polymer Nanostructures

Co-localization of monomers, crosslinkers, and chain-transfer agents (CTA) within self-assembled bilayers in an aqueous suspension enabled the successful directed assembly of nanocapsules using a reversible addition-fragmentation chain transfer (RAFT) process without compromising the polymerization kinetics. This study uncovered substantial influence of the organized medium on the course of the reaction, including differential reactivity based on placement and mobility of monomers, crosslinkers, and CTAs within the bilayer.

Supramolecular Assembly of Amino Acid Based Cationic Polymer for Efficient Gene Transfection Efficiency in Triple Negative Breast Cancer

The success of gene therapy is enormously dependent on an efficient gene carrier, and in this context, cationic polymers still continue to play a major role particularly with respect to the safety issue compared to viral vectors. Developing an efficient gene carrier system having promising gene transfection efficiency with low toxicity is the foremost impediment associated with a nonviral carrier. Here, we explored amino acid based biocompatible polymers synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization where glycine (Gly), leucine (Leu), and phenyl alanine (Phe) amino acids were used as the pendent groups of the polymeric brushes. The presence of both a hydrophobic group (long chain aliphatic group associated with the RAFT agent) and hydrophilic amino groups was associated with the supramolecular assembly of the polymeric chain having hydrodynamic sizes within the range of 150-300 nm with a positive zeta potential of 30 ± 5 mV. All polymers showed very low toxicity and possessed >80% cell viability even at a very high concentration of 1000 μg/mL against both normal and cancerous cells. In addition to this, the polymers also showed excellent blood compatibility, and negligible hemolysis was observed at the concentration of 500 μg/mL. All polymers showed efficient DNA complexation capability as well as excellent protection of DNA against highly negatively charged surfactant and enzymatic digestion, although the efficiency was dependent on the N/P ratio of polymer/DNA complexes. Interestingly, the phenyl alanine moiety containing polymer brush P(HEMA-Phe-NH₂) showed a hexagonal shaped nanoparticle after complexation with pDNA and consequently showed higher cellular uptake, resulting in a higher transfection efficiency in a triple negative breast cancer cell, the MDA-MB-231 cell. Therefore, the synthesized polymer containing an amino acid pendent group, especially the phenyl alanine moiety, may be a promising nonviral gene carrier system in gene therapy application in the future.

Tryptophan-based styryl homopolymer and polyzwitterions with solvent-induced UCST, ion-induced LCST and pH-induced UCST

A multi-stimuli responsive tryptophan-based styryl homopolymer and polyzwitterions with solvent-induced UCST, ion-induced LCST and pH-induced UCST under different conditions are presented.

Facile Synthesis of Chiral Polymers with Defined Architecture via Cooperative Assembly of Confined Templates

Herein is presented the synergistically self-assembled system as biomimetic polymerization media. This approach allows the facile synthesis of chiral amino acid-based polymers with high molecular weight and low dispersity inside of the bilayer of catanionic vesicles by using a conventional radical polymerization under moderate conditions.

Poly(N-isopropylacrylamide-co-2-((diethylamino)methyl)-4-formyl-6-methoxyphenyl acrylate) Environmental Functional Copolymers: Synthesis, Characterizations, and Grafting with Amino Acids

Vanillin was used to synthesize a new derivative with an active aldehyde group and response to pH. It is named 2-((diethylamino) methyl)-4-formyl-6-methoxyphenyl acrylate, abbreviated to DEAMVA. The chemical structures were evaluated by ¹H, 13C nuclear magnetic resonance (NMR), infrared (IR), and UV-Vis-spectroscopy, and all results demonstrated good statement. In order to achieve the dual responsive behavior thermo-pH with functionality, free radical polymerization of N-isopropylacrylamide with DEAMVA in different molar ratios (5, 10, 15 mol%) has been used, with azobisisobutyronitrile (AIBN) as the initiator. The chemical structure of the polymers was investigated by ¹H NMR and IR. The dual responsive functional copolymer was exposed to a grafted process with tryptophan and tyrosine, both of which were also evaluated by ¹HNMR and IR. Copolymers before and after grafting were physically investigated by size exclusion chromatography (SEC) for estimation of the molecular weight, the glass transition temperature by differential scanning calorimeter (DSC) and scanning electron microscope (SEM) for the surface morphology. The phase separation or lower critical solution temperature (LCST) (Tc) of the polymer solution was determined not only by a turbidity method using the change in the transmittance with temperature, but also by micro-DSC. The conversion to an amino acid-grafted polymer was detected through Beer's law for the absorption of the ⁻CH=N- imine group by UV-Vis-Spectroscopy.

Leucine-Based Polymer Architecture-Induced Antimicrobial Properties and Bacterial Cell Morphology Switching

To evaluate the comparative antibacterial activity of leucine-based cationic polymers having linear, hyperbranched, and star architectures containing both hydrophilic and hydrophobic segments against Gram-negative bacterium, Escherichia coli (E. coli), herein we performed zone of inhibition study, minimum inhibitory concentration (MIC) calculation, and bacterial growth experiment. The highest antibacterial activity in terms of the MIC value was found in hyperbranched and star architectures because of the greater extent of cationic and hydrophobic functionality, enhancing cell wall penetration ability compared to that of the linear polymer. The absence of the bacterial regrowth stage in the growth curve exhibited the highest bactericidal capacity of star polymers, when untreated cells (control) already reached to the stationary phase, whereas the bacterial regrowth stage with a delayed lag phase was critically observed for linear and hyperbranched architectures displaying lower bactericidal efficacy. Coagulation of E. coli cells, switching of cell morphology from rod to sphere, and lengthening due to stacking in an antimicrobial polymer-treated environment at the bacterial regrowth stage in liquid media were visualized critically by field emission scanning electron microscopy and confocal fluorescence microscopy instruments in the presence of 4',6-diamidino-2-phenylindole stain.

Side-Chain Amino Acid-Based Cationic Antibacterial Polymers: Investigating the Morphological Switching of a Polymer-Treated Bacterial Cell

Synthetic polymer-based antimicrobial materials destroy conventional antibiotic resistant microorganisms. Although these antibacterial polymers imitate the properties of antimicrobial peptides (AMPs), their effect on bacterial cell morphology has not been studied in detail. To investigate the morphology change of a bacterial cell in the presence of antimicrobial polymer, herein we have designed and synthesized side-chain amino acid-based cationic polymers, which showed efficient antibacterial activity against Gram-negative (Escherichia coli), as well as Gram-positive (Bacillus subtilis) bacteria. Morphological switching from a rod shape to a spherical shape of E. coli cells was observed by field emission-scanning electron microscopy analysis due to cell wall disruption, whereas the B. subtilis cell structure and size remained intact, but stacks of the cells formed after polymer treatment. The zone of inhibition experiment on an agar plate for E. coli cells exhibited drastic morphological changes at the vicinity of the polymer-treated portion and somewhat less of an effect at the periphery of the plate.

Intracellular plasmid DNA delivery by self-assembled nanoparticles of amphiphilic PHML-b-PLLA-b-PHML copolymers and the endocytosis pathway analysis

This work presents a new series of polycationic nanoparticles of (l-)-lysine conjugated amphiphilic triblock copolymer poly(hydroxyletheyl methacrylate-L-lysine)-b-poly(L-lactide)-b-poly(hydroxyletheyl methacrylate-L-lysine)s (PHML-b-PLLA-b-PHML) as potent low cytotoxic vectors for intracellular plasmid DNA delivery. First, the triblock PHML-b-PLLA-b-PHML copolymers were prepared via a combination of metal-free controlled ring opening polymerization and successive atom transfer radical polymerization. Then the cationic PHML-b-PLLA-b-PHML nanoparticles were further prepared by solution self-assembly. The particle size, zeta potential and morphology of as-prepared PHML-b-PLLA-b-PHML nanoparticles were characterized by dynamic light scattering and atomic force microscopy, respectively. The plasmid DNA binding affinities and polyplex stabilities were separately explored by agarose gel retardation and DNase I degradation assays. Then in vitro cytotoxicity and gene transfection efficiency of the PHML-b-PLLA-b-PHML nanoparticles vectors as well as relevant polyplex endocytosis pathway were investigated with H1299 cells. It was revealed that the PHML-b-PLLA-b-PHML nanoparticles exhibited low cytotoxicity, strong plasmid DNA binding affinity, high polyplex stability and efficient plasmid DNA transfection even under serum conditions (10% FBS). Moreover, the endocytosis analysis results disclosed that the PHML₃₀-b-PLLA-b-PHML₃₀ nanoparticle/plasmid DNA polyplexes were predominantly involved in lipid-raft-mediated endocytosis pathway, similar to that of SV40 virus-based vectors.

Amino Acid Functional Polymers: Biomimetic Polymer Design Enabling Catalysis, Chiral Materials, and Drug Delivery

Amino acids are the natural building blocks for the world around us. Highly functional, these small molecules have unique catalytic properties, chirality, and biocompatibility. Imparting these properties to surfaces and other macromolecules is highly sought after and represents a fast-growing field. Polymers functionalized with amino acids in the side chains have tunable optical properties, pH responsiveness, biocompatibility, structure and self-assembly properties. Herein, we review the synthesis of amino acid functional polymers, discuss manipulation of available strategies to achieve the desired responsive materials, and summarize some exciting applications in catalysis, chiral particles, and drug delivery.

Solution processible hyperbranched inverse-vulcanized polymers as new cathode materials in Li–S batteries

Highly soluble inverse-vulcanized hyperbranched polymers were synthesized as cathode-active materials in Li–S batteries.

Remarkable swelling capability of amino acid based cross-linked polymer networks in organic and aqueous medium

This work reports design and synthesis of side chain amino acid based cross-linked polymeric gels, able to switch over from organogel to hydrogel by a simple deprotection reaction and showing superabsorbancy in water. Amino acid based methacrylate monomers, tert-butoxycarbonyl (Boc)-l/d-alanine methacryloyloxyethyl ester (Boc-l/d-Ala-HEMA), have been polymerized in the presence of a cross-linker via conventional free radical polymerization (FRP) and the reversible addition-fragmentation chain transfer (RAFT) technique for the synthesis of cross-linked polymer gels. The swelling behaviors of these organogels are investigated in organic solvents, and they behave as superabsorbent materials for organic solvents such as dichloromethane, acetone, tetrahydrofuran, etc. Swollen cross-linked polymer gels release the absorbed organic solvent rapidly. After Boc group deprotection from the pendant alanine moiety, the organogels transform to the hydrogels due to the formation of side chain ammonium (-NH3(+)) groups, and these hydrogels showed a significantly high swelling ratio (∼560 times than their dry volumes) in water. The morphology of organogels and hydrogels is studied by field emission scanning electron microscopy (FE-SEM). Amino acid based cross-linked gels could find applications as absorbents for oil spilled on water as well as superabsorbent hydrogels.

Cytocompatibility of novel extracellular matrix protein analogs of biodegradable polyester polymers derived from α-hydroxy amino acids

One of the challenges in regenerative medicine is the development of novel biodegradable materials to build scaffolds that will support multiple cell types for tissue engineering. Here we describe the preparation, characterization, and cytocompatibility of homo- and hetero-polyesters of α-hydroxy amino acid derivatives with or without lactic acid conjugation. The polymers were prepared by a direct condensation method and characterized using gel permeation chromatography, (1)H-nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, optical activity, and solubility. The surface charge of the polymers was evaluated using zeta potential measurements. The polymers were coated onto glass cover slips followed by characterization using nano-surface profiler, thin film reflectometry, and atomic force microscopy (AFM). Their interaction with endothelial and neuronal cells was assessed using adhesion, proliferation, and differentiation assays. Of the characterized polymers, Poly-HOVal-LA, but not Poly-(D)HOPhe, significantly augmented nerve growth factor (NGF)-induced neuronal differentiation of the PC12 pheochromcytoma cells. In contrast, Poly-HOLeu increased by 20% the adhesion of endothelial cells, but did not affect PC12 cell differentiation. NGF-induced Erk1/2 phosphorylation in PC12 cells grown on the different polymers was similar to the effect observed for cells cultured on collagen type I. While no significant association could be established between charge and the differentiative/proliferative properties of the polymers, AFM analysis indicated augmentation of NGF-induced neuronal differentiation on smooth polymer surfaces. We conclude that overall selective cytocompatibility and bioactivity might render α-hydroxy amino acid polymers useful as extracellular matrix-mimicking materials for tissue engineering.

Controlled synthesis of amino acid-based pH-responsive chiral polymers and self-assembly of their block copolymers

Leucine/isoleucine side chain polymers are of interest due to their hydrophobicity and reported role in the formation of α-helical structures. The synthesis and reversible addition-fragmentation chain transfer (RAFT) polymerization of amino acid-based chiral monomers, namely Boc-L-leucine methacryloyloxyethyl ester (Boc-L-Leu-HEMA, 1a), Boc-L-leucine acryloyloxyethyl ester (Boc-L-Leu-HEA, 1b), Boc-L-isoleucine methacryloyloxyethyl ester (Boc-L-Ile-HEMA, 1c), and Boc-L-isoleucine acryloyloxyethyl ester (Boc-L-Ile-HEA, 1d), are reported. The controlled nature of the polymerization of the said chiral monomers in N, N-dimethylformamide (DMF) at 70 °C is evident from the formation of narrow polydisperse polymers, the molecular weight controlled by the monomer/chain transfer agent (CTA) molar ratio and the linear relationship between molecular weight and monomer conversion. The resulting well-defined polymers were used as macro-CTAs to prepare corresponding diblock copolymers by RAFT polymerization of methyl (meth)acrylate monomers. Deprotection of Boc groups in the homopolymers and block copolymers under acidic conditions produced cationic, pH-responsive polymers with primary amine moieties at the side chains. The optical activity of the homopolymers and block copolymers were studied using circular dichroism (CD) spectroscopy and specific rotation measurements. The self-assembling nature of the block copolymers to produce highly ordered structures was illustrated through dynamic light scattering (DLS) and atomic force microscopy (AFM) studies. The side chain amine functionality instills pH-responsive behavior, which makes these cationic polymers attractive candidates for drug delivery applications, as well as for conjugation of biomolecules.

Preparation of functional water-soluble low-cytotoxic poly(methacrylate)s with pendant cationic L-lysines for efficient gene delivery

In this work, we present the preparation of water-soluble poly(methacrylate)s with pendant cationic L-lysines PHMLs(6-30 K). Plasmid DNA binding affinity as well as particle sizes and zeta potentials of the polyplexes were examined for these PHML vectors, and their cytotoxicities were assayed with HeLa cells by CCK-8 and lactate dehydrogenase kits. Gene transfection efficacy and intracellular uptake of the polyplexes by the PHML vectors were also studied with HeLa cells. As a result, it was revealed that the low cytotoxic PHMLs tended to exhibit gene transfection efficiencies significantly higher than those of the linear structural PLL (15-30 K) control, in particular the molecular weight of a PHML vector remarkably influenced its pDNA binding affinity, transfection efficacy and intracellular uptake of the polyplexes.