Carbohydrate-Conjugated Amino Acid-Based Fluorescent Block Copolymers: Their Self-Assembly, pH Responsiveness, and/or Lectin Recognition

An effective strategy has been documented to combine both carbohydrate and amino acid biomolecules in a single synthetic polymeric system via a reversible addition–fragmentation chain transfer (RAFT) polymerization technique. The resultant unique block copolymer was engineered to form uniform micelles with the desired projection of either selective or both amino acid/sugar residues on the outer surface with multivalency, providing pH-based stimuli-responsiveness and/or lectin recognition. The self-assembly process was studied in detail by field emission scanning electron microscopy (FE-SEM), dynamic light scattering (DLS), and UV–visible spectroscopy. The enhanced lectin binding behavior was observed for glyconanoparticles with both amino acid/sugar entities on the shell as compared to the only glycopolymer nanoparticle because of the higher steric hindrance factor in the case of only the glycopolymer nanoparticle. Fluorophore conjugation by postpolymerization functionalization was further exploited by fluorescence spectroscopy for evidencing the lectin recognition process

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, <i>tert</i>-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

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

Supramolecular Interaction-Assisted Fluorescence and Tunable Stimuli-Responsiveness of l‑Phenylalanine-Based Polymers

Supramolecular host–guest interactions between randomly methylated β-cyclodextrin (RM β-CD) and side-chain phenylalanine (Phe) and Phe–Phe dipeptide-based homopolymers have been employed for the amplification of fluorescence emission of otherwise weakly fluorescent amino acid Phe. The host–guest complex has been characterized by ¹H and ¹³C NMR spectroscopy, two-dimensional rotating-frame overhauser spectroscopy, Fourier-transform infrared spectroscopy, UV–visible spectroscopy, and fluorescence spectroscopy. To gain insights into the origin of fluorescence in homopolymers, density functional theory calculations were performed where phenyl moieties inside the less polar core of β-CD were observed to form a π–π coupled complex resulting in an enhanced emission. Furthermore, the complex-forming ability of Phe, the guest molecule, has been employed in tuning the cloud point temperature (<i>T</i>_CP) of statistical copolymers derived from side-chain Phe/Phe–Phe-based methacrylate monomers and <i>N</i>-isopropylacrylamide. By varying the co-monomer feed ratios in the statistical copolymer and hence the concentration of RM β-CD throughout the polymer chain, host–guest interaction-assisted broad tunability in <i>T</i>_CP of the supramolecular polymeric complex has been achieved

Side-Chain Amino-Acid-Based pH-Responsive Self-Assembled Block Copolymers for Drug Delivery and Gene Transfer

Developing safe and effective nanocarriers for multitype of delivery system is advantageous for several kinds of successful biomedicinal therapy with the same carrier. In the present study, we have designed amino acid biomolecules derived hybrid block copolymers which can act as a promising vehicle for both drug delivery and gene transfer. Two representative natural chiral amino acid-containing (l-phenylalanine and l-alanine) vinyl monomers were polymerized via reversible addition–fragmentation chain transfer (RAFT) process in the presence of monomethoxy poly­(ethylene glycol) based macro-chain transfer agents (mPEG_<i>n</i>-CTA) for the synthesis of well-defined side-chain amino-acid-based amphiphilic block copolymers, monomethoxy poly­(ethylene glycol)-<i>b</i>-poly­(Boc-amino acid methacryloyloxyethyl ester) (mPEG_<i>n</i>-<i>b</i>-P­(Boc-AA-EMA)). The self-assembled micellar aggregation of these amphiphilic block copolymers were studied by fluorescence spectroscopy, atomic force microscopy (AFM) and scanning electron microscopy (SEM). Potential applications of these hybrid polymers as drug carrier have been demonstrated <i>in vitro</i> by encapsulation of nile red dye or doxorubicin drug into the core of the micellar nanoaggregates. Deprotection of side-chain Boc- groups in the amphiphilic block copolymers subsequently transformed them into double hydrophilic pH-responsive cationic block copolymers having primary amino groups in the side-chain terminal. The DNA binding ability of these cationic block copolymers were further investigated by using agarose gel retardation assay and AFM. The <i>in vitro</i> cytotoxicity assay demonstrated their biocompatible nature and these polymers can serve as “smart” materials for promising bioapplications

Swelling-Induced Optical Anisotropy of Thermoresponsive Hydrogels Based on Poly(2-(2-methoxyethoxy)ethyl methacrylate): Deswelling Kinetics Probed by Quantitative Mueller Matrix Polarimetry

Thermodynamically favored polymer–water interactions below the lower critical solution temperature (LCST) caused swelling-induced optical anisotropy (linear retardance) of thermoresponsive hydrogels based on poly­(2-(2-methoxyethoxy)­ethyl methacrylate). This was exploited to study the macroscopic deswelling kinetics quantitatively by a generalized polarimetry analysis method, based on measurement of the Mueller matrix and its subsequent inverse analysis via the polar decomposition approach. The derived medium polarization parameters, namely, linear retardance (δ), diattenuation (<i>d</i>), and depolarization coefficient (Δ), of the hydrogels showed interesting differences between the gels prepared by conventional free radical polymerization (FRP) and reversible addition–fragmentation chain transfer polymerization (RAFT) and also between dry and swollen state. The effect of temperature, cross-linking density, and polymerization technique employed to synthesize hydrogel on deswelling kinetics was systematically studied via conventional gravimetry and corroborated further with the corresponding Mueller matrix derived quantitative polarimetry characteristics (δ, <i>d</i>, and Δ). The RAFT gels exhibited higher swelling ratio and swelling-induced optical anisotropy compared to FRP gels and also deswelled faster at 30 °C. On the contrary, at 45 °C, deswelling was significantly retarded for the RAFT gels due to formation of a skin layer, which was confirmed and quantified via the enhanced diattenuation and depolarization parameters

CdS Quantum Dots Doped Tuning of Deswelling Kinetics of Thermoresponsive Hydrogels Based on Poly(2-(2-methoxyethoxy)ethyl methacrylate)

Thermoresponsive poly­(2-(2-methoxyethoxy)­ethyl methacrylate) (PMEO₂MA) based hybrid nanocomposite hydrogels (NCH) were synthesized by dispersing preformed cadmium sulfide (CdS) quantum dots (QDs) in the reaction mixture followed by polymerization via reversible addition–fragmentation chain transfer (RAFT) technique. High doping capacity and negligible QDs leakage were observed for hydrophilic QDs doped hydrogels (hpl-NCH) due to H-bonding interactions between QDs and pendant groups of hydrogel network. The hpl-NCH networks showed improved structural/orientational order and swelling ratios with increasing doping concentration compared to the organic hydrogel (OH). Opposite trends were observed for bulk-CdS (NCH-bulk) and 1-dodecanethiol capped CdS (NCH-DDT) doped hydrogels. Swelling induced linear retardance and quenching of photoluminescence (PL) intensity for hydrogels were exploited to study the deswelling kinetics respectively by Mueller matrix polarimetry and solid state fluorimetry, which were further corroborated with gravimetric analysis. For all the NCH, deswelling process significantly decreased with increasing temperature, which followed the order: 30 > 45 > 60 °C. Slower deswelling was observed for NCH-bulk and hpl-NCH compared to the OH, and also with increase in doping concentration due to the formation of skin layer. However, NCH-DDT exhibited accelerated deswelling process and the order was reversed with respect to doping concentration due to DDT mediated enhanced hydrophobic aggregation and water leakage channels created by long hydrophobic free-mobile nature of QDs surface tethered DDT molecules. The presented methodology provides tunable deswelling of PMEO₂MA based hydrogels by doping with hydrophilically/hydrophobically modified CdS QDs

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, <b>1a</b>), Boc-l-leucine acryloyloxyethyl ester (Boc-l-Leu-HEA, <b>1b</b>), Boc-l-isoleucine methacryloyloxyethyl ester (Boc-l-Ile-HEMA, <b>1c</b>), and Boc-l-isoleucine acryloyloxyethyl ester (Boc-l-Ile-HEA, <b>1d</b>), are reported. The controlled nature of the polymerization of the said chiral monomers in <i>N</i>, <i>N</i>-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

Recyclable Thermoresponsive Polymer−β-Glucosidase Conjugate with Intact Hydrolysis Activity

β-Glucosidase (BG) catalyzes the hydrolysis of cellobiose to glucose and is a rate-limiting enzyme in the conversion of lignocellulosic biomass to sugars toward biofuels. Since the cost of enzyme is a major contributor to biofuel economics, we report the bioconjugation of a temperature-responsive polymer with the highly active thermophilic β-glucosidase (B8CYA8) from Halothermothrix orenii toward improving enzyme recyclability. The bioconjugate, with a lower critical solution temperature (LCST) of 33 °C withstands high temperatures up to 70 °C. Though the secondary structure of the enzyme in the conjugate is slightly distorted with a higher percentage of β-sheet like structure, the stability and specific activity of B8CYA8 in the conjugate remains unaltered up to 30 °C and retains more than 70% specific activity of the unmodified enzyme at 70 °C. The conjugate can be reused for β-glucosidic bond cleavage of cellobiose for at least four cycles without any significant loss in specific activity