Priyadarsi De

Temperature-Regulated Activity of Responsive Polymer−Protein Conjugates Prepared by Grafting-from via RAFT Polymerization

A facile route to well-defined smart polymer-protein conjugates with tunable bioactivity is reported. Protein modification with a reversible addition-fragmentation chain transfer (RAFT) agent and subsequent room temperature polymerization in aqueous media led to conjugates of poly(N-isopropylacrylamide) and a model protein. Representing the first example of polymer-protein conjugation with RAFT agent immobilization via the R-group approach, high molecular weight and reductively stable conjugates were accessible without extensive purification or adverse effects on the protein structure. An increase in molecular weight with conversion was observed for the chains grafted from the protein surface, confirming the controlled nature of the polymerization. The responsive behavior of the immobilized polymer facilitated conjugate isolation and also allowed environmental modulation of bioactivity.

Folate-Conjugated Thermoresponsive Block Copolymers: Highly Efficient Conjugation and Solution Self-Assembly

A combination of controlled radical polymerization and azide-alkyne click chemistry was employed to prepare temperature-responsive block copolymer micelles conjugated with biological ligands with potential for active targeting of cancer tissues. Block copolymers of N-isopropylacrylamide (NIPAM) and N,N-dimethylacrylamide (DMA) were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization with an azido chain transfer agent (CTA). Pseudo-first-order kinetics and linear molecular weight dependence on conversion were observed for the RAFT polymerizations. CuI-catalyzed coupling with propargyl folate resulted in folic acid residues being efficiently conjugated to the alpha-azido chain ends of the homo and block copolymers. Temperature-induced self-assembly resulted in aggregates capable of controlled release of a model hydrophobic drug. CuI-catalyzed azide-alkyne cycloaddition has proven superior to conventional methods for conjugation of biological ligands to macromolecules, and the general strategy presented herein can potentially be extended to the preparation of folate-functionalized assemblies with other stimuli susceptibility (e.g., pH) for therapeutic and imaging applications.

Conjugation of RAFT-generated polymers to proteins by two consecutive thiol–ene reactions

Well-defined temperature-responsive polymers were covalently conjugated to model proteins by two consecutive Michael addition thiol–ene reactions. Poly(N-isopropylacrylamide) (PNIPAM) prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization was aminolyzed to yield thiol-terminated chains that were subsequently reacted with excess 1,8-bis-maleimidodiethyleneglycol. The resulting maleimide-terminated polymer was reacted with bovine serum albumin and ovalbumin to yield polymer–protein conjugates by a “grafting-to” approach. The thermoresponsive nature of PNIPAM was conferred to the conjugate, as demonstrated by dynamic light scattering analysis that indicated the formation of intermolecular aggregates at elevated temperatures.

Thermoresponsive block copolymer-protein conjugates prepared by grafting-from via RAFT polymerization.

Well-defined smart block copolymer-protein conjugates were prepared by two consecutive grafting-from reactions via reversible addition-fragmentation chain transfer (RAFT) polymerization. The initiating portion (R-group) of the RAFT agent was anchored to a model protein such that the thiocarbonylthio moiety was readily accessible for chain transfer with propagating chains in solution. Well-defined polymer-protein conjugates of poly(N-isopropylacrylamide) (PNIPAM) and bovine serum albumin (BSA) were prepared at room temperature in aqueous media. The retained trithiocarbonate moiety on the free end group of the immobilized polymer allowed the homopolymer conjugate to be extended by polymerization of N,N-dimethylacrylamide. Polyacrylamide gel electrophoresis, size exclusion chromatography, and NMR spectroscopy confirmed the synthesis of the various conjugates and revealed that the polymerizations were well controlled. As expected, the resulting block copolymer-protein conjugates demonstrated thermoresponsive behavior due to the temperature-sensitivity of the PNIPAM block, as evidenced by turbidity measurements and dynamic light scattering analysis.

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.

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 (mPEGn-CTA) for the synthesis of well-defined side-chain amino-acid-based amphiphilic block copolymers, monomethoxy poly(ethylene glycol)-b-poly(Boc-amino acid methacryloyloxyethyl ester) (mPEGn-b-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 in vitro 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 in vitro cytotoxicity assay demonstrated their biocompatible nature and these polymers can serve as smart materials for promising bioapplications.

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.

Polyisobutylene-Based pH-Responsive Self-Healing Polymeric Gels

This work demonstrates the successful application of dynamic covalent chemistry for the construction of self-healing gels from side-chain primary amine leucine pendant diblock copolymers of polyisobutylene (PIB) ((P(H2N-Leu-HEMA)-b-PIB)) in the presence of PIB based dialdehyde functionalized cross-linker (HOC-PIB-CHO) through imine (-HC═N-) bond formation without aiding any external stimuli. Gels were synthesized in 1,4-dioxane at room temperature at varied wt % of gelator concentration, [H2N]/[CHO] ratios and molecular weight of the block segments. The mechanical property of gels was examined by rheological measurements. We observed higher value of storage modulus (G) than the loss modulus (G″) within the linearity limits of deformation, indicating the rheological behavior in the gel is dominated by an elastic property rather than a viscous property. The G values significantly depend upon the extent of cross-linking in the gel network. To establish self-healing property of the gels, rheology analysis through step-strain measurements (strain = 0.1 to 200%) at 25 °C was performed. The polymeric gel network shows reversible sol-gel transition for several cycles by adjusting the pH of the medium with the help of hydrochloric acid (HCl) and triethylamine (Et3N) triggers. FT-IR spectroscopy established formation of imine bonds in the gel network and these gels showed poor swelling behavior in various organic solvents because of the small interstitial porosity, confirmed by field emission-scanning electron microscopy (FE-SEM).

Synthesis, structural characterization, thermal studies and chain dynamics of poly(methacrylonitrile peroxide) by NMR spectroscopy

Poly(methacrylonitrile peroxide) (PMNP) has been synthesized from methacrylonitrile by free radical initiated oxidative polymerization and characterized by different spectroscopic methods. NMR spectroscopy confirmed the alternating copolymer structure with labile peroxy bonds in the main chain. The extreme instability of PMNP was noted from FTIR spectroscopy. Thermal degradation studies by using differential scanning calorimetry and thermogravimetry have revealed that PMNP degrades highly exothermically and the heat of degradation, 42.5 kcal mol−1, is of the same order as that reported for other vinyl polyperoxides. Mass spectral fragmentation pattern under electron impact (EI) condition has also been investigated. The mechanism of the primary exothermic degradation has been substantiated by thermochemical calculations. The chain dynamics of the polyperoxide chain has been studied by means of 13C spin–lattice relaxation times (T1) of the main chain as well as the side chain carbons. The temperature dependence of the spin–lattice relaxation times shows that the PMNP is more flexible compared to the analogous poly(styrene peroxide).

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.