Α-amino Acid N-carboxyanhydrides Research Articles

Comb-like amphiphilic polypeptide-based copolymer nanomicelles for co-delivery of doxorubicin and P-gp siRNA into MCF-7 cells

A comb-like amphiphilic copolymer methoxypolyethylene glycol-graft-poly(l-lysine)-block-poly(l-phenylalanine) (mPEG-g-PLL-b-Phe) was successfully synthesized. To synthesize mPEG-g-PLL-b-Phe, diblock copolymer PLL-b-Phe was first synthesized by successive ring-opening polymerization of α-amino acid N-carboxyanhydrides followed by the removal of benzyloxycarbonyl protecting groups, and then mPEG was grafted onto PLL-b-Phe by reductive amination via Schiff's base formation. The chemical structures of the copolymers were identified by 1H NMR. mPEG-g-PLL-b-Phe copolymer had a critical micelle concentration of 6.0mg/L and could self-assemble in an aqueous solution into multicompartment nanomicelles with a mean diameter of approximately 78nm. The nanomicelles could encapsulate doxorubicin (DOX) through hydrophobic and π–π stacking interactions between DOX molecules and Phe blocks and simultaneously complex P-gp siRNA with cationic PLL blocks via electrostatic interactions. The DOX/P-gp siRNA-loaded nanomicelles showed spherical morphology, possessed narrow particle size distribution and had a mean particle size of 120nm. The DOX/P-gp siRNA-loaded nanomicelles exhibited pH-responsive release behaviors and displayed accelerated release under acidic conditions. The DOX/P-gp siRNA-loaded nanomicelles were efficiently internalized into MCF-7 cells, and DOX released could successfully reach nuclei. In vitro cytotoxicity assay demonstrated that the DOX/P-gp siRNA-loaded nanomicelles showed a much higher cytotoxicity in MCF-7 cells than DOX-loaded nanomicelles due to their synergistic killing effect and that the blank nanomicelles had good biocompatibility. Thus, the novel comb-like mPEG-g-PLL-b-Phe nanomicelles could be a promising vehicle for co-delivery of chemotherapeutic drug and genetic material.

Phenyl Trimethylsilyl Sulfide-Mediated Controlled Ring-Opening Polymerization of α-Amino Acid N-Carboxyanhydrides.

We describe here the first example of trimethylsilyl sulfide (S-TMS) mediated controlled ring-opening polymerization (ROP) of α-amino acid N-carboxyanhydrides (NCAs). We show that phenyl trimethylsilyl sulfide (PhS-TMS), an inexpensive and commercially available compound, mediates rapid ROP of a broad scope of NCA monomers, produces functional poly(amino acids) (PAAs) with controllable molecular weights (MWs), narrow polydispersity index (PDI), and an in situ generated phenyl thioester group at the C-terminus (PAA-SPhs). PhS-TMS offers more rapid chain initiation than previously reported hexamethyldisilazane (HMDS) initiator, ensuring a living polymerization with better control. Mechanistic studies suggest that a reactive trimethylsilyl carbamate (TMSC) was generated during the chain initiation and continued to regulate the chain propagation through a TMS transfer process. Considering the versatility of NCAs, and the potential of leveraging the C-terminal phenyl thioester for native chemical ligation (NCL), we believe this method may offer a powerful platform enabling the rapid generation of functional PAAs and their C-terminal conjugates for numerous biological applications.

Preparation and characterization of foamy poly(γ-benzyl-l-glutamate-co-l-phenylalanine)/bioglass composite scaffolds for bone tissue engineering

Novel foamy scaffolds of poly(γ-benzyl-l-glutamate) and poly(γ-benzyl-l-glutamate-co-l-phenylalanine) were fabricatedviaa combination of a sintered NaCl templating method and ring-opening polymerization of α-amino acid N-carboxyanhydrides.

Stimuli-Triggered Sol-Gel Transitions of Polypeptides Derived from α-Amino Acid N-Carboxyanhydride (NCA) Polymerizations.

The past decade has witnessed significantly increased interest in the development of smart polypeptide-based organo- and hydrogel systems with stimuli responsiveness, especially those that exhibit sol-gel phase-transition properties, with an anticipation of their utility in the construction of adaptive materials, sensor designs, and controlled release systems, among other applications. Such developments have been facilitated by dramatic progress in controlled polymerizations of α-amino acid N-carboxyanhydrides (NCAs), together with advanced orthogonal functionalization techniques, which have enabled economical and practical syntheses of well-defined polypeptides and peptide hybrid polymeric materials. One-dimensional stacking of polypeptides or peptide aggregations in the forms of certain ordered conformations, such as α helices and β sheets, in combination with further physical or chemical cross-linking, result in the construction of three-dimensional matrices of polypeptide gel systems. The macroscopic sol-gel transitions, resulting from the construction or deconstruction of gel networks and the conformational changes between secondary structures, can be triggered by external stimuli, including environmental factors, electromagnetic fields, and (bio)chemical species. Herein, the most recent advances in polypeptide gel systems are described, covering synthetic strategies, gelation mechanisms, and stimuli-triggered sol-gel transitions, with the aim of demonstrating the relationships between chemical compositions, supramolecular structures, and responsive properties of polypeptide-based organo- and hydrogels.

Synthesis and Characterization of pH-Responsive Copolypeptides Vesicles for siRNA and Chemotherapeutic Drug Co-Delivery.

Novel diblock copolypeptides are synthesized and evaluated as a pH-sensitive drug delivery vector for anticancer drugs and siRNA co-delivery. The synthetic process involve the ring opening polymerization of α-amino acid N-carboxyanhydrides as well as aminolysis and deprotection reactions, and the structures are characterized. The copolypeptides can self-assemble into stable vesicles at neutral pH, and disassemble in acidic endosomal/lysosomal, which shown the pH-responsive behavior. The cell uptake results indicated that the vesicles can co-deliver DOX and siRNA inside the same cell and exhibit a pH-sensitive release behavior. Therefore, the novel polymeric vesicles are a promising carrier for co-delivery anticancer drug and siRNA to overcome the drug resistance and enhance cancer treatment.

Polypeptide brushes grown via surface-initiated ring-opening polymerization of α-amino acid N-carboxyanhydrides

Polypeptide brushes are attractive platforms to generate functional and responsive interfaces that are of potential interest due to their possible biodegradability, biocompatibility and tunable secondary structures. Surface-initiated ringopening polymerization (SI-ROP) of α-amino acid N-carboxyanhydrides represents a powerful and versatile strategy to generate polypeptide brushes. This review is an attempt to capture the state-of-the-art in this field and highlights the latest developments in several selected areas. In addition to presenting an overview of the synthetic methods that have been used to generate polypeptide brushes via SI-ROP, this article will discuss the preparation of patterned polypeptide brushes, the conformational properties of surface-tethered polypeptides, ways to control chain orientation at surfaces as well as properties and applications of these thin polymer films.

Azo-capped polysarcosine-b-polylysine as polypeptide gene vector: A new strategy to improve stability and easy optimization via host–guest interaction

Polypeptide has been extensively researched in gene/drug delivery system due to the good biocompatibility. Herein, we synthesized total-polypeptide copolymers, i.e. Azo(azobenzene)-capped polysarcosine-b-polylysine (ASL) with narrow molecular weight distribution by α-amino acid N-carboxyanhydride (NCA) polymerization. Although the molecular weight of PLL segment was only about 6kDa, ASL could condense DNA effectively and form about 150nm spherical nanoparticles at N/P ratio of 15. The surface charge was significantly reduced due to the shielding effect of polysarcosine (PSAR) shell. ASL/DNA PeptoPlexes showed good colloidal stability under physiological salt condition and complexation competition stability in the presence of counter polyanion, which might improve the circulation time in vivo. The tip design of azobenzene provided a facile way for ligand modification via host–guest interaction, which could be flexibly optimized by changing its functional tags responding to a request. Our data showed that the introduction of CD-R8 could promote the internalization of gene into cytoplasm and even nucleus owing to the membrane penetrating effect of R8. Cell culture experiments indicated as a total-polypeptide system, ASL showed good cellular viability and comparable gene transfection level as PLL with molecular weight of 50kDa. Overall, PSAR served as an ideal alternative of PEG and this total-polypeptide system showed us a good direction for gene carrier design.

Controlled synthesis of phosphorylcholine derivatives of poly(serine) and poly(homoserine).

We report methods for the synthesis of polypeptides that are fully functionalized with desirable phosphorylcholine, PC, groups. Because of the inherent challenges in the direct incorporation of the PC group into α-amino acid N-carboxyanhydride (NCA) monomers, we developed a synthetic approach that combined functional NCA polymerization with efficient postpolymerization modification. While poly(L-phosphorylcholine serine) was found to be unstable upon synthesis, we successfully prepared poly(L-phosphorylcholine homoserine) with controlled chain lengths and found these to be water-soluble with disordered chain conformations.

Fast and living ring-opening polymerization of α-amino acid N-carboxyanhydrides triggered by an "alliance" of primary and secondary amines at room temperature.

A novel highly efficient strategy, based on an "alliance" of primary and secondary amine initiators, was successfully developed allowing the fast and living ring-opening polymerization (ROP) of α-amino acid N-carboxyanhydrides (NCAs) at room temperature.

Organocatalysis by hydrogen-bonding: a new approach to controlled/living polymerization of α-amino acid N-carboxyanhydrides

Hydrogen-bonding organocatalytic controlled/living ROP of α-amino acid N-carboxyanhydrides.

Polymer-<I>Block</I>-Polypeptides and Polymer-Conjugated Hybrid Materials as Stimuli-Responsive Nanocarriers for Biomedical Applications

Stimuli-responsive nanocarriers are a class of soft materials that includes natural polymers, synthetic polymers, and polypeptides. Recently, modern synthesis tools such as atom transfer radical polymerization, reversible addition-fragmentation chain transfer polymerization, nitroxide-mediated radical polymerization, ring-opening polymerization of α-amino acid N-carboxyanhydrides, and various "click" chemistry strategies were simultaneously employed for the design and synthesis of nanosized drug delivery vehicles. Importantly, the research focused on the improvement of the nanocarrier targetability and the site-specific, triggered release of therapeutics with high drug loading efficiency and minimal drug leakage during the delivery to specific targets. In this context, nanocarriers responsive to common stimuli such as pH, temperature, redox potential, light, etc. have been widely used for the controlled delivery of therapeutics to pathological sites. Currently, different synthesis and self-assembly strategies improved the drug loading efficacy and targeted delivery of therapeutic agents to the desired site. In particular, polypeptide-containing hybrid materials have been developed for the controlled delivery of therapeutic agents. Therefore, stimuli-sensitive synthetic polypeptide-based materials have been extensively investigated in recent years. This review focuses on recent advances in the development of polymer-block-polypeptides and polymer-conjugated hybrid materials that have been designed and evaluated for various stimuli-responsive drug and gene delivery applications.

Directed interactions of block copolypept(o)ides with mannose-binding receptors: PeptoMicelles targeted to cells of the innate immune system.

Core-shell structures based on polypept(o)ides combine stealth-like properties of the corona material polysarcosine with adjustable functionalities of the polypeptidic core. Mannose-bearing block copolypept(o)ides (PSar-block-PGlu(OBn)) have been synthesized using 11-amino-3,6,9-trioxa-undecyl-2,3,4,6-tetra-O-acetyl-O-α-D-mannopyranoside as initiator in the sequential ring-opening polymerization of α-amino acid N-carboxyanhydrides. These amphiphilic block copolypept(o)ides self-assemble into multivalent PeptoMicelles and bind to mannose-binding receptors as expressed by dendritic cells. Mannosylated micelles showed enhanced cell uptake in DC 2.4 cells and in bone marrow-derived dendritic cells (BMDCs) and therefore appear to be a suitable platform for immune modulation.

Synthesis and sequential deprotection of triblock copolypept(o)ides using orthogonal protective group chemistry.

The synthesis of triblock copolymers based on polysarcosine, poly-N-ε-t-butyloxycarbonyl-l-lysine, and poly-N-ε-t-trifluoroacetyl-l-lysine by ring-opening polymerization of the corresponding α-amino acid N-carboxyanhydrides (NCAs) is described. For the synthesis of N-ε-t-butyloxycarbonyl-l-lysine (lysine(Boc)) NCAs, an acid-free method using trimethylsilylchloride/triethylamine as hydrochloric acid (HCl) scavengers is presented. This approach enables the synthesis of lysine(Boc) NCA of high purity (melting point 138.3 °C) in high yields. For triblock copolypept(o)ides, the degree of polymerization (Xn ) of the polysarcosine block is varied between 200 and 600; poly-N-ε-t-butyloxycarbonyl-l-lysine and poly-N-ε-t-trifluoroacetyl-l-lysine blocks are designed to have a Xn in the range of 10-50. The polypeptide-polypeptoid hybrids (polypept(o)ides) can be synthesized with precise control of molecular weight, high end group integrity, and dispersities indices between 1.1 and 1.2. But more important, the use of tert-butyloxycarbonyl- and trifluoroacetyl-protecting groups allows the selective, orthogonal deprotection of both blocks, which enables further postpolymerization modification reactions in a block-selective manner. Therefore, the presented synthetic approach provides a versatile pathway to triblock copolypept(o)ides, in which functionalities can be separated in specific blocks.

Synthesis of α-hydroxy-ω-aminotelechelic polypeptide from α-amino acid N-carboxyanhydrides catalyzed by alkali-metal borohydrides

It is reported that alkali-metal borohydrides (MBH4, M = Li, Na and K) are efficient catalysts for ring opening polymerization (ROP) of α-amino acid N-carboxyanhydrides (NCAs). Polypeptides are prepared in quantitative yields with relatively narrow molecular weight distributions (MWDs = 1.1∼1.5) which depend on the reaction temperature. End groups of the produced polypeptide are studied in detail by MALDI-ToF MS, 1H-NMR, 13C-NMR, 1H-1H COSY and 1H-13C HMQC analyses. The results indicate that α-hydroxy-ω-aminotelechelic polypeptides are formed which are suitable for postpolymerization functionalization.

Analogues of Poly(l-phosphoserine) via Living Polymerization of Phosphonate-Containing N-Carboxyanhydride Monomers.

We have synthesized new phosphonate-containing α-amino acid N-carboxyanhydride (NCA) monomers and used these to prepare well-defined phosphonate-containing polypeptides and block copolypeptides. These NCAs were found to undergo living polymerization to high molecular weights with narrow chain length distributions. The methods described here demonstrate that phosphono polypeptides are also readily incorporated into block copolymers with controlled architecture and composition. One polymer, poly(l-phosphonohomoalanine) (poly(Pha)), is an isosteric analogue of poly(l-phosphoserine) and was found to display a pH-responsive conformational change. The pH-responsive α-helical conformation of poly(Pha) is unprecedented and may prove to be useful in the preparation of stimuli-responsive block copolypeptides containing poly(Pha) segments.

Peptide Hydrogels Assembled from Nonionic Alkyl-polypeptide Amphiphiles Prepared by Ring-Opening Polymerization

Three alkyl-polypeptide (AP) amphiphiles were prepared using ring-opening polymerization of α-amino acid N-carboxyanhydride. The polypeptide segment was composed of diethylene-glycol-monomethyl-ether-functionalized poly-L-glutamate (poly-L-EG(2)Glu). These AP amphiphiles can spontaneously self-assemble into transparent hydrogels in water. These hydrogels showed shear thinning properties, and their strength can be modulated by hydrophobic alkyl tails. CryoTEM and AFM characterizations suggested that these hydrogels were formed by nanoribbons arising from intermolecular interactions between nonionic poly-L-EG(2)Glu segments.

Assembly of Free‐Standing Polypeptide Films via the Synergistic Combination of Hyperbranched Macroinitiators, the Grafting‐From Approach, and Cross‐Chain Termination

Cross-linked polypeptide-based films are fabricated via a novel and robust method employing surface-initiated ring opening polymerization of α-amino acid N-carboxyanhydrides (NCA-ROP). The judicious combination of amine-based hyperbranched macroinitiators and benzyl ester-protected NCA derivatives promotes network formation by cross-chain terminations, which allows the formation of stable cross-linked peptide-based capsules in a one-pot system.

Dual Stimuli-Responsive Poly(N-isopropylacrylamide)-b-poly(l-histidine) Chimeric Materials for the Controlled Delivery of Doxorubicin into Liver Carcinoma

A series of dual stimuli responsive synthetic polymer bioconjugate chimeric materials, poly(N-isopropylacrylamide)55-block-poly(L-histidine)n [p(NIPAM)55-b-p(His)n] (n=50, 75, 100, 125), have been synthesized by employing reversible addition-fragmentation chain transfer polymerization of NIPAM, followed by ring-opening polymerization of α-amino acid N-carboxyanhydrides. The dual stimuli responsive properties of the resulting biocompatiable and membrenolytic p(NIPAM)55-b-p(His)n polymers are investigated for their use as a stimuli responsive drug carrier for tumor targeting. Highly uniform self-assembled micelles (∼55 nm) fabricated by p(NIPAM)55-b-p(His)n polymers display sharp thermal and pH responses in aqueous media. An anticancer drug, doxorubicin (Dox), is effectively encapsulated in the micelles and the controlled Dox release is investigated in different temperature and pH conditions. Antitumor effect of the released Dox is also assessed using the HepG2 human hepatocellular carcinoma cell lines. Dox molecules released from the [p(NIPAM)55-b-p(His)n] micelles remain biologically active and have stimuli responsive capability to kill cancer cells. The self-assembling ability of these hybrid materials into uniform micelles and their efficiency to encapsulate Dox makes them a promising drug carrier to cancer cells. The new chimeric materials thus display tunable properties that can make them useful for a molecular switching device and controlled drug delivery applications needing responses to temperature and pH for the improvement of cancer chemotherapy.

ChemInform Abstract: Synthesis of Polypeptides by Ring‐Opening Polymerization of α‐Amino Acid N‐Carboxyanhydrides

AbstractReview: 70 refs.

Environmentally benign peptide synthesis using liquid-assisted ball-milling: application to the synthesis of Leu-enkephalin

This paper describes an original methodology for peptide bond synthesis avoiding toxic solvents and reactants. Ball-milling stoichiometric amounts of Boc-protected α-amino acid N-carboxyanhydrides (Boc-AA-NCA) or Boc-protected α-amino acid N-hydroxysuccinimide esters (Boc-AA-OSu) with α-amino acid alkyl ester salts in the presence of NaHCO3 and a minimal quantity of EtOAc led to the production of di- to pentapeptides in an efficient and environmentally benign manner. This approach was successfully applied to the synthesis of Leu-enkephalin.