Transition Metal−Amine Initiators for Preparation of Well-Defined Poly(γ-benzyl l-glutamate)

Well-defined star (co)polypeptides via a fast, efficient, and metal-free strategy

Polypeptides, especially star polypeptides, as a unique kind of biological macromolecules have broad applications in biomedical fields such as drug release, gene delivery, tissue engineering, and regenerative medicines due to their close structural similarity to naturally occurring peptides and proteins, biocompatibility, and amino acid functionality. However, the synthesis of star polypeptide mainly relies on the conventional primary amine-initiated ring-opening polymerization (ROP) of N-carboxyanhydrides (NCA) and suffers from low polymerization activity and limited controllability. This study proposes a fast, efficient and metal-free strategy to access star (co)polypeptides by combining the Michael reaction between acrylates and secondary aminoalcohols with the hydrogen-bonding organocatalytic ROP of NCA. This approach enables the preparation of a library of star (co)polypeptides with predesigned molecular weights, narrow molecular weight distributions, tunable arm number, and arm compositions. Importantly, this method exhibits high activity and selectivity at room temperature, making it both practical and versatile in synthesis applications.

Precision Synthesis of Polypeptides via Living Anionic Ring-Opening Polymerization of N-Carboxyanhydrides by Tri-thiourea Catalysts

The chemistry of α-amino acid N-carboxyanhydrides (NCAs) has a history of over 100 years, but precise and efficient ring-opening polymerization methods for NCAs remain highly needed to facilitate the studies of polypeptides─that is, mimics of natural proteins─in various disciplines. Moreover, the universally accepted NCA polymerization mechanisms are largely limited to the "amine" and the "activated monomer" mechanisms, and the anionic ring-opening polymerization of NCAs has so far not been invoked. Herein, we show an unprecedented anion-binding catalytic system combining tripodal tri-thiourea with sodium thiophenolate that enables the fast and selective anionic ring-opening polymerization of NCAs. This method leads to the precision construction of various polypeptides with living polymerization behavior and is evidenced by narrow molecular weight distributions (M_w/M_n < 1.2), chain extension experiments, and minimal "activated monomer" pathway. Calculations and experimental results elucidate a living anionic polymerization mechanism, and high selectivities for monomer propagation relative to other deleterious side reactions, such as the "activated monomer" pathway, are attributed to the enhanced stabilization of the propagating carbamate anion, which is enforced by an intramolecular hydrogen bond within the tri-thiourea structure.

Recent Advances in Thermoresponsive OEGylated Poly(amino acid)s

Thermoresponsive polymers have been widely studied in the past decades due to their potential applications in biomedicine, nanotechnology, and so on. As is known, poly(N-isopropylacrylamide) (PNIPAM) and poly(oligo(ethylene glycol)methacrylates) (POEGMAs) are the most popular thermoresponsive polymers, and have been studied extensively. However, more advanced thermoresponsive polymers with excellent biocompatibility, biodegradability, and bioactivity also need to be developed for biomedical applications. OEGylated poly(amino acid)s are a kind of novel polymer which are synthesized by attaching one or multiple oligo(ethylene glycol) (OEG) chains to poly(amino acid) (PAA).These polymers combine the great solubility of OEG, and the excellent biocompatibility, biodegradability and well defined secondary structures of PAA. These advantages allow them to have great application prospects in the field of biomedicine. Therefore, the study of OEGylated poly(amino acid)s has attracted more attention recently. In this review, we summarized the development of thermoresponsive OEGylated poly(amino acid)s in recent years, including the synthesis method (such as ring-opening polymerization, post-polymerization modification, and Ugi reaction), stimuli-response behavior study, and secondary structure study. We hope that this periodical summary will be more conducive to design, synthesis and application of OEGylated poly(amino acid)s in the future.

Fast and selective organocatalytic ring-opening polymerization by fluorinated alcohol without a cocatalyst

Organocatalysis is an important branch of catalysis for various organic transformations and materials preparation. Polymerizations promoted by organic catalysts can produce polymeric materials without any metallic residues, providing charming materials for high-value and sensitive domains such as biomedical applications, microelectronic devices and food packaging. Herein, we describe a fluorinated alcohol based catalytic system for polypeptide synthesis via catalytic ring-opening polymerization (ROP) of α-amino acid N-carboxyanhydride (NCA), fulfilling cocatalyst free, metal free, high rate and high selectivity. During polymerization, the fluorinated alcohol catalyst forms multiple dynamic hydrogen bonds with the initiator, monomer and propagating polymer chain. These cooperative hydrogen bonding interactions activate the NCA monomers and simultaneously protect the overactive initiator/propagating polymer chain-ends, which offers the whole polymerization with high activity and selectivity. Mechanistic studies indicate a monocomponent-multifunctional catalytic mode of fluorinated alcohol. This finding provides a metal free and fast approach to access well-defined polypeptides.

Polymerizations promoted by organic catalysts can produce polymeric materials without any metallic residues contamination. Here the authors show a fluorinated alcohol based catalytic system for polypeptide synthesis from α-amino acid N-carboxyanhydride, fulfilling cocatalyst and metal free conditions with high rate and selectivity.

Lithium hexamethyldisilazide initiated superfast ring opening polymerization of alpha-amino acid N-carboxyanhydrides

Polypeptides have broad applications and can be prepared via ring-opening polymerization of α-amino acid N-carboxyanhydrides (NCAs). Conventional initiators, such as primary amines, give slow NCA polymerization, which requires multiple days to reach completion and can result in substantial side reactions, especially for very reactive NCAs. Moreover, current NCA polymerizations are very sensitive to moisture and must typically be conducted in a glove box. Here we show that lithium hexamethyldisilazide (LiHMDS) initiates an extremely rapid NCA polymerization process that is completed within minutes or hours and can be conducted in an open vessel. Polypeptides with variable chain length (DP = 20–1294) and narrow molecular weight distribution (Mw/Mn = 1.08–1.28) were readily prepared with this approach. Mechanistic studies support an anionic ring opening polymerization mechanism. This living NCA polymerization method allowed rapid synthesis of polypeptide libraries for high-throughput functional screening.

Ring-opening polymerizations of α-amino acid N-carboxyanhydrides to form polypeptides are usually sensitive to moisture, slow and can undergo side reactions. Here the authors use lithium hexamethyldisilazide to initiate α-amino acid N-carboxyanhydride polymerizations that is very fast and can be conducted in an open vessel.

A S-Sn Lewis Pair-Mediated Ring-Opening Polymerization of α-Amino Acid N-Carboxyanhydrides: Fast Kinetics, High Molecular Weight, and Facile Bioconjugation

The rapid and controlled generation of polypeptides with ultrahigh molecular weight (MW) and well-defined chain end functionality has been a great challenge. To tackle this problem, we report here an initiation system based on a S-Sn Lewis pair, trimethylstannyl phenyl sulfide (PhS-SnMe₃), for the ring-opening polymerization (ROP) of α-amino acid N-carboxyanhydrides (NCAs). This initiator displays a strong solvent effect, and can yield polypeptides with high MW (>1.0 × 10⁵ g·mol^-1) and low polydispersity index within a few hours. The MWs of the obtained polypeptides are strongly dependent on the THF/DMF ratio. The polymerization follows a typical first-order kinetic character with respect to the monomer concentration in mixed THF and DMF. Moreover, a highly reactive phenyl thioester is in situ generated at the C-terminus of the polypeptides, which is readily accessible for native chemical ligation affording high MW and site-specific protein-polypeptide conjugates. Together, this initiator sheds light on regulating the ROP of NCAs via appropriate Lewis pair and solvent selection, and is particularly useful in preparing ultrahigh MW polypeptides within a short period of time.

Synthesis and Cell Interaction of Statistical l-Arginine-Glycine-l-Aspartic Acid Terpolypeptides

Copolymerizations and terpolymerizations of N-carboxyanhydrides (NCAs) of glycine (Gly), N_δ-carbobenzyloxy-l-ornithine (Z-Orn), and β-benzyl-l-aspartate (Bz-Asp) were investigated. In situ ¹H NMR spectroscopy was used to monitor individual comonomer consumptions during binary and ternary copolymerizations. The six relevant reactivity ratios were determined from copolymerizations of the NCAs of amino acids via nonlinear least-squares curve fitting. The reactivity ratios were subsequently used to maximize the occurrence of the Asp-Gly-Orn ( DGR') sequence in the terpolymers. Terpolymers with variable probability of occurrence of DGR' were prepared in the lab. Subsequently, the ornithine residues on the terpolymers were converted to l-arginine (R) residues via guanidination reaction after removal of the protecting groups. The resulting DGR terpolymers translate to traditional peptides and proteins with variable RGD content, due to the convention in nomenclature that peptides are depicted from N- to C-terminus, whereas the NCA ring-opening polymerization is conducted from C- to N-terminus. The l-arginine containing terpolymers were evaluated for cell interaction, where it was found that neuronal cells display enhanced adhesion and process formation when plated in the presence of statistical DGR terpolymers.

Synthesis and Application of Aurophilic Poly(Cysteine) and Poly(Cysteine)-Containing Copolymers

The redox capacity, as well as the aurophilicity of the terminal thiol side groups, in poly(Cysteine) lend a unique characteristic to this poly(amino acid) or polypeptide. There are two major application fields for this polymer: (i) biomedical applications in drug delivery and surface modification of biomedical devices and (ii) as coating for electrodes to enhance their electrochemical sensitivity. The intended application determines the synthetic route for p(Cysteine). Polymers to be used in biomedical applications are typically polymerized from the cysteine N-carboxyanhydride by a ring-opening polymerization, where the thiol group needs to be protected during the polymerization. Advances in this methodology have led to conditions under which the polymerization progresses as living polymerization, which allows for a strict control of the molecular architecture, molecular weight and polydispersity and the formation of block copolymers, which eventually could display polyphilic properties. Poly(Cysteine) used as electrode coating is typically polymerized onto the electrode by cyclic voltammetry, which actually produces a continuous, pinhole-free film on the electrode via the formation of covalent bonds between the amino group of Cysteine and the carbon of the electrode. This resulting coating is chemically very different from the well-defined poly(Cysteine) obtained by ring-opening polymerizations. Based on the structure of cysteine a significant degree of cross-linking within the coating deposited by cyclic voltammetry can be assumed. This manuscript provides a detailed discussion of the ring-opening polymerization of cysteine, a brief consideration of the role of glutathione, a key cysteine-containing tripeptide, and examples for the utilization of poly(Cysteine) and poly(Cysteine)-containing copolymers, in both, the biomedical as well as electrochemical realm.

Process signatures in glatiramer acetate synthesis: structural and functional relationships

Glatiramer Acetate (GA) is an immunomodulatory medicine approved for the treatment of multiple sclerosis, whose mechanisms of action are yet to be fully elucidated. GA is comprised of a complex mixture of polypeptides with different amino acid sequences and structures. The lack of sensible information about physicochemical characteristics of GA has contributed to its comprehensiveness complexity. Consequently, an unambiguous determination of distinctive attributes that define GA is of highest relevance towards dissecting its identity. Herein we conducted a study of characteristic GA heterogeneities throughout its manufacturing process (process signatures), revealing a strong impact of critical process parameters (CPPs) on the reactivity of amino acid precursors; reaction initiation and polymerization velocities; and peptide solubility, susceptibility to hydrolysis, and size-exclusion properties. Further, distinctive GA heterogeneities were correlated to defined immunological and toxicological profiles, revealing that GA possesses a unique repertoire of active constituents (epitopes) responsible of its immunological responses, whose modification lead to altered profiles. This novel approach established CPPs influence on intact GA peptide mixture, whose physicochemical identity cannot longer rely on reduced properties (based on complete or partial GA degradation), providing advanced knowledge on GA structural and functional relationships to ensure a consistent manufacturing of safe and effective products.

One-pot synthesis of alternating peptides exploiting a new polymerization technique based on Ugi's 4CC reaction

A catalyst-free, one-pot synthetic technique for alternating peptides was developed on the basis of Ugi's 4 component condensation reaction.

Polypeptides by light: photo-polymerization of N-carboxyanhydrides (NCA)

Synthesis of well-defined synthetic polypeptides by in situ UV-triggered formation of amine initiators.

Synthetic Biomaterials from Metabolically Derived Synthons

The utility of metabolic synthons as the building blocks for new biomaterials is based on the early application and success of hydroxy acid based polyesters as degradable sutures and controlled drug delivery matrices. The sheer number of potential monomers derived from the metabolome (e.g., lactic acid, dihydroxyacetone, glycerol, fumarate) gives rise to almost limitless biomaterial structural possibilities, functionality, and performance characteristics, as well as opportunities for the synthesis of new polymers. This review describes recent advances in new chemistries, as well as the inventive use of traditional chemistries, toward the design and synthesis of new polymers. Specific polymeric biomaterials can be prepared for use in varied medical applications (e.g., drug delivery, tissue engineering, wound repair, etc.) through judicious selection of the monomer and backbone linkage.

Directed Growth of Polymer Nanorods Using Surface-Initiated Ring-Opening Polymerization of N-Allyl N-Carboxyanhydride

A stepwise chemistry route was used to prepare arrays of polymer nanostructures of poly(N-allyl glycine) on Si(111) using particle lithography. The nanostructures were used for studying surface reactions with advanced measurements of atomic force microscopy (AFM). In the first step to fabricate the surface platform, isolated nanopores were prepared within a thin film of octadecyltrichlorosilane (OTS). The OTS served as a surface resist, and the areas of nanopores provided multiple, regularly shaped sites for further reaction. An initiator, (3-aminopropyl)triethoxysilane (APTES), was grown selectively inside the nanopores to define sites for polymerization. The initiator attached selectively to the sites of nanopores indicating OTS prevented nonspecific adsorption. Surface-initiated ring-opening polymerization of N-allyl N-carboxyanhydride with APTES produced polymer nanorods on the nanodots of APTES presenting amine functional groups. The surface changes for each step were monitored using high resolution atomic force microscopy (AFM). Slight variations in the height of the poly(N-allyl glycine) nanorods were observed which scale correspondingly to the initial dimensions of nanopores. The distance between adjacent polymer nanorods was controlled by the size of mesoparticle masks used in the experiment. This surface platform has potential application in biotechnology for smart coatings or biosensors.

Synthesis and characterization of conducting polymers containing polypeptide and ferrocene side chains as ethanol biosensors

A novel approach for the fabrication of a biosensor from a conducting polymer bearing polypeptide segments and ferrocene moieties is reported.

Elucidation of the structure of poly(γ-benzyl-l-glutamate) nanofibers and gel networks in a helicogenic solvent

The synthesis, characterization, self-assembly, and gel formation of poly(γ-benzyl-l-glutamate) (PBLG) in a molecular weight range from ca. 7,000–100,000 g/mol and with narrow molecular weight distribution are described. The PBLG is synthesized by the nickel-mediated ring-opening polymerization and is characterized by size-exclusion chromatography coupled with multiple-angle laser light scattering, NMR, and Fourier transform infrared spectroscopy. The self-assembly and thermoreversible gel formation in the helicogenic solvent toluene is investigated by transmission electron microscopy, atomic force microscopy, small-angle X-ray scattering, and synchrotron powder X-ray diffraction. At concentrations significantly below the minimum gelation concentration, spherical aggregates are observed. At higher concentrations, gels are formed, which show a 3D network structure composed of nanofibers. The proposed self-assembly mechanism is based on a distorted hexagonal packing of PBLG helices parallel to the axis of the nanofiber. The gel network forms due to branching and rejoining of bundles of PBLG nanofibers. The network exhibits uniform domains with a length of 200 ± 42 nm composed of densely packed PBLG helices.

Silica-polypeptide composite particles: controlling shell growth

A method is presented for preparing core-shell silica-polypeptide composite particles with variable and controllable shell growth. The procedure is demonstrated using poly(carbobenzoxy-L-lysine) and poly(benzyl-L-glutamate); after deprotection, these can lead to the most common basic and acidic homopolypeptides, poly(L-lysine) and poly(L-glutamic acid). Control over shell thickness is made possible by sequential addition of N-carboxyanhydride peptide monomer to surfaces that have been functionalized with an amino initiator combined with a surface passivation agent. This results in a series of particles having different shell thicknesses. Variation of shell thickness was evident both in light scattering and in thermogravimetric assays. The shells were visible by transmission electron microscopy; these images along with light scattering measurements suggest the polymers in the shells are highly solvated.

Bis[2-(2H-benzotriazol-2-yl)-4-methylphenolato]palladium(II)

In the title complex, [Pd(C(13)H(10)N(3)O)(2)], the Pd(II) atom is tetra-coordinated by two N atoms and two O atoms from two bidentate 2-(2H-benzotriazol-2-yl)-4-methylphenolate ligands, forming a square-planar environment. The asymmetric unit contains one half mol-ecule in which the Pd atom lies on a centre of symmetry.

Glucose-based amphiphilic telomers designed to keep membrane proteins soluble in aqueous solutions: synthesis and physicochemical characterization

A novel class of nonionic amphipols (NAPols) designed to handle membrane proteins in aqueous solutions has been synthesized, and its solution properties have been examined. These were synthesized through free radical cotelomerization of glucose-based hydrophilic and amphiphilic monomers derived from tris(hydroxymethyl)acrylamidomethane using azobisisobutyronitrile as the initiator and thiol as the transfer agent. The molecular weight and the hydrophilic/lipophilic balance of the cotelomers were modulated by varying the thiol/monomers and the hydrophilic monomer/amphiphilic monomer ratios, respectively, and were characterized by 'H NMR, UV, gel permeation chromatography, and Fourier transform infrared spectroscopy. Their physicochemical properties in aqueous solution were studied by dynamic light scattering, aqueous size-exclusion chromatography, analytical ultracentrifugation, and surface-tension measurements. NAPols are highly soluble in water and form, within a large concentration range, well-defined supramolecular assemblies with a diameter of approximately 6-7 nm, a narrow particle size distribution, and an average molecular weight close to 50 x 10(3) g x mol(-1). Varying the hydrophilic/amphiphilic monomer ratio of NAPols in the range of 3.0-4.9, the degree of polymerization in the range of 51-78, and the resulting average molar mass in the range of 20-29 x 10(3) g x mol(-1) has little incidence on their solution properties. Glucose-based NAPols efficiently kept soluble in aqueous solutions two test membrane proteins: bacteriorhodopsin and the transmembrane domain of Escherichia coli's outer membrane protein A.

Improved synthesis of O-linked, and first synthesis of S- linked, carbohydrate functionalised N-carboxyanhydrides (glycoNCAs)

An improved method for the synthesis of glycosylated N-carboxyanhydrides, which are monomers for glycopeptide synthesis, is presented.

Polypeptides and 100 years of chemistry of alpha-amino acid N-carboxyanhydrides

Syntheses and polymerizations of alpha-amino acid N-carboxyanhydrides (NCAs) were reported for the first time by Hermann Leuchs in 1906. Since that time, these cyclic and highly reactive amino acid derivatives were used for stepwise peptide syntheses but mainly for the formation of polypeptides by ring-opening polymerizations. This review summarizes the literature after 1985 and reports on new aspects of the polymerization processes, such as the formation of cyclic polypeptides or novel organometal catalysts. Polypeptides with various architectures, such as diblock, triblock, and multiblock sequences, and star-shaped or dendritic structures are also mentioned. Furthermore, lyotropic and thermotropic liquid-crystalline polypeptides will be discussed and the role of polypeptides as drugs or drug carriers are reviewed. Finally, the hypothetical role of NCAs in molecular evolution on the prebiotic Earth is discussed.