Recent advances in post-polymerization modifications on polypeptides: synthesis and applications

Polypeptides, a kind of very promising biomaterial, have shown a wide range of applications due to their excellent biocompatibility, easy accessibility, and structural variability. To synthesize polypeptides with desired functions, post-polymerization modification (PPM) plays an important role in introducing novel chemical structure on their side-chains. The key of PPM strategy is to develop highly selective and efficient reactions that can couple the additional functional moieties with pre-installed side-chain functionalities on polypeptides. In this minireview, classic PPM reactions and especially their recent progresses are summarized, including different modification approaches for unsaturated alkyl group, oxygen-containing functional group, nitrogen-containing functional group, sulfur-containing functional group and other special functional group on side chains. In addition, this review also highlights the applications of structure-diversified polypeptides generated via PPM strategy in the field of biomaterial.

Structure and Oxidation Effects on Conformation and Thermoresponsiveness of the OEGylated Poly(glutamic acid)-Bearing Side-Chain Thioether Linkers

A series of side-chain thioether-linked OEGylated poly(glutamic acid) (PGAs) have been synthesized by "thiol-ene" synthetic methodology, where both the oligo-ethylene glycol (OEG) length and the hydrophobic linkers at the side chains are varied to learn how these structural features affect the secondary structure and thermoresponsive behaviors in water. Before side-chain oxidation, the structural factors affecting the α-helicity include the backbone length, the OEG length, and the hydrophobic linkers' length at the side chains; however, the OEG length plays the most crucial role among these factors because longer OEG around the peripheral side chains can stop water penetration into the backbone to disturb the intramolecular H bonds, which finally allows stabilizing the α-helix; after the oxidation, the polypeptides show increased α-helicity because of the enhanced hydrophilicity. More interestingly, a rare oxidation-induced conformation transition from the ordered β-sheet to the ordered α-helix can be achieved. In addition, only the OEGylated poly(glutamic acids) (PGAs) with shorter hydrophobic linkers and longer OEG can display the thermoresponsive properties before the oxidation but the subsequent oxidation can cause the polypeptides bearing longer hydrophobic linkers to exhibit the thermosensitivity since sulfone formation at the side chain can lead to final hydrophilicity-hydrophobicity balance. This work is meaningful to understand the secondary structure-associated solution behaviors of the synthetic polypeptides.

Using Polypeptide Bearing Furan Side Chains as a General Platform to Achieve Highly Effective Preparation of Smart Glycopolypeptide Analogue-Based Nano-Prodrugs for Cancer Treatment

Although several synthetic polypeptide-based nano-prodrugs (NPDs) have entered clinical trials for cancer treatment, achieving a highly effective production of the NPDs for clinical translation remains a challenge. Herein, we develop a typical preparation of pH/glutathione (GSH) dual-responsive glycopolypeptide analogue NPDs having a high drug capsulation/loading efficiency of ca. 93% and ca. 27% even based on ring-opening polymerization (ROP) of a novel and general furan-containing N-carboxyanhydride (NCA) monomer, which facilitates the Diels-Alder (D-A) side-chain functionalization by maleimide modified chemotherapy drug without using any reactive additives. High reactivity of the D-A reaction resulting in the high preparation efficiency of the NPDs is confirmed by ¹H NMR and density functional theory (DFT) calculations. The self-assembled properties as well as the dual-responsiveness of the NPDs are systemically studied by particle size and zeta potential assay, transmission electron microscopy and drug-delivery dynamics. The cell uptake mechanism, intracellular drug distribution, in vitro/vivo antitumor activity evaluations and the main organ damages of the NPDs are all investigated. Our work can provide a good solution to solve the inefficient fabrication of the smart synthetic polypeptide-based micelles for cancer treatment by following this general and sophisticated platform.

Self-assisted membrane-penetrating helical polypeptides mediate anti-inflammatory RNAi against myocardial ischemic reperfusion (IR) injury

Aromatically-modified helical polypeptide mediates membrane-penetrating RAGE siRNA delivery toward anti-inflammatory treatment against myocardial IR injury.

Functional biodegradable polymers via ring-opening polymerization of monomers without protective groups

Biodegradable polymers are of current interest and chemical functionality in such materials is often demanded in advanced biomedical applications. Functional groups often are not tolerated in the polymerization process of ring-opening polymerization (ROP) and therefore protective groups need to be applied. Advantageously, several orthogonally reactive functions are available, which do not demand protection during ROP. We give an insight into available, orthogonally reactive cyclic monomers and the corresponding functional synthetic and biodegradable polymers, obtained from ROP. Functionalities in the monomer are reviewed, which are tolerated by ROP without further protection and allow further post-modification of the corresponding chemically functional polymers after polymerization. Synthetic concepts to these monomers are summarized in detail, preferably using precursor molecules. Post-modification strategies for the reported functionalities are presented and selected applications highlighted.

Diphenylpyrenylamine-functionalized polypeptides: secondary structures, aggregation-induced emission, and carbon nanotube dispersibility

In this study we prepared-through ring-opening polymerization of γ-benzyl-l-glutamate N-carboxyanhydride (BLG-NCA) initiated by N,N-di(4-aminophenyl)-1-aminopyrene (pyrene-DPA-2NH2)-poly(γ-benzyl-l-glutamate) (PBLG) polymers with various degrees of polymerization (DP), each featuring a di(4-aminophenyl)pyrenylamine (DPA) luminophore on the main backbone. The secondary structures of these pyrene-DPA-PBLG polypeptides were investigated using Fourier transform infrared spectroscopy and wide-angle X-ray diffraction, revealing that the polypeptides with DPs of less than 19 were mixtures of α-helical and β-sheet conformations, whereas the α-helical structures were preferred for longer chains. Interestingly, pyrene-DPA-2NH2 exhibited weak photoluminescence (PL), yet the emission of the pyrene-DPA-PBLG polypeptides was 16-fold stronger, suggesting that attaching PBLG chains to pyrene-DPA-2NH2 turned on a radiative pathway for the non-fluorescent molecule. Furthermore, pyrene-DPA-2NH2 exhibited aggregation-caused quenching; in contrast, after incorporation into the PBLG segments with rigid-rod conformations, the resulting pyrene-DPA-PBLG polypeptides displayed aggregation-induced emission. Transmission electron microscopy revealed that mixing these polypeptides with multiwalled carbon nanotubes (MWCNTs) in DMF led to the formation of extremely dispersible pyrene-DPA-PBLG/MWCNT composites. The fabrication of MWCNT composites with such biocompatible polymers should lead to bio-inspired carbon nanostructures with useful biomedical applications.

Strategies to Fabricate Polypeptide-Based Structures via Ring-Opening Polymerization of N-Carboxyanhydrides

In this review, we provide a general and clear overview about the different alternatives reported to fabricate a myriad of polypeptide architectures based on the ring-opening polymerization of N-carbonyanhydrides (ROP NCAs). First of all, the strategies for the preparation of NCA monomers directly from natural occurring or from modified amino acids are analyzed. The synthetic alternatives to prepare non-functionalized and functionalized NCAs are presented. Protection/deprotection protocols, as well as other functionalization chemistries are discussed in this section. Later on, the mechanisms involved in the ROP NCA polymerization, as well as the strategies developed to reduce the eventually occurring side reactions are presented. Finally, a general overview of the synthetic strategies described in the literature to fabricate different polypeptide architectures is provided. This part of the review is organized depending on the complexity of the macromolecular topology prepared. Therefore, linear homopolypeptides, random and block copolypeptides are described first. The next sections include cyclic and branched polymers such as star polypeptides, polymer brushes and highly branched structures including arborescent or dendrigraft structures.

Engineering the Aromaticity of Cationic Helical Polypeptides toward "Self-Activated" DNA/siRNA Delivery

The development of potent yet nontoxic membrane-penetrating materials is in high demand for effective intracellular gene delivery. We have recently developed α-helical polypeptides which afford potent membrane activities to facilitate intracellular DNA delivery via both endocytosis and the nonendocytic "pore formation" mechanism. Endocytosis will cause endosomal entrapment of the DNA cargo, while excessive "pore formation" would cause appreciable cytotoxicity. Additionally, helical polypeptides with stiff, rodlike structure suffer from low siRNA binding affinity. To address such critical issues, we herein incorporated various aromatic domains (benzyl, naphthyl, biphenyl, anthryl, and pyrenyl) into the side-chain terminals of guanidine-rich, helical polypeptides, wherein the flat-rigid shape, π-electronic structures of aromatic motifs "self-activated" the membrane-penetrating capabilities of polypeptides to promote intracellular gene delivery. Benzyl (Bn)- and naphthyl (Naph)-modified polypeptides demonstrated the highest DNA uptake level that outperformed the unmodified polypeptide, P2, by ∼4 fold. More importantly, compared with P2, Bn- and Naph-modified polypeptides allowed more DNA cargos to be internalized via the nonendocytic pathway, which significantly bypassed the endosomal entrapment and accordingly enhanced the transfection efficiency by up to 42 fold, outperforming PEI 25k as the commercial reagent by 3-4 orders of magnitude. The aromatic modification also improved the siRNA condensation capability of polypeptides, achieving notably enhanced gene-silencing efficiency against tumor necrosis factor-α to treat acute hepatic inflammation. Furthermore, we revealed that aromaticity-augmented membrane activity was accompanied by comparable or even significantly reduced "pore formation" capability, thus leading to diminished cytotoxicity at high concentrations. This study therefore provides a promising approach to manipulate the membrane activities and penetration mechanisms of polycations, which overcomes the multiple critical barriers preventing effective and safe gene delivery.

Orthogonally reactive amino acids and end groups in NCA polymerization

We summarize recent strategies for the synthesis of orthogonally reactive polypeptides and polypeptoids by direct and post-polymerization approaches.

Hydrogen bonding interactions affect the hierarchical self-assembly and secondary structures of comb-like polypeptide supramolecular complexes displaying photoresponsive behavior

Hierarchical lamellae-within-lamellae structure for the PTyr/AzoPy-C16 supramolecular complex, featuring long-range-ordered lamellae arising from the PTyr within lamellae arising from AzoPy-C16 units oriented in a perpendicular manner.

Well-defined (co)polypeptides bearing pendant alkyne groups

A novel metal-free strategy, using hydrogen-bonding catalytic ring opening polymerization of alkyne-functionalized N-carboxy anhydrites of α-amino acids, was developed for the synthesis of well-defined polypeptides bearing pendant alkyne groups.

Biodegradable thermal- and redox-responsive poly(l-glutamate) with Y-shaped oligo(ethylene glycol) side-chain and tunable phase transition temperature

A series of poly(l-glutamate) bearing Y-shaped oligo(ethylene glycol)_x side-chains were synthesized via a combination of ROP and thiol–yne photoaddition. The polypeptides showed dual thermal and redox-responsive properties.

Ionic α-helical polypeptides toward nonviral gene delivery

The advent of polymeric materials has significantly promoted the development and rapid growth of various technologies in biomedical applications, such as tissue engineering and controlled drug and gene delivery. Water-soluble polypeptides bearing functional side chains and adopting stable secondary structures are a new class of functional polymeric materials of potentially broad applications in medicine and biotechnology. In this article, we summarize our recent effort on the design and synthesis of the water-soluble α-helical ionic polypeptides originally developed in our laboratory and highlight their applications in cell membrane penetration and nonviral gene/small interfering RNA (siRNA) delivery.

Comb-like poly(l-cysteine) derivatives with different side groups: synthesis via photochemistry and click chemistry, multi-responsive nanostructures, triggered drug release and cytotoxicity

A series of comb-like graft polypeptides having different side groups and tunable grafting ratios were prepared by sequential photocleavage reactions and Michael-type thiol–ene addition, which provides a promising platform for on-demand nanomedicine and cancer therapy.

Construction of a versatile and functional nanoparticle platform derived from a helical diblock copolypeptide-based biomimetic polymer

Sequential polymerization of N-carboxyanhydrides accelerated by nitrogen flow is utilized to generate a novel well-defined diblock copolypeptide (PDI = 1.08), with incorporation of alkyne-functionalized side-chain groups allowing for rapid and efficient thiol-yne click-type modifications, followed by self-assembly into nanopure water to construct a helical polypeptide-based versatile and functional nanoparticle platform.

ROMP (co)polymers with pendent alkyne side groups: post-polymerization modification employing thiol–yne and CuAAC coupling chemistries

The synthesis of a series of copolymers via ring-opening metathesis polymerization (ROMP) containing pendent trimethylsilyl-protected alkyne functional groups is described.

Stimuli responsive triblock copolymers by chain-growth polymerization from telechelic macroinitiators prepared via a step-growth polymerization

The synthesis of stimuli-responsive ABA tri-block copolymers using a step-growth polmerization followed by a chain-growth polymerization.

Polypeptide/doxorubicin hydrochloride polymersomes prepared through organic solvent-free technique as a smart drug delivery platform

Rapid and efficient side-chain functionalization of polypeptide with neighboring carboxylgroups is achieved via the combination of ring-opening polymerization and subsequent thiol-yne click chemistry. The spontaneous formation of polymersomes with uniform size is found to occur in aqueous medium via electrostatic interaction between the anionic polypeptide and cationic doxorubicin hydrochloride (DOX·HCl). The polymersomes are taken up by A549 cells via endocytosis, with a slightly lower cytotoxicity compared with free DOX ·HCl. Moreover, the drug-loaded polymersomes exhibit the enhanced therapeutic efficacy, increase apoptosis in tumor tissues, and reduce systemic toxicity in nude mice bearing A549 lung cancer xenograft, in comparison with free DOX ·HCl.

Soluble, Clickable Polypeptides from Azide-Containing N-Carboxyanhydride Monomers

We report a method for the synthesis of soluble, well-defined, azide-functionalized polypeptides via living polymerization of new azide-containing amino acid N-carboxyanhydride (NCA) monomers. Homo and diblock azidopolypeptides were prepared with controlled segment lengths using (PMe₃)₄Co initiator and were subsequently modified by reaction with functional alkyne reagents. The azide groups were found to be quantitatively converted to the corresponding triazole derivatives, and the functionalized polymers were obtained in high yield. This methodology provides a facile and straightforward method for preparation of functional and side-chain reactive, high molecular weight polypeptides.

Facile functionalization of polyesters through thiol-yne chemistry for the design of degradable, cell-penetrating and gene delivery dual-functional agents

Synthesis of polyesters bearing pendant amine groups with controlled molecular weights and narrow molecular weight distributions was achieved through ring-opening polymerization of 5-(4-(prop-2-yn-1-yloxy)benzyl)-1,3-dioxolane-2,4-dione, an O-carboxyanhydride derived from tyrosine, followed by thiol-yne "click" photochemistry with 2-aminoethanethiol hydrochloride. This class of biodegradable polymers displayed excellent cell penetration and gene delivery properties with low toxicities.

Thiol-yne coupling: revisiting old concepts as a breakthrough for up-to-date applications

Radical thiol-yne coupling (TYC) has emerged as one of the most appealing click chemistry procedures, appearing as a sound candidate for replacing/complementing other popular click reactions such as the thiol-ene coupling (TEC) and the Cu-catalysed azide-alkyne cycloaddition (CuAAC). Radical TYC is indeed a metal-free reaction suitable for biomedical applications, and its mechanistic features often make it more efficient than its TEC sister reaction and more suitable for multifaceted derivatisations in the materials chemistry and bioconjugation realms. This article reviews the fascinating results obtained in those fields in very recent years.