Effects of Secondary Structures and pH on the Self-Assembly of Poly(ethylene glycol)-b-polytyrosine.

Different from conventional synthetic polymers, polypeptides exhibit a distinguishing characteristic of adopting specific secondary structures, including random coils, α-helixes, and ß-sheets. The conformation determines the rigidity and solubility of polypeptide chains, which further direct the self-assembly and morphology of the nanostructures. We studied the effect of distinct secondary structures on the self-assembly behavior of polytyrosine (PTyr)-derived amphiphilic copolymers. Two block copolymers of enantiopure poly(ethylene glycol)-b-poly(l-tyrosine) (PEG-b-P(l-Tyr)) and racemic poly(ethylene glycol)-b-poly(dl-tyrosine) (PEG-b-P(dl-Tyr)) were synthesized through the ring-opening polymerization of l-tyrosine N-thiocarboxyanhydride (l-Tyr-NTA) and dl-tyrosine N-thiocarboxyanhydride (dl-Tyr-NTA), respectively, by using poly(ethylene glycol) amine as the initiator. PEG44-b-P(l-Tyr)10 adopts a ß-sheet conformation and self-assembles into rectangular nanosheets in aqueous solutions, while PEG44-b-P(dl-Tyr)9 is primarily in a random coil conformation with a tiny content of ß-sheet structures, which self-assembles into sheaf-like nanofibrils. A pH increase results in the ionization of phenolic hydroxyl groups, which decreases the ß-sheet content and increases the random coil content of the PTyr segments. Accordingly, PEG44-b-P(l-Tyr)10 and PEG44-b-P(dl-Tyr)9 self-assemble to form slender nanobelts and twisted nanoribbons, respectively, in alkaline aqueous solutions. The secondary structure-driven self-assembly of PTyr-derived copolymers is promising to construct filamentous nanostructures, which have potential for applications in controlled drug release.

An Inspection into Multifarious Ways to Synthesize Poly(Amino Acid)s.

Poly(α-amino acid)s (PAAs) attract growing attention due to their essential role in the application as biomaterials. To synthesize PAAs with desired structures and properties, scientists have developed various synthetic techniques with respective advantages. Here, different approaches to preparing PAAs are inspected. Basic features and recent progresses of these methods are summarized, including polymerizations of amino acid N-carboxyanhydrides (NCAs), amino acid N-thiocarboxyanhydrides (NTAs), and N-phenoxycarbonyl amino acids (NPCs), as well as other synthetic routes. NCA is the most classical monomer to prepare PAAs with high molecular weights (MWs). NTA polymerizations are promising alternative pathways to produce PAAs, which can tolerate nucleophiles including alcohols, mercaptans, carboxyl acids, and water. By various techniques including choosing appropriate solvents or using organic acids as promoters, NTAs polymerize to produce polypeptoids and polypeptides with narrow dispersities and designed MWs up to 55.0 and 57.0 kg mol-1 , respectively. NPC polymerizations are phosgene-free ways to synthesize polypeptides and polypeptoids. For the future prospects, detail investigations into polymerization mechanisms of NTA and NPC are expected. The synthesis of PAAs with designed topologies and assembly structures is another intriguing topic. The advantages and unsettled problems in various synthetic ways are discussed for readers to choose appropriate approaches for PAAs.

Helical Self-Assembly of Amphiphilic Chiral Azobenzene Alternating Copolymers.

Imposing chirality to supramolecular architectures is an important step forward toward understanding and utilization of chiral nanomaterials. This article reports the self-assembly of amphiphilic chiral alternating copolymers of poly(binaphthyl azobenzene-alt-hexaethylene glycol) (P(BNPAzo-alt-EG6)) into helical supramolecular rods. Unlike conventional chiral assembly of copolymers largely through intermolecular organization, the intrachain stacking of chiral units along the main chain into single molecular micelles with amplified axial chirality of binaphthyl is key to the formation of helical supramolecular rods, which takes advantage of the particular chiral unit and soft unit alternating topological structure of the backbones. Moreover, the supramolecular self-assembly is light reversible because the azobenzene rings in the backbone scarcely execute trans- to cis-isomerization upon UV irradiation, and therefore the supramolecular rods keep their sublevel chirality even though the helical appearance was destroyed. This work paves an effective route to construct and regulate chiral supramolecular architectures and reveals an insight into natural and artificial chiral self-assembly.

Polymersomes with aggregation-induced emission based on amphiphilic block copolypeptoids.

Biocompatible polymersomes are prepared from amphiphilic block copolypeptoids with aggregation-induced emission, where the hydrophobic block P(TPE-NAG) is a tetraphenylethylene (TPE)-modified poly(N-allylglycine) and the hydrophilic block is polysarcosine. These nanoparticles are non-cytotoxic and show strong fluorescence emission in aqueous solution.

Oxidation-Sensitive Polymersomes Based on Amphiphilic Diblock Copolypeptoids.

Stimuli-responsive polymersomes formed by amphiphilic block copolymers have attracted substantial attention as smart and robust containers for drug delivery and nano/microreactors. Biosourced amphiphilic diblock copolypeptoids were developed that can self-assemble into oxidation-responsive unilamellar vesicles. These vesicles can burst under the action of reactive oxygen species which can be the hydrogen peroxide or the singlet oxygen produced by light-activation of a photosensitizer with spatiotemporal control. Polysarcosine (PSar, also called poly(N-methyl glycine)) was selected as the hydrophilic block because of its resistance to protein adsorption and low toxicity, similar to poly(ethylene glycol) (PEG). We designed and synthesized poly(N-3-(methylthio)propyl glycine) as the hydrophobic block. Its polyglycine backbone is the same as that of PSar, and especially, its hydrophobic N-substituents, thioether side chains, can be oxidized to hydrophilic sulfoxides. These oxidation-responsive polymersomes entirely based on N-substituted poly(amino acid)s were biocompatible as confirmed by cell viability tests and may find applications in drug delivery, biosensing, biodetection, and nano/microreactors.

Identifying the Hydrolysis of Carbonyl Sulfide as a Side Reaction Impeding the Polymerization of N-Substituted Glycine N-Thiocarboxyanhydride.

Polypeptoids are noticeable biological materials due to their versatile properties and various applications in drug delivery, surface modification, self-assembly, etc. N-Substituted glycine N-thiocarboxyanhydrides (NNTAs) are more stable monomers than the corresponding N-carboxyanhydrides (NNCAs) and enable one to prepare polypeptoids via ring-opening polymerization even in the presence of water. However, larger amounts of water (>10,000 ppm) cause inhibition of the polymerization. Herein, we discover that during polymerization hydrogen sulfide evolves from the hydrolysis of carbonyl sulfide, which is the byproduct of ring-opening reaction, and reacts with NNTA to produce cyclic oligopeptoids. The capture of N-ethylethanethioic acid as an intermediate product confirms the reaction mechanism together with density functional theory quantum computational results. By bubbling the polymerization solution with argon, the side reaction can be suppressed to allow the synthesis of polysarcosine with high molar mass ( Mn = 11,200 g/mol, D = 1.25) even in the presence of ∼10,000 ppm of water.

Fe3+@polyDOPA-b-polysarcosine, a T1-Weighted MRI Contrast Agent via Controlled NTA Polymerization.

α-Amino acid N-thiocarboxyanhydrides (NTAs) are promising cyclic monomers to synthesize polypeptides and polypeptoids via controlled ring-opening polymerizations. Superior to N-carboxyanhydrides requiring protection on hydroxyl groups, NTAs are able to tolerate such nucleophiles. In this work, we report the synthesis of NTA monomers containing unprotected phenolic hydroxyl groups of 3,4-dihydroxy-l-phenylalanine (DOPA) and l-tyrosine (Tyr). Their controlled ROPs and sequential copolymerizations with polysarcosine (PSar) yield PDOPA, PTyr, and PDOPA-b-polysarcosine (PDOPA-b-PSar) products quantitatively with designable degrees of polymerization. Micellar nanoparticles of Fe3+@PDOPA-b-PSar have been prepared thanks to the strong chelation of iron(III) cation by catechol ligands that act as T1-weighted magnetic resonance imaging (MRI) contrast agents. For instance, Fe3+@PDOPA10-b-PSar50 exhibits higher longitudinal relaxivity (r1 = 5.6 mM-1 s-1) than commercial Gd3+-based compounds. Effective MRI contrast enhancement in vivo of nude mice with a moderate duration (150 min) and 3D magnetic resonance angiography in rabbit illustrated by using volume rendering and maximal intensity projection techniques ignite the clinical application of Fe3+-based polypept(o)ides in diagnostic radiology as Gd-free MRI contrast agents.

Polysarcosine brush stabilized gold nanorods for in vivo near-infrared photothermal tumor therapy.

Gold nanorods (AuNRs) are suitable candidates for photothermal therapy in vivo, because of their excellent ability to transfer near-infrared (NIR) light into heat. However, appropriate surface should be generated on AuNRs before their in vivo application because of the low colloidal stability in complicate biological environment and relatively strong toxicity compared to their pristine stabilizer cetyltrimethylammonium bromide. In the current study, polysarcosine (PS), a non-ionic hydrophilic polypeptoid whose structure is similar to polypeptides, bearing repeating units of natural α-amino acid, was used to stabilize AuNRs due to its excellent hydrophilicity and biocompatibility. Polysarcosine with optimized molecular weight was synthesized and used to modify AuNRs by traditional ligand exchange. The grafting of PS on AuNRs was evidenced by fourier transform infrared (FTIR) spectroscopy and the alternation of surface zeta potential. The polysarcosine coated AuNRs (Au@PS) showed good stabilities in wide pH range and simulated physiological buffer with the ligand competition of dithiothreitol (DTT). The Au@PS NRs had neglectable cytotoxicity and showed efficient ablation of tumor cells in vitro. Moreover, Au@PS NRs had a longer circulation time in body that resulted in a higher accumulation in solid tumors after intravenous injection, compared to AuNRs capped with polyethylene glycol (PEG). Photothermal therapy in vivo demonstrated that the tumors were completely destroyed by single-time irradiation of NIR laser after one-time injection of the polysarcosine capped AuNRs. The Au@PS NRs did not cause obvious toxicity in vivo, suggesting promising potential in cancer therapy. STATEMENT OF SIGNIFICANCE: In current study, polysarcosine (PS), a non-ionic hydrophilic polypeptoid whose structure is similar to polypeptides, bearing repeating units of natural α-amino acid, was used to stabilize AuNRs due to its excellent hydrophilicity and biocompatibility. The polysarcosine coated AuNRs (Au@PS) showed good stabilities in wide pH range and simulated physiological buffer. The Au@PS NRs had very low cytotoxicity and showed high efficacy for the ablation of cancer cells in vitro. Moreover, Au@PS NRs had a longer circulation time in blood that led to a higher accumulation in tumors after intravenous injection, compared to AuNRs capped with polyethylene glycol (PEG). In vivo photothermal therapy showed that tumors were completely cured without reoccurrence by one-time irradiation of NIR laser after a single injection of the polysarcosine modified AuNRs.

Gold nanoparticles coated with polysarcosine brushes to enhance their colloidal stability and circulation time in vivo.

Polysarcosine (PS), a non-ionic hydrophilic polypeptoid whose structure is similar to polypeptides, bearing repeating units of natural α-amino acid, has been used to stabilize gold nanoparticles (AuNPs) due to its excellent hydrophilicity and biocompatibility. Disulfide functionalized polysarcosines with different molecular weight were synthesized and used to cap AuNPs by traditional ligand exchange. The grafting of PS on AuNPs was evidenced by Fourier transform infrared (FTIR) spectroscopy and the alternation of surface zeta potential. The polysarcosine coated AuNPs (Au@PS) showed good stabilities in wide pH range and saline condition. They had strong resistance to ligand competition of dithiothreitol (DTT). They showed good stability in serum, with a molecular weight dependent interaction pattern with proteins. The Au@PS had very low cytotoxicity and cell uptake in vitro. Based on the results in vitro, polysarcosine with molecular weight of 5kD with the best ability to stabilize AuNPs was used for in vivo test. The Au@PS had a longer circulation time in blood after intravenous injection than that of Au@PEG, indicating a better stealth-like property of polysarcosine. The Au@PS did not cause obvious toxicity in vivo, suggesting potential applications in disease diagnosis and therapy.

Poly(ε-caprolactone)-block-polysarcosine by Ring-Opening Polymerization of Sarcosine N-Thiocarboxyanhydride: Synthesis and Thermoresponsive Self-Assembly.

Biocompatible amphiphilic block copolymers composed of polysarcosine (PSar) and poly(ε-caprolactone) (PCL) were synthesized using ring-opening polymerization of sarcosine N-thiocarboxyanhydride initiated by oxyamine-ended PCL and characterized by NMR, SEC, and DSC. Self-assembling of two triblock copolymers PSar8-b-PCL28-b-PSar8 (CS7) and PSar16-b-PCL40-b-PSar16 (CS10) in dilute solution was studied in detail toward polymersome formation using thin-film hydration and nanoprecipitation techniques. A few giant vesicles were obtained by thin-film hydration from both copolymers and visualized by confocal laser scanning microscope. Unilamellar sheets and nanofibers (with 8-10 nm thickness or diameter) were obtained by nanoprecipitation at room temperature and observed by Cryo-TEM. These lamellae and fibrous structures were transformed into worm-like cylinders and spheres (D∼30-100 nm) after heating to 65 °C (>Tm,PCL). Heating CS10 suspensions to 90 °C led eventually to multilamellar polymersomes (D∼100-500 nm). Mechanism II, where micelles expand to vesicles through water diffusion and hydrophilic core forming, was proposed for polymersome formation. A cell viability test confirmed the self-assemblies were not cytotoxic.

PEG-amine-initiated polymerization of sarcosine N-thiocarboxyanhydrides toward novel double-hydrophilic PEG-b-polysarcosine diblock copolymers.

Amino acid N-thiocarboxyanhydride (NTA), the thioanalog of N-carboxyanhydride (NCA), is much more stable than NCA against moisture and heat. The convenient monomer synthesis without rigorous anhydrous requirements makes the ring-opening polymerization of NTA a competitive alternative to prepare polypeptoid-containing materials with potential of large-scale production. Polysarcosines (PSars) with high yields (>90%) and low polydispersities (<1.2) are synthesized from sarcosine N-thiocarboxyanhydride (Sar-NTA) at 60 °C initiated by primary amines including poly(ethylene glycol) amine (PEG-NH2 ). The lengths of PSar segments are controlled by various feed ratios of Sar-NTA to initiator. PEG-b-PSar products, a class of novel double-hydrophilic diblock copolymers, are effective in stabilizing oil-in-water emulsions at nano- and microscale, which demonstrates promising encapsulation applications in food, cosmetics, and drug delivery. Due to the different solubility of PEG and PSar blocks, PEG-b-PSar copolymers form micelles in organic solvents with the capability to incorporate metal cations including Cu(2+) and Ni(2+) .