Transition of Conformation and Solubility in β-Sheet-Structured Poly(l-cysteine)s with Methylthio or Sulfonium Pendants

Poly(l-cysteine) Peptide Amphiphile Derivatives Containing Disulfide Bonds: Synthesis, Self-Assembly-Induced β-Sheet Nanostructures, pH/Reduction Dual Response, and Drug Release

Three disulfide bond-containing peptide amphiphiles (PA1-3) with different lengths of alkyl tails (PA1 for C₆, PA2 for C₁₂, and PA3 for C₁₈) were synthesized by ring-opening polymerization of α-amino acid N-carboxyanhydride followed by post-polymerization modification. The peptide segments were composed of 3-mercaptopropionic acid-modified poly(l-cysteine) [P(Cys-SS-CH₂CH₂COOH)]. We characterized the chemical structure and molecular parameters by ¹H NMR, ¹³C NMR, gel permeation chromatography, Fourier transform infrared spectroscopy, and circular dichroism spectroscopy. It is found that alkyl-P(Cys-SS-CH₂CH₂COOH) mainly presents a β-sheet conformation at the solid state. However, the PAs present predominant random coils at pH 7.4 in aqueous solutions. The β-sheet conformation increased dramatically when the concentration of the PA exceeded its critical micelle concentration (ca. 0.3 mg/mL for PA3), indicating the formation of self-assembly-induced β-sheet nanostructures. Elongation of the alkyl chain length or a decrease of the pH of the PA solution can promote the formation of the β-sheet conformation. The three PAs can self-assemble into spherical micelles or nanofibrous hydrogels, which can be utilized as nanocarriers for systemic drug delivery or implants for localized drug delivery, respectively. Cisplatin (CDDP) was loaded as a model medicine to examine the drug delivery potential of PA3. We found that the CDDP-loaded PA3 micelles are stable at pH 7.4, have a spherical morphology with a hydrodynamic diameter of ca. 52 nm, and accomplish pH/reduction dual-responsive release of CDDP. In addition, alkyl-P(Cys-SS-CH₂CH₂COOH) can self-assemble into nanofibrous hydrogels at pH 5.0-6.0 or upon the addition of certain metal ions and show excellent dynamic reversibility. Moreover, the CDDP-loaded PA3 hydrogel exhibits a sustained release profile and a nearly linear release over 48 h. In vitro cytotoxicity of PA3 also indicates its nontoxicity. Thus, our findings suggest that alkyl-P(Cys-SS-CH₂CH₂COOH) has great potential for both systemic and localized delivery of therapeutics.

A sulfonate-based polypeptide toward infection-resistant coatings

Multifunctional coatings have gained significant attention for their promising potential to address the issue of medical device-related infections. However, they usually have multiple components in one layer which decreases the density of functional groups on surfaces and hence reduces the biological properties. Herein, we report a mono-component and sulfonate-based anionic polypeptide coating with on-demand antibacterial activity, antifouling property, and biocompatibility. The anionic polypeptide was prepared by ring-opening polymerization of L-cysteine-based N-carboxyanhydride (NCA) with allyl groups and a subsequent thiol-ene reaction to incorporate the sulfonate pendants. It adopted a 17.1-19.5% β-sheet conformation and self-assembled into a spherical nanoparticle. The polypeptide coating showed excellent in vitro antibacterial activity against both Gram-positive (i.e., S. aureus) and Gram-negative bacteria (i.e., E. coli) with >99% killing efficacy after acidic solution treatment and prominent antifouling property and biocompatibility after weak base treatment. An in vivo study revealed that the sulfonate-based polypeptide-coated polydimethylsiloxane (PDMS) exhibited good anti-infection property and histocompatibility.

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.