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.

Liang DONG

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