Synthesis of poly(pro-hyp-gly)_n by direct polycondensation of (pro-hyp-gly)_n, where n = 1, 5, and 10, and stability of the triple-helical structure

Pro-Hyp-Gly is a characteristic amino acid sequence found in fibrous collagens, and (Pro-Hyp-Gly)₁₀, which has been widely used as a collagen-model peptide, forms a stable triple-helical structure. Here, we synthesized polypeptides consisting of the Pro-Hyp-Gly sequence by direct polycondensation of (Pro-Hyp-Gly)_n, where n = 1, 5, and 10, using 1-hydroxybenzotriazole and 1-ethyl-3-(3-dimethyl-aminopropyl)-carbodiimide hydrochloride in both phosphate buffer (pH = 7.4) and dimethylsulfoxide (DMSO) solutions for 48 h at 20°C. The reaction of (Pro-Hyp-Gly)₅ and (Pro-Hyp-Gly)₁₀ in DMSO successfully gave polypeptides with molecular weights over 10,000, whereas low molecular weight products were obtained by reaction in phosphate buffer (pH = 7.4). In contrast, Pro-Hyp-Gly at a concentration of 50 mg/mL in phosphate buffer (pH = 7.4) gave polypeptides with molecular weights over 10,000. The Fourier transform infrared (FTIR) and ¹H nuclear magnetic resonance (NMR) spectra of poly(Pro-Hyp-Gly) ₁₀ revealed that the polymerization of (Pro-Hyp-Gly)₁₀ described in this report had no side reactions. Each polypeptide obtained shows a collagen-like triple-helical structure, and the triple-helical structures of poly(Pro-Hyp-Gly) and poly(Pro-Hyp-Gly)₁₀ were stable up to T = 80°C, which suggests that the high molecular weight promotes stability of the triple-helical structure, in addition to the high Hyp content. Furthermore, transmission electron microscopy (TEM) observations show that poly(Pro-Hyp-Gly)₁₀ aggregates to form nanofiber-like structures about 10 nm in width, which suggests that a Pro-Hyp-Gly repeating sequence contains enough information for triple-helix formation, and for subsequent nanofiber-like structure formation.