Complex Formation of Collagen Model Peptides with Polyelectrolytes and Stabilization of the Triple Helical Structure

Small-angle X-ray scattering (SAXS) measurements were made for three collagen model peptides, H–(Gly-Pro-4-(R)-Hyp)9–OH, H–(Pro-4-(R)-Hyp-Gly)9–OH, and H–(Gly-4-(R)-Hyp-4-(R)-Hyp)9–OH with or without sodium polyacrylate (NaPAA) in 20 mM and 100 mM aqueous NaCl at 15 and 75 °C. At 15 °C, almost all triple helical peptides form a complex with NaPAA when the molar ratio of acrylic acid unit to peptide molecules is larger than 10 whereas they are molecularly dispersed at 75 °C. Furthermore, the attached triple helices appreciably extend the main chain of NaPAA, and the radius of gyration for the complex is at most twice larger than the single NaPAA chain. Circular dichroism measurements demonstrated that the complexation noticeably stabilizes the triple helical structure and the melting midpoint temperature T m is 2–10 °C higher than that for the solution without NaPAA. This stabilization was also observed for negatively charged sodium carboxymethylamylose and sodium hyaluronate as well as for positively charged poly(vinyl ammonium) chloride, but no appreciable stabilization of the triple helical structure by the polyelectrolytes was observed for an end-capped peptide Ac–(Gly-Pro-4-(R)-Hyp)9-NH2. These results indicate that the complex formation is due to the electrostatic attraction between polyelectrolytes and the opposite charges at the end of uncapped triple helical peptides.