Effects of salt concentrations on the structural transitions of peptide-amphiphile solution

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Effects of salt concentrations on the structural transitions of peptide-amphiphile solution Takahiro Otsuka and Atsushi Hotta Graduate School of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan ABSTRACT A new peptide amphiphile (PA) called C16-W3K has hierarchical structures, presenting unique solution states, micelle structures, and secondary structures. In this work, the effects of salt (sodium dihydrogenorthophosphate) concentration on the hierarchical structural transitions of the C16-W3K solution due to its active hydrogen bonding in the peptide were discussed. In order to analyze the effects of salt on the structural transitions, the mechanical and structural analyses were conducted by viscosity measurements, transmission electron microscopy (TEM), and circular dichroic (CD) spectroscopy. It was found that the C16-W3K solutions with different salt concentrations presented different multi-scale structural transitions from spherical micelles with α-helix molecular conformations in the sol state to wormlike micelles with β-sheet conformations in the gel state. Additionally, we found that the speed of transition increased as the salt concentration increased and the conformational ratio of ȕ-sheet to Į-helix in the solutions increased with the increase in the salt concentration. INTRODUCTION The self-assembly of peptide amphiphiles (PA) has been an attractive feature of nanostructured materials, which is crucial for biological activities to construct supramolecular structures. PA is generally a block copolymer having a hydrophilic peptide segment coupled to a hydrophobic alkyl tail. Due to the hydrogen bonding in the peptide segment, PA can form several unique secondary and micelle structures. For example, the peptide segment of PA forms secondary structures such as Į-helices and ȕ-sheets [1]. As for the micelle structures, due to the non-covalent interactions that drive the self-assembly, PA in water occasionally forms spherical micelles, wormlike micelles, and vesicles. At the same time, PA would change its solution state (sol or gel state) depending on their secondary and micelle structures: PA in water self-assembles to form nanofibers originating from wormlike micelles, rod-like micelles, and/or nanoribbons, which induces gelation due to the entangled nanofibers. As mentioned above, PA solution has hierarchical three-scale structures which are closely related to each other: conformational secondary structures of peptide, self-assembled micelle structures, and macroscopic solution states. Since PA possesses high biocompatibility and a narrow distribution of molecular weight by smart synthesis, it is considered that PA is one of the most prospective materials for e.g. cell scaffolds in tissue engineering aiming functional regeneration[2]. The secondary structures of a peptide, the self-assembled structures, and the solution states of PA including the structural transitions of PA crucially depend on key chemical parameters such as PA concentration [3], temperature [3],

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