Chemical Self-assembly of Textured Peptide Tapes with Nanoscale Order

초록

Multilayered amorphous mm to cm long "tapes" can be obtained as precipitates from several designed peptides with amphiphilic sequences. Acid blocks on the ends of the peptide sequences have been used to functionalize the "tapes" with inorganic ions. The peptide tapes are self-limiting in width and thickness, but grow lengthwise to form long coils. A hierarchical, or multiscale, pattern of undulations and ridges is observed on the surface of the tapes, and is correlated to the birefringence observed for the tapes. This suggests a structural origin for the texture - molecular or cluster orientation - rather than a purely topographic or environmental origin. Even though the sequences are looslely based on silks, and "silk-like" crystal structures have been obtained for these molecules under different conditions, no silk-like crystal structures are observed in the tapes. FTIR data suggest a "silk I"-like secondary structure. Strong X-ray scattering at positions corresponding to several nanometers suggests a folded structure, arranged in layers. Evidence for chiral layers (a solid version of a chiral smectic phase) is observed in high resolution SEM. The folded supersecondary structure suggested by these data tapes resembles the folded beta-sheet structures in amyloids, but the beta-strand conformation and crystalline local order are missing. The occurrence of the tapes under conditions very close to those required for beta-sheet crystallization suggests a "trapped" precursor to the folded beta-sheet structure.