Packing Structure of Antiparallel β-Sheet Polyalanine Region in a Sequential Model Peptide of Nephila clavipes Dragline Silk Studied Using 13C Solid-State NMR and MD Simulation.

Packing structures of polyalanine regions, which are considered to be the reason for the extremely high strength of spider dragline silks, were studied using a series of sequential peptides: (Glu)4GlyGlyLeuGlyGlyGlnGlyAlaGly(Ala)nGlyGlyAlaGlyGlnGlyGlyTyrGlyGly(Glu)4 (n = 3-8) using 13C solid-state NMR spectroscopy. The conformations of (Ala)n in the freeze-dried peptides changed gradually with increasing n from random coils to α-helices with partial antiparallel β-sheet (AP-β) structures. Conversely, all the insolubilized peptides, n = 6-8 after low-pH treatment and n = 4-8 after formic acid/methanol treatment, formed AP-β structures with significant amounts of staggered packing arrangements. These results are different from previously obtained results for pure alanine oligopeptides, that is, AP-β (Ala)n formed rectangular packing for less than n = 6 but staggered packings for n ≥ 7. The 13C-labeled peptides were also used to confirm the staggered packing arrangements from NMR dynamics. Furthermore, a MD simulation supported the observed results.