A DOQSY approach for the elucidation of torsion angle distributions in biopolymers: Application to silk

Silk from the wild silkworm Samia cynthia ricini with a molecular mass of about 300 kDa consists of alternating repeats of nano-crystalline poly-(Ala) and non-crystalline glycine-rich domains. The backbone torsion angles between pairs of these two amino acids is determined by DOQSY solid-state NMR spectroscopy: the alanine-rich domains are predominantly in a β-sheet conformation, whereas the glycine-rich domains are found to be partially in an extended β-sheet conformation and partially in an approximately 31-helical conformation. In the cast film from liquid silk significantly different secondary structures were found: the alanine-rich domains are α-helical conformation, whereas the results for glycine-labelled sample are explained by a random-coil state. A detailed error analysis of the technique is presented.