Structure of silk I (Bombyx mori silk fibroin before spinning) in the dry and hydrated states studied using C solid-state NMR spectroscopy.

Nowadays, much attention has been paid to Bombyx mori silk fibroin (SF) by many researchers because of excellent physical properties and biocompatibility. These superior properties originate from the structure of SF and therefore, the structural analysis is a key to clarify the superiority. Here we concentrated on silk I structure (SF structure before spinning). We showed that silk I* (the structure of (GAGAGS) which is a main part of SF) is a repeated type II β-turn, neither α-helix nor random coil, from the conformation-dependent C NMR chemical shift data. This conclusion is different from that obtained using IR by many researchers. Next, the formation of silk I* structure was investigated at molecular level using C solid-state NMR spectroscopy. Three kinds of C INEPT, CP/MAS and DD/MAS NMR spectra were observed for SF, [3-C]Ser- and [3-C]Tyr-SF, the crystalline fraction obtained by chymotrypsin treatment of SF and their model peptide with silk I structures in the dry and hydrated states. Especially, the presence of the sequences containing Tyr, (((GX)GY) where X = A or V) with random coil conformations adjacent to (GAGAGS) is an essence to get water-soluble SF and the formation of silk I* structure of (GAGAGS).