Observation of Silk I Conformation in Bombyx Mori Liquid Silk with NMR

Abstract

A Bombyx mori Silkworm larve keeps silk in silkglnad at high concentrated (∼20% w/w) aqueous solution state before spinning. Liquid silk is expected to form some ordered structure since liquid silk transforms to β-sheet riched silk fiber with high strength. in a moment under normal temperature and pressure. In fact, observation of α-helix-like pattern in the CD spectra of liquid silk at the concentrated state was reported. On the other hand, the 13C chemical shift of liquid silk indicate random coil although liquid silk extracted from a different kind of silkworm, S.c.ricini, which has polyalanine sequences (Ala)12 clearly shows α-helix in the three carbons of Ala residues. In this study, atomic-leveled conformation of Bombyx mori liquid silk was analyzed with several solution NMR techniques. 1H, 13C and 15N chemical shifts of repeated motifs in silk fibroin primary sequence, GAGAGS, GAGAGY, GAAS were obtained from 1H-1H NOESY, TOCSY, 13C HSQC, 15N HSQC HCACO and HNCO. The dihedral angles of these motifs were proposed with talos+ program which predicts the dihedral angles from chemical shifts. The dihedral angles of GAGAGS and GAGAGY showed repeated type II β-turn structure although that of GAAS showed random coil. The NOE signals between (i)Hα and (i+2)HN also support the type II β-turn structure for these two motifs. This is the first time that the generation of Silk I structure in the native B.mori liquid silk was detected and confirmed.