Real-Time Monitoring of the Structural Transition of Bombyx mori Liquid Silk under Pressure by Solid-State NMR

Silk fibroin is stored in the silk glands of Bombyx mori silkworms as a condensed aqueous solution called liquid silk. It is converted into silk fibers at the silkworm's spinnerets under mechanical forces including shear stress and pressure. However, the detailed mechanism of the structural transition of liquid silk to silk fibers under pressure is not well understood. Magic angle spinning (MAS) in solid-state nuclear magnetic resonance (NMR) can exert pressure on liquid samples in a quantitative manner. In this study, solid-state NMR was used to quantitatively analyze the impact of pressure on the structural transition of liquid silk. A combination of ¹³C DD-MAS and CP-MAS NMR measurements enabled the conformation and dynamics of the crystalline region of the silk fibroin (both before (Silk I_p) and after (Silk II_p) the structural transition) to be detected in real time with atomic resolution. Spectral analyses proposed that the pressure-induced structural transition from Silk I_p to Silk II_p proceeds by a two-step autocatalytic reaction mechanism. The first reaction step is a nucleation step in which Silk I_p transforms to single lamellar Silk II_p, and the second is a growth step in which the single lamellar Silk II_p acts as a catalyst that reacts with Silk I_p molecules to further form Silk II_p molecules, resulting in stacked lamellar Silk II_p. Furthermore, the rate constant in the second step shows a significant pressure dependence, with an increase in pressure accelerating the formation of large stacked lamellar Silk II_p.