Hydration of Bombyx mori silk cocoon, silk sericin and silk fibroin and their interactions with water as studied by 13C NMR and 2H NMR relaxation

The mechanical properties of Bombyx mori silk fibers, such as elasticity and tensile strength, change remarkably upon hydration. However, changes in the local conformation and dynamics of individual amino acid residues and change in the dynamics of water molecules due to hydration are not currently well understood on the molecular level. In this work, the conformations and dynamics of the hydrated Bombyx mori silk fibers, including silk cocoon (SC), silk sericin (SS) and silk fibroin (SF), were determined after sustained immersion in water by using C-13 refocused insensitive nuclei enhanced by polarization transfer (INEPT) NMR, C-13 cross-polarization/magic angle spinning (CP/MAS) NMR and C-13 dipolar decoupled-magic angle spinning (DD/MAS) NMR. The C-13 INEPT NMR spectrum reflects their mobile domain, the C-13 CP/MAS NMR spectrum their rigid domain, and the C-13 DD/MAS NMR spectrum both domains. The mobile domain of the hydrated SC fiber originates mainly from the hydrated SS part and the rigid domain of the hydrated SC fiber from the hydrated SF part. Moreover, the dynamics of mobile water molecules interacting with the silk fiber was studied by H-2 solution NMR relaxation measurements in the silk fiber-(H2O)-H-2 system. Using an inverse Laplace transform algorithm, we were able to identify distinct mobile components in the relaxation times for (H2O)-H-2. Our measurements provide new insight relating to the characteristics of the hydrated structure of SC, SS and SF fibers, and the water molecules that interact with them in water. The information is relevant in light of current interest in the design of novel silk-based biomaterials which are usually in contact with blood and other body fluids.