Hydrogen-Bonding Structure of Serine Side Chains in Bombyx mori and Samia cynthia ricini Silk Fibroin Determined by Solid-State 2H NMR

Deuterium solid-state NMR was used to study the dynamics and molecular structure of the serine (Ser) side chains in silk fibroin from Bombyx mori and from Samia cynthia ricini. Samples were selectively labeled with [3,3- 2 H 2 ]Ser, and the 2 H NMR powder spectra were analyzed by line shape simulation. Two types of motion could be characterized quantitatively: one component undergoing a rapid three-site jump (25%) and a second component representing a slow exchange between sites with unequal occupancies and with a small amplitude of libration (75%). From these results, it is concluded that approximately 75% of the Ser residues contribute to the formation of hydrogen bonds in both kinds of silk fibroin from B. mori and S. cynthia ricini. Furthermore, uniaxially oriented silk fibers were used to determine the side-chain conformation and the orientational distribution of the Ser residues in the slow motional component of B. mori silk fibroin. The gauche+ conformer around N-C α -C β -O was found to be dominant, suggesting that the hydroxyl groups of Ser interact with carbonyl groups on adjacent chains and thereby contribute to the intermolecular hydrogen-bonding network of the fiber.