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.