Hydrogen-bonding structure of serine side chains in Bombyx mori and Samia cynthia ricini silk fibroin determined by solid-state H-2 NMR

Deuterium solid-state NMR was used to study the dynamics and molecular stru cture of the serine (Ser) side chains in silk fibroin from Bombyx mori and from Samia cynthia ricini. Samples were selectively labeled with [3,3-H-2(2 )]Ser, and the H-2 NMR powder spectra were analyzed by line shape simulatio n. Two types of motion could be characterized quantitatively: one component undergoing a rapid three-site jump (25%) and a second component representi ng a slow exchange between sites with unequal occupancies and with a small amplitude of libration (75%). From these results, it is concluded that appr oximately 75% of the Ser residues contribute to the formation of hydrogen b onds in both kinds of silk fibroin from B. mori and S. cynthia ricini. Furt hermore, uniaxially oriented silk fibres were used to determine the side-ch ain conformation and the orientational distribution of the Ser residues in the slow motional component of B. mori sills, fibroin. The gauche+ conforme r around N-C-alpha-C-beta-O was found to be dominant, suggesting that the h ydroxyl groups of Ser interact; with carbonyl groups on adjacent chains and thereby contribute to the intermolecular hydrogen-bonding network of the f iber.