Intermolecular Packing in B. mori Silk Fibroin: Multinuclear NMR Study of the Model Peptide (Ala-Gly)15 Defines a Heterogeneous Antiparallel Antipolar Mode of Assembly in the Silk II Form

We have previously suggested that crystalline Bombyx mori silk in silk II form (the silk structure after spinning) is not a simple antiparallel β-sheet but is intrinsically heterogeneous. Using the peptide (AG)15, we have obtained the first fully assigned high resolution solid state 1H NMR spectrum. Distinct heterogeneity was observed, in both 1H and 13C CP/MAS signals. Based on these results, a new model is proposed that contains two different packing arrangements of antiparallel β-sheets. The structures were energetically minimized by CASTEP calculation and used to calculate the solid state 1H, 13C, and 15N NMR chemical shifts using the GIPAW method. This new model was supported by good agreement between the calculated and observed 1H, 13C, and 15N chemical shifts and relative 1H–1H proximities obtained from 2D 1H DQMAS experiments. We conclude that the intermolecular packing of B. mori silk fibroin has been finally resolved.