Local Structure and Dynamics of Serine in the Heterogeneous Structure of the Crystalline Domain of Bombyx mori Silk Fibroin in Silk II Form Studied by 2D 13C-13C Homonuclear Correlation NMR and Relaxation Time Observation

The crystalline fraction (Cp fraction) of silk fibroin in silk II form from the silkworm Bombyx mori is the classic example of antiparallel beta-sheet and consists mainly of Ala, Ser, and Gly. In the C-13 CP/MAS NMR spectrum, the Ala C beta, Ser C alpha, and Ser C beta peaks are asymmetric, showing the heterogeneous nature of the structure, which is shown to consist of two different packing geometries, denoted domains A and B. In this work, these peaks were resolved and assigned using 2D C-13-C-13 homonuclear correlation NMR spectra collected with different dipolar- assisted rotational resonance (DARR) mixing times. The local structure and dynamics of serine in the Cp fraction are discussed in detail. Model peptides of the Cp fraction, [3-C-13]Ser(AGSGAG)(5) with different [3-C-13]Ser labeling positions, were prepared to clarify that the individual peaks of the Ser C beta carbons come from different domains, not from different positions within a chain. The dynamics of these individual peaks were studied by measuring C-13 T-1 values, which provide information on serine side chain dynamics and thus on the formation of intermolecular hydrogen bonding through the Ser OH group. Moreover, cross peaks between domains A and B were observed in the Ala C beta DARR spectrum with a long mixing time of 400 ms, indicating a close contact between the two geometries. We conclude that the crystalline region is heterogeneous, comprising two closely associated packing geometries that form separate but small domains.