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鈥?sup>13C Homonucle

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• 作者：Keiko Okushita ; Atsushi Asano ; Michael P. Williamson ; Tetsuo Asakura
• 刊名：Macromolecules
• 出版年：2014
• 出版时间：July 8, 2014
• 年：2014
• 卷：47
• 期：13
• 页码：4308-4316
• 全文大小：506K
• ISSN：1520-5835

文摘

The crystalline fraction (Cp fraction) of silk fibroin in silk II form from the silkworm Bombyx mori is the classic example of antiparallel 尾-sheet and consists mainly of Ala, Ser, and Gly. In the ¹³C CP/MAS NMR spectrum, the Ala C尾, Ser C伪, and Ser C尾 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 ¹³C鈥?sup>13C 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, [3-¹³C]Ser-(AGSGAG)₅ with different [3-¹³C]Ser labeling positions, were prepared to clarify that the individual peaks of the Ser C尾 carbons come from different domains, not from different positions within a chain. The dynamics of these individual peaks were studied by measuring ¹³C T₁ 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尾 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.