Macromolecules, Vol.46, No.19, 8046-8050, 2013
Determination of Accurate H-1 Positions of (Ala-Gly)n as a Sequential Peptide Model of Bombyx mori Silk Fibroin before Spinning (Silk I)
The accurate H-1 positions of alanine-glycine alternating copolypeptide, (AG)(15) with Silk I structure were determined. For the purpose, the geometry optimization was performed starting with the atomic coordinates of the hetero atoms reported previously (Macromolecules 2005, 38, 7397-7403) and applied only for protons under periodic boundary conditions. The agreement between the calculated and observed chemical shifts of all H-1,C-13 and N-15 nuclei was excellent, indicating strongly that the determination of all the atomic-coordinate including H-1 nuclei was performed with high accuracy. Here the H-1 chemical shift was obtained by using both 1 mm microcoil MAS NMR probe-head for mass-limited solid-state samples developed by us and ultrahigh field NMR at 920 MHz. The DQ correlations in the H-1 DQMAS NMR spectra were also used to confirm the intra- and intermolecular structures obtained here. The characteristic structure of Silk I which can be easily converted to Silk II by external forces was discussed together with the generation of Silk I structure from the aqueous solution of the silk fibroin.