Determination of Accurate 1H Positions of (Ala-Gly)n as a Sequential Peptide Model of Bombyx mori Silk Fibroin before Spinning (Silk I)

The accurate 1 H 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 1 H, 13 C and 15 N nuclei was excellent, indicating strongly that the determination of all the atomic-coordinate including 1 H nuclei was performed with high accuracy. Here the 1 H 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 1 H 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.