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

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