Determination of Accurate 1H Positionsof (Ala-Gly)n as a Sequential Peptide Model of BombyxmoriSilk Fibroin before Spinning (Silk I).

Theaccurate 1H positions of alanine-glycine alternating copolypeptide,(AG)15with Silk I structure were determined. For the purpose,the geometry optimization was performed starting with the atomic coordinatesof the hetero atoms reported previously (Macromolecules2005, 38, 7397−7403) and applied only for protons under periodic boundaryconditions. The agreement between the calculated and observed chemicalshifts of all 1H,13C and 15N nucleiwas excellent, indicating strongly that the determination of all theatomic-coordinate including 1H nuclei was performed withhigh accuracy. Here the 1H chemical shift was obtainedby using both 1 mm microcoil MAS NMR probe-head for mass-limited solid-statesamples developed by us and ultrahigh field NMR at 920 MHz. The DQcorrelations in the 1H DQMAS NMR spectra were also usedto confirm the intra- and intermolecular structures obtained here.The characteristic structure of Silk I which can be easily convertedto Silk II by external forces was discussed together with the generationof Silk I structure from the aqueous solution of the silk fibroin. [ABSTRACT FROM AUTHOR]