Determination of the mutual orientation of the 15N and 13C NMR chemical shift tensors of 13-15N double labeled model peptides for silk fibroin from the dipolar-coupled powder patterns

Abstract The 15 N and 13 C chemical shift tensors, and the orientation of the principal axis system relative to the molecular symmetry axes were determined for 15 N and 13 C carbonyl carbon sites of 13 C 15 N double labeled model peptides for Bombyx mori silk fibroin, that is, Boc-[1- 13 C]Ala[ 15 N]Gly-OMe, Boc-[1- 13 C]Ala[ 15 N]GlyAlaGly-OPac, Boc-AlaGly[1- 13 C]Ala[ 15 N]GlyAlaGly-OPac, Boc-[1- 13 C]Gly[ 15 N]AlaGlyAla-OPac, Boc-GlyAla[1- 13 C]Gly[ 15 N]AlaGlyAla-OPac and Boc-[1- 13 C]Gly[ 15 N]ValGlyAla-OPac, where Boc is t-butoxycarbonyl, OMe is methyl ester, OPac is phenacyl ester, Ala is alanine, Gly is glycine and Val is valine. From the comparisons of the 15 N chemical shift tensors and the orientations of the principal axis system relative to the molecular symmetry axes among three compounds having [1- 13 C]Ala[ 15 N]Gly units, it is concluded that the intermolecular interactions such as hydrogen bonding are different between Boc-[1- 13 C]Ala[ 15 N]Gly-OMe and two compounds, Boc-[1- 13 C]Ala[ 15 N]GlyAlaGly-OPac and Boc-AlaGly[1- 13 C]Ala[ 15 N]GlyAlaGly-OPac although the latter two compounds have similar structures. A similar conclusion has also been obtained from the 13 C chemical shift tensors of these compounds.