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

The N-15 and C-13 chemical shift tensors, and the orientation of the principal axis system relative to the molecular symmetry axes were determined for N-15 and C-13 carbonyl carbon sites of C-13-N-15 double labeled model peptides for Bombyx mori silk fibroin, that is, Boc-[1-C-13]Ala[N-15]Gly-OMe, Boc-[1-C-13]Ala[N-15]GlyAlaGly-OPac, Boc-AlaGly[1-C-13]Ala[N-15]GlyAlaGly-OPac, Boc-[1-C-13]Gly[N-15]AlaGlyAla-OPac, Boc-GlyAla[1-C-13]Gly[N-15]AlaGlyAla-OPac and Boc-[1-C-13]Gly[N-15]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 N-15 chemical shift tensors and the orientations of the principal axis system relative to the molecular symmetry axes among three compounds having [1-C-13]Ala[N-15]Gly units, it is concluded that the intermolecular interactions such as hydrogen bonding are different between Boc-[1-C-13]Ala[N-15]Gly-OMe and two compounds, Boc-[1-C-13]Ala[ N-15]GlyAlaGly-OPac and Boc-AlaGly[1-C-13]Ala[N-15]GlyAlaGly-OPac although the latter two compounds have similar structures. A similar conclusion has also been obtained from the C-13 chemical shift tensors of these compounds. (C) 1998 Elsevier Science B.V.