Abstract
To investigate local structures of left-handed α-helix (αL) and left-handed ω-helix (ωL), the principal values (δ11, δ22, and δ33) of the 13C chemical shift anisotropy (CSA) tensors of the main-chain carbonyl (C═O) carbons in poly(β-benzyl l-aspartate) were determined by 13C switching-angle sample-spinning solid-state NMR. Further, the observed CSA tensor values for αR, αL, and ωL are compared with those obtained by quantum chemical calculation for a model peptide. Although the isotropic chemical shifts of the C═O carbons in αR, αL, and ωL lie within ±2 ppm, the δ22 values, whose axis is close to the direction of the hydrogen bond (C═O···H–N), are markedly different. In contrast to the relation established for the δ22 values in various αR helices, that is, an increase of the hydrogen-bond length leads to an upfield shift of δ22, the δ22 values for αL and ωL show a downfield shift for the longer hydrogen-bond length.
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