To investigate local structures of left-handed a-helix (alpha(L)) and left-handed pi-helix (pi(L)), the principal values (delta(11), delta(22), and delta(33)) of the C-13 chemical shift anisotropy (CSA) tensors of the main-chain carbonyl (C=O) carbons in poly(beta-benzyl l-aspartate) were determined by C-13 switching-angle sample-spinning solid-state NMR. Further, the observed CSA tensor values for alpha(R), alpha(L), and pi(L) are compared with those obtained by quantum chemical calculation for a model peptide. Although the isotropic chemical shifts of the C=O carbons in alpha(R), alpha(L), and pi(L) lie within +/- 2 ppm, the delta(22) values, whose axis is close to the direction of the hydrogen bond (C-(OHN)-H-...), are markedly different. In contrast to the relation established for the delta(22) values in various aR helices, that is, an increase of the hydrogen-bond length leads to an upfield shift of delta(22), the delta(22) values for alpha(L) and pi(L) show a downfield shift for the longer hydrogen-bond length.