C-13 chemical shift tensors and secondary structure of poly-L-alanine by solid-state two-dimensional spin-echo NMR and ab initio chemical shielding calculation

Analyses of C-13 chemical shielding tensors for powder samples of alpha-hel ix-rich poly-L-alanine (pALA) and the mechanically stretched beta-sheet pAL A films were performed by solid-state two-dimensional spin-echo (2DSE) NMR spectroscopy. From the iterative fitting of 2DSE spectra with spectrum simu lations, information was obtained about the principal values of the chemica l shift tensors (CST) for nuclei with small chemical shift anisotropy such as CH and CH3 carbons. The experimental results show that the lowest shield ing component, delta(11), of L-alanine residue alpha- and beta-carbons are sensitive to the conformational changes and responsible for the behavior of the conformation-dependent isotropic chemical shifts. Ab initio gauge-inva riant atomic orbital chemical shielding calculations were carried out in or der to understand the correlation between the CST and the polypeptide confo rmation.