Glycyl C(alpha) chemical shielding in tripeptides: measurement by solid-state NMR and correlation with X-ray structure and theory.

We report here 13Cα chemical shielding parameters for central Gly residues in tripeptides adopting α-helix, β-strand, polyglycine II, and fully extended 2° structures. To assess experimental uncertainties in the shielding parameters and the effects of 14N−13Cα or 15N−13Cα dipolar coupling, stationary and magic angle spinning (MAS) spectra with and without 15N decoupling were obtained from natural abundance and double-labeled samples containing [2-13C, 15N]Gly. We find that accurate (<1 ppm uncertainty) shielding parameters are measured with good sensitivity and resolution in 15N decoupled 1D or 2D MAS spectra of double-labeled samples. Compared to variations of isotropic shifts with peptide angles, those of 13Cα shielding anisotropy and asymmetry are greater. Trends relating shielding parameters to the 2° structure are apparent, and the correlation of the experimental values with unscaled ab initio shielding calculations has an rms error of 3 ppm. Using the experimental data and the ab initio shielding va...