Glycyl CαChemical Shielding in Tripeptides: Measurement by Solid-State NMR and Correlation with X-ray Structure and Theory

Abstract

We report here ¹³C^α 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−¹³C^α or ¹⁵N−¹³C^α 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 (15N decoupled 1D or 2D MAS spectra of double-labeled samples. Compared to variations of isotropic shifts with peptide angles, those of ¹³C^α 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 values, the empirical trends relating the 2° structure to shielding are extended to the larger range of torsion angles found in proteins.