Measurement of Proton, Nitrogen, and Carbonyl Chemical Shielding Anisotropies in a Protein Dissolved in a Dilute Liquid Crystalline Phase

Abstract

The changes in a solute's chemical shifts between an isotropic and a liquid crystalline phase provide information on the magnitude and orientation of the chemical shielding tensors relative to the molecule's alignment frame. Such chemical shift changes have been measured for the polypeptide backbone C‘, N, and HN resonances in the protein ubiquitin. Perdeuterated ubiquitin was dissolved in a medium containing a small volume fraction of phospholipid bicelles, which switches from an isotropic to a liquid crystalline phase at ca. 25 °C. The one-bond 1H−15N dipolar couplings provide a reference to determine the protein alignment tensor, using a vibrationally corrected 1H−15N bond length of 1.04 A, corresponding to rCN = 1.33 A. Assuming all atoms of a given type have the same chemical shielding anisotropy (CSA) tensor, the average C‘ tensor values are σ11 = −75 ppm, σ22 = −12 ppm, and σ33 = 87 ppm for 13C‘ with an angle between σ11 and the C‘−N bond of 38°, and σ33 orthogonal to the peptide plane. Similarly, ...