Measurement of ~(15)N chemical shift anisotropy in a protein dissolved in a dilute liquid crystalline medium with the application of magic angle sample spinning

摘要

The chemical shifts of nuclei that have chemical shielding anisotropy, such as the ~(15)N amide in a protein, show significant changes in their chemical shifts when the sample is altered from an isotropic state to an aligned state. Such orientation-dependent chemical shift changes provide information on the magnitudes and orientation of the chemical shielding tensors relative to the molecule's alignment frame. Because of the extremely high sensitivity of the chemical shifts to the sample conditions, the changes in chemical shifts induced by adding aligned bicelles do not arise only from the protein alignment but should also include the accumulated effects of environmental changes including protein-bicelle interactions. With the aim of determining accurate ~(15)N chemical shielding tensor values for solution proteins, here we have used magic angle sample spinning (MAS) to observe discriminately the orientation-dependent changes in the ~(15)N chemical shift. The application of MAS to an aligned bicelle solution removes the torque that aligns the bicelles against the magnetic filed. Thus, the application of MAS to a protein in a bicelle solution eliminates only the molecular alignment effect, while keeping all other sample conditions the same. THe observed chemical shift differences between experiments with an without MAS therefore provide accurate values of the orientation-dependent ~(15)N chemical shifts. From the values for ubiquitin in a 7.5% (w/v) bicelle medium, we determined the ~(15)N chemical shielding anisotropy (CSA) tensor. For this evaluation, we considered uncertainties in measuring the ~1H-~(15)N dipolar couplings and the ~(15)N chemical shifts and also structural noise present in the reference X-ray structure, assuming a random distribution of each NH bond vector in a cone with 5° deviation from the original orientation. Taking into account these types of noise, we determined the average ~(15)N CSA tensor for the residues in ubiquitin as △σ = -162.0 ± 4.3 ppm, η = 0.18 ± 0.02, and β = 18.6 ± 0.5°, assuming a ~1H-~(15)N bond length of 1.02 A. These tensor values are consistent with those obtained from solid-state NMR experiments.