Molecular-orientation analysis based on alignment-induced TROSY chemical shift changes

We present a new NMR technique for determining the alignment tensor of a weakly aligned protein using only alignment-induced ¹⁵N transverse relaxation optimized spectroscopy (TROSY) chemical shift changes. Alignment-induced TROSY chemical shift changes reflect the combined contributions from two different anisotropic spin interactions including the residual dipolar couplings (RDCs) and the residual chemical shift anisotropy effects (RCSAs). We show here that these two residual anisotropic spin interactions’ values, encoded in the TROSY chemical shift changes, can be used to determine a weakly aligned protein’s alignment tensor. To prove the significance of this method, we show that our TROSY-based analysis gives the consistent alignment angles with those determined using RDCs for ¹⁵N-labeled ubiquitin (8.6 kDa) in an aligned medium, within an uncertainty range estimated by considering experimental and structural noises, being 5° at most. Because our approach requires a pre-determined ¹⁵N CSA tensor value, we also estimated the uncertainties associated with the resultant alignment tensor values caused by variation in ¹⁵N CSA tensors. In spite of the significant variations in literature-reported ¹⁵N CSA tensors, they gave consistent orientation angles within an uncertainty range. These results ensure that our TROSY-based approach is a useful alternative to the RDC-based method to determine the alignment angles especially for large proteins in a weakly aligned state.