Incorporation of residual chemical shift anisotropy into the treatment of N pseudocontact shifts for structural refinement.

Paramagnetic NMR experiments, including the pseudocontact shift experiment, have seen increasing use due to recently developed probes and labeling strategies. The pseudocontact shift experiment can provide valuable intra- or inter-molecular distance and orientation information. However, the use of H/C or H/N PCS data in structure calculations is currently complicated by the contribution of residual chemical shift anisotropy to the C or N datasets. Here, we present a corrected PCS energy term for the software package Xplor-NIH with the appropriate residual chemical shift anisotropy correction and show its suitability for model refinements of ubiquitin labeled at residue 57 with a Tm-M8-SPy tag. For data taken at 800 MHz, the improvement with the corrected energy term is sufficient to make the quality of the fit for the N dataset comparable to that of the H dataset, for which no correction is needed. The corrected energy term is expected to become more relevant with increased use of higher field instruments and as new paramagnetic probes with larger magnetic susceptibility tensors continue to be developed.