Weak alignment of paramagnetic proteins warrants correction for residual CSA effects in measurements of pseudocontact shifts

Paramagnetic metal ions can induce molecular alignment with respect to the magnetic field. This alignment generates residual anisotropic chemical shifts (RACS) due to nonisotropic averaging over the molecular orientations. Using a 30 kDa protein-protein complex, the RACS effects are shown to be significant for heteronuclear spins with large chemical shift anisotropies, lanthanide ions with large anisotropic magnetic susceptibility tensors, and measurements at high magnetic field. Therefore, RACS must be taken into account when pseudocontact shifts are measured by comparison of chemical shifts observed between complexes with paramagnetic and diamagnetic lanthanide ions. The results are of particular importance when different pseudocontact shifts measured for the 1HN, 15N, and 13C' spins of a peptide group are used to restrain its orientation with respect to the electronic magnetic susceptibility tensor in structure calculations.