Lanthanide induced NMR perturbations of hew lysozyme: Evidence for nonaxial symmetry

Attention has recently been focused on the use of the trivalent lanthanides ions as probes in NMR studies of macromolecules. Previous analyses of the shifts induced by these paramagnetic ions have assumed that the magnetic susceptibility tensor of the metals is axially symmetric. This study has tested the validity of this assumption by analyzing reported NMR shift and relaxation data for various lanthanide complexes with lysozyme. For the Gd³⁺-lysozyme complex the broadening data, fit to a ( 1 r⁶) model, places the metal ion in close proximity to the reported x-ray position. Contrary to current assumptions, the shifts induced by Nd³⁺ and Ce³⁺ exhibit considerable non-axial character. On the basis of a statistical hypothesis test, the axially symmetric model was rejected with more than 97% confidence.