An analysis of the cobalt(2+) ion-induced nuclear magnetic resonance perturbations of hen egg white lysozyme

Abstract

A general methodology is presented for analyzing dipolar shifts induced by paramagnetic ions in the NMR specta of ligand molecules. The method is applied to the shift perturbations induced by Co2+ in the spectrum of hen egg white lysozyme. A hypothesis testing scheme is employed to evaluate statistically the relative precision with which the axially symmetric and non-axially symmetric forms of the dipolar shift equation fit the observed data. The assumption of axial symmetry for the magnetic susceptibility tensor of Co2+ is rejected at the confidence level of 99%. Since the results presented here are similar to those reached in an analysis of lanthanide-induced shifts, the assumption of axial symmetry may, in general, not hold. Similar conclusions have been reached by other investigators in studies of paramagnetic metal binding to model systems. The 3 Co2+ coordinates were included in an 8 parameter fit of the Co2+ shift data. The Co2+ position obtained from this fit is in statistical agreement with the position inferred from X-ray data. Thus, the analysis of shift data may furnish a means for determining the site of metal complexation in macromolecules whose structure has been determined by X-ray crystallography.