Transient versus steady state NOE in paramagnetic molecules Cu2Co2SOD as an example

Truncated, steady state and transient NOE experiments have been performed on bovine Cu2Co2 superoxide dismutase. The effectiveness of the different NOE experiments in the general case of paramagnetic macromolecules is discussed. It is concluded that steady state NOEs give superior results. The validity of the two spins approximation is discussed, and NOE values for a fully coupled set of nuclei have been calculated. Transient NOE experiments, when properly performed, confirm the previous assignment of the hyperfine shifted signals in Cu2Co2SOD based on steady state NOE measurements [(1989) Inorg. Chem. 28, 4650] and eliminate any further reason for controversy on an important issue as the assignment of the 1H NMR signals of protons of metal-coordinated imidazoles.