MULTIQUANTUM EPR OF THE MIXED-VALENCE COPPER SITE IN NITROUS-OXIDE REDUCTASE

This work demonstrates the use of multiquantum EPR to study the magnet ic properties of copper complexes and copper proteins. Pure absorption spectra are obtained because of the absence of field modulation. The signal intensity of 3-quantum spectra is proportional to the spin latt ice relaxation time T1, while its linewidth in a frequency difference sweep is T1(-1). A change in lineshape for the EPR detectable mixed va lence [Cu(1.5) ... Cu(1.5)] site in nitrous oxide reductase is attribu ted to suppression of the forbidden transitions. The data confirm the unusually fast relaxation time for this site, which requires temperatu res of less than 100 K to hyperfine structure. The T1's for the mixed valence [Cu(1.5) ... Cu(1.5)] site in nitrous oxide reductase are very similar to T1's for the Cu(a) site in cytochrome c oxidase. The simil ar relaxation properties, together with previous multifrequency EPR re sults, support the hypothesis that the EPR detectable sites in cytochr ome c oxidase and nitrous oxide reductase are mixed valence [Cu(1.5) . .. Cu(1.5)] configurations.