H-1 NOE AND LIGAND-FIELD STUDIES OF COPPER COBALT SUPEROXIDE-DISMUTASE WITH ANIONS

The 1H NOE's on bovine erythrocyte Cu(II)2Co(II)2 superoxide dismutase in the presence of saturating amounts of azide have been measured and compared with those in the absence of azide. The interproton distances involving the histidines coordinated to copper(II) are quite similar. Efforts are made to minimize the errors in distances in order to obtain a reliable picture of the reciprocal positions of the above histidines. The conclusion that the histidine positions do not change upon anion binding is extended to the cases of CN−, NCO−, NCS−, and F−. Within this frame the essential disappearance of the hyperfine coupling between the protons of histidine-48 and the unpaired electron on copper(II) upon azide binding and the large variations of circular dichroism spectra are accounted for on the basis of an angular overlap treatment. The same model is used to discuss the spectral variations occurring in the cyanide and cyanate derivatives. It is proposed that the copper-nitrogen (His-48) distance increases upon anion binding as a result of a movement of the copper ion. The fluoride and thiocyanate derivatives are included in the present general model. © 1990, American Chemical Society. All rights reserved.