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d-d bands due to the Ni2+ chromophore are consistent with a tetrahedral or distorted tetrahedral geometry,12 as is generally observed for metal ions bound to the native zinc site of this protein.1 Addition of azide to either of these derivatives causes no change in their spectra, consistent with the absence of a ligand-binding site on the Ni2+ ion when it is bound to the native zinc site. This behavior also is similar to that observed when other metal ions are bound to that site.The NMR spectra of Ag2Ni2SOD (Figure 2E) and of reduced Cu2Ni2SOD (Figure 2D) are also extremely similar.There are 15 detectable paramagnetically shifted proton NMR signals which we assign to the three histidyl and one aspartyl residues, which are presumably coordinated to Ni2+. By considering the line width (which is contributed by the distancedependent hyperfine interactions and chemical exchanges) of the isotropic shifts, the sharper signals can be tentatively assigned to Cj-H or N-H protons and the broad signals to Ce-H protons.3 We assign resonances, b, h, and j to N-H protons since they are not observed in D20. The upfield-shifted signals are tentatively assigned as Cj3-H2 protons of coordinated histidyl and aspartyl residues by comparison with paramagnetically shifted resonances of other proteins.3 Several other Ni2+-substituted derivatives of SOD have also been prepared in our laboratory. Their characterization and a fuller assignment of the NMR spectra of the above-described nickel derivatives are now in progress.Acknowledgment. Financial support from the USPHS (GM 28222) is gratefully acknowledged. We also thank Dr. James A. Roe for many helpful discussions.(12) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 4th ed.:

Paramagnetic Ce(ffl), ( ), Nd(EI), Sm(ni), ( ), Tb(m), Dy(ffl), ( ), Er(EI), Tm(m), and Yb(m) texaphyrins were studied in solution using NMR spectroscopic techniques. Key spectroscopic features for the dinitrate complexes LnTxfNOa): were assigned on the basis of ID NOE, COSY, and ROESY experiments as well as line width and isotropic shift analysis. The observed isotropic shifts can be fit to theoretical models, assuming dipolar contributions are dominant for all but the imino protons. The resulting calculated values are consistent with

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