A chemically reduced CopZ, pH 7.0; B, chemically reduced CopZ, pH 10; C, C77S variant; D, C75S variant; E, C109S variant; F, C119S variant; G, after 77 K cryoreduction (CR) of diferric CopZ. Sharp feature at ~ 1500 G in some spectra is non-heme Fe(III). The sharp feature at ~ 1500 G in some spectra is non-heme Fe(III). Conditions are as follows: T=10 K; frequency, 9.372 GHz; power, 1 milliwatt; modulation amplitude, 5 G. WT, wild type.