Cu-A, an electron transfer center present in cytochrome c oxidase, COX, and nitrous oxide reductase, N-2-OR, is a dimeric copper complex with four ligands, two cysteine thiols bridging the metal ions and two terminal histidine residues. The center cycles between the mixed-valence state [Cu(I),Cu(II)] and the reduced state [Cu(I), Cu(I)]. The EPR, optical absorption, low-temperature magnetic circular dichroism, and CD spectra of three proteins containing the mixed-valence state of Cu-A have been measured between 33 000 and 5000 cm(-1). These results point to two forms of the chromophore, one in the enzyme N(2)OR of Pseudomonas stutzeri, lacking its catalytic center, and also in a water soluble domain of subunit II of Paracoccus denitrificans COX and the other, referred to as Cu-A*, in a site engineered into a soluble domain of subunit II of the quinol oxidase in Escherichia coli.