Cu(II)-Carbon Bonding in Cyanide Complexes of Copper Enzymes

The electron spin resonance (ESR) signals of the cyanide complexes of 63Cu(II) or 65Cu(II) human carbonic anhydrase B show a five-line multiplet on each of the four g|| copper nuclear hyperfine lines indicating the presence of two magnetically equivalent nitrogen atoms as ligands. Formation of the complex with 13C14N- splits each multiplet into two identical multiplets separated by 112 gauss. In contrast 12C15N- has no effect on the spectrum. Thus, the cyanide ion is coordinated to the Cu(II) ion via the carbon atom and the nitrogen nucleus of cyanide does not contribute to the nitrogen superhyperfine structure of the spectrum. Analogous effects of 13C14N- and 12C15N- are observed on the ESR spectra of the Cu(II) ion in the cyanide complex of bovine superoxide dismutase. Only 13C14N- affects the Cu(II) g|| nuclear hyperfine lines (seven-line multiplets reflecting coordination to three magnetically equivalent nitrogen atoms) of native superoxide dismutase. Each g|| line is split into two separated by 45 gauss. In mixed cyanide-metal-enzyme complexes C coordination of the CN- ion appears to be preferred. These findings confirm the previous assumptions concerning the cyanide complexes of Cu(II) carbonic anhydrase and superoxide dismutase.