X-ray absorption spectra are reported for the native, azide-bound, and cyanide-bound forms of bovine erythrocyte superoxide dismutase (SOD). Absorption edge data suggest a similar geometric structure for the native enzyme and the azido derivative, but a different coordination structure for the cyano derivative. Analysis of the EXAFS, using a recently developed multiple-scattering method for treating systems containing metal-histidine coordination, has allowed accurate simulation of the raw unfiltered EXAFS data over the range k = 3–13 A '1. The analysis has provided structural details of the coordinated imidazole rings in addition to structural parameters for the coordinated anions. Thus azide is coordinated in an equatorial position of a square-pyramidal site, with Cu-N (azide) = 1.99 A and a Cu-N-N angle of less than ca. 135°. The cyanide group is also coordinated in an equatorial position, with Cu-C = 1,97 A and a Cu-C-N angle of 180°. The multiple-scattering analysis of the linear Cu-C-N group has allowed determination of the C-N distance of the cyano group as 1.18 A. Unlike azido-SOD, which retains an axial imidazole group at 2.27 A, the cyano derivative appears to be 4-coordinate, with no evidence for an additional coordinated histidine. Detailed analysis of the outer-shell contributions from the coordinated histidine groups indicates that the imidazole rings are asymmetrically coordinated, with the degree of asymmetry depending on the identity of the derivative. It is suggested that the latter effect results from alteration in N-H-bonding interactions of the coordinated imidazoles with the polypeptide chain.
ASJC Scopus subject areas
- Colloid and Surface Chemistry