Cytochrome oxidase (CcO) is known to possess four metal centers - two heme and two Cu sites. The catalytic center of CcO is defined by the two metal sites, heme a3 and CuB, which are associated to form a binuclear center. Numerous studies have propose that the two metal centers be closely spaced and bridged, but the identity of the bridging ligand in the """"""""as-isolated"""""""" enzyme remains unresolved. Work at Harvard has given rise to several binuclear synthetic complexes which are viable analogs of the CcO site. They include a bridged Fe(III)-O-Cu(II) complex which reproduces the most prominent electronic features of the oxidized CcO site, and a CN- bridged analog which has potential relevance to cyanide toxicity. These analogues present a significant challenge for EXAFS analysis because: i) from the Fe viewpoint the octaethylporphrin ligand is rigid and contributes significant multiple scattering (MS) terms to the overall EXAFS signal, ii) a mono bridge, such as O2- will contribute strong MS as the bridge angle approaches linearity and be a significant contributor from either the Cu or the Fe EXAFS viewpoint, and iii) multinuclear bridges, such as CN- could have very complicated 4-body MS interactions (again angularly dependent). We intend to perform complete MS XAS analysis on these models and enzymes.
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