Abstract
There is increasing interest in oxo-bridged polyiron centers in biology. Binuclear units of this kind occur in hemerythrin, the marine invertebrate oxygen carrier, in ribonucleotide reductase, and in purple acid phosphatases. The reversible binding of dioxygen in hemerythrin has been postulated to occur where the bridging carboxylate groups are contributed from glutamate and aspartate residues of the protein. Following the synthesis of realistic model compounds for both oxidized and reduced forms of the hemerythrin iron core, as well as analogues of the former in which either the oxo bridge was protonated or the bridging carboxylate ligands were replaced by diphenylphosphate groups, they have been working to establish their relevance to the chemical and physical properties of the proteins. Previously, they showed close similarities between the molecular geometry, magnetic exchange interactions, and optical spectroscopic and vibrational properties of azidomethemerythrin and the model compounds (Fe/sub 2/O(O/sub 2/CCH/sub 3/)/sub 2/(HB(pz)/sub 3/)/sub 2/) and (Fe/sub 2/O-(O/sub 2/CCH/sub 3/)/sub 2/(TACN)/sub 2/)/sup 2 +/, where HB(pz)/sub 3//sup -/ = hydrotris(1-pyrazolyl)borate and TACN = 1,4,7-triazacyclononane. They now report the results of extended X-ray absorption fine structure (EXAFS) studies of these two complexes as well as the derivatives (Fe/sub 2/O)O/sub 2/P(OC/sub 6/H/sub 5/)/sub 2/)/sub 2/(HB(pz)/sub 3/)/sub 2/) andmore » (Fe/sub 2/(OH)(O/sub 2/CCH/sub 3/)/sub 2/(HB(pz)/sub 3/)/sub 2/)/sup +/. This work addresses in well-defined model systems how sensitive EXAFS spectroscopy is to the important question of modulations in the iron-to-bridging oxygen and nonbonded, e.g. Fe...Fe or Fe...P, distances. Such geometric information is necessary to understand the oxygen binding reaction of hemerythrin as well as the structural biology of ribonucleotide reductase and purple acid phosphatase.« less
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