Hemerythrin Models

A suitable model for the oxygen carrier protein hemerythrin is [Fe2(Et-HPTB)(OBz)](BF4)2 (Et-HPTB = AWAT,iV -tetrakis[(N-ethyl-2-benzimidazolyl)methyl]-2-hydroxy-l,3-diaminopropane, OBz = benzoate). It can mimic the formation of a binuclear peroxo iron complex in the natural system (101). The measured value of -12.8 cm3 mol1 for the activation volume of the oxidation reaction together with the negative value of the activation entropy confirm the highly structured nature of the transition state. 

A new class of metalloprotelns containing polynuclear, non-heme oxo-bridged iron complexes has emerged recently. Dinuclear centers are present in hemerythrin (Hr), ribonucleotide reductase (RR), purple acid phosphatases (PAP) and, possibly, methane monooxygenase (MMO) these centers as well as model compounds are reviewed in Chapter 8. 

Binuclear iron(II) complexes in which a hydroxide bridge is supported by the dinucleating bis-carboxylate ligand dibenzofuran-4,6-bis(diphenylacetate), (217), have proved useful models for hemerythrin. The nature of the binuclear iron center in hemerythrin itself, and in other metalloproteins, has been reviewed, the binding of O2, NO, N3, and NCS to the iron of hemerythrin discussed, " and the volume profile for hemerythrin reacting with O2 established. Bulky tolyl-substituted carboxylate ligands, both bridging and terminal, and... 

Like hemerythrin, hemocyanin is an oxygen transport non-heme-containing protein found in some arthropods and molluscs (104,105). In the 02-bound form, hemocyanin contains an antiferromagnetically coupled binuclear copper(II) system (106) ligated by histidine residues, with a sideways / 2-v2 V2 peroxo group bound to both Cu11 centers (104), which superseded the previous model (107). 

A (/t-oxo)bis(t<-carboxylato)diiron(III) core in a model for met hemerythrin has been prepared by spontaneous self-assembly in aqueous solutions of iron(III) perchlorate, sodium acetate and sodium tri-1 -pyrazolylborate.384 The resulting binuclear complex was shown to be very similar in structure and properties to that of met hemerythrin. Thus, the two Fe111 atoms are linked via a n-oxo... 

In summary, the variety of oxidation levels readily obtainable by these /z-oxide-/i-carboxylate complexes and their reactivity with dioxygen analogs suggest that they may well prove to possess a high degree of structural correspondence to the Mn2 site within the catalase enzyme. This belief is supported by the tertiary structural similarity between catalase and hemerythrin, and the similar structure of the Fe20( i-02CR)2 unit in the latter protein and its synthetic models. 

The complex HB(pz)3Fe(p-CH3C00)2(p-0)Fe(pz)3BH, prepared in one step as a model for the binuclear iron center in hemerythrin, contains two CH3COO and one O bridge between the two Fe(III) atoms. The same holds true for the similarly synthesized diformato and dibenzoato analogs The oxygen bridge in these compounds... 

N and O donors typically hexacoordinate, these can have extended carboxylate bridged stractures. The stracture and reactivity of such complexes have been examined as model systems in the search for explanations of the biological activation and transport of oxygen. The oxygen carrying proteins hemerythrins contain active centers " " in which two iron atoms are linked by both /x-oxo and carboxylate bridges provided by amino acid groups - these are discussed elsewhere in this Encyclopedia. 

Comparison of Structural Data for Hemerythrins and Model Complexes (L = MesOlaneNs)... 

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