Iron-porphyrin complex, reversible binding

III. Reversible Binding of Superoxide to Iron-Porphyrin Complex 86... 

Simple iron(II) porphyrins cannot reversibly bind oxygen, except at low temperature. At room temperature, and in the absence of a large excess of a sixth ligand, formation of the six-coordinate iron(II) dioxygen species is immediately followed by attack of a second five-coordinate iron(II) complex to give the p-peroxo bisiron(III) complex. This rapidly breaks down, presumably via a ferryl intermediate to give a p-oxo bisiron(III) complex in which the iron has been irreversibly oxidized to the ferric form (Scheme 1) " ... 

A very important question is, how does the particular combination of protein and iron-porphyrin allow myoglobin to reversibly bind molecular oxygen The answer to this question is not known in all its details, but it is well established that Fe(lI)-porphyrins will complex readily and reversibly with oxygen. There are two additional coordination sites around the iron in heme besides the four ring nitrogens. These are indicated below as the general ligands L ... 

Although many interesting iron(II) porphyrin complexes have been synthesized that bind dioxygen reversibly, they are not suitable for achieving the cooperative O2 binding profile of hemoglobin or myoglobin. However, entirely synthetic... 

As part of the work on model heme FeNO complexes, mechanistic studies on the reversible binding of nitric oxide to metmyoglobin and water soluble Fe, Co and Fe porphyrin complexes in aqueous solution, ligand-promoted rapid NO or NO2 dissociation from Fe porphyrins, reductive nitrosylation of water-soluble iron porphyrins, activation of nitrite ions to carry out O-atom transfer by Fe porphyrins, demonstration of the role of scission of the proximal histidine-iron bond in the activation of soluble guanylyl cyclase through metalloporphyrin substitution studies, reactions of peroxynitrite with iron porphyrins, and the first observation of photoinduced nitrosyl linkage isomers of FeNO heme complexes have been reported. 

In the presence of O2, nonbiological copper(I) [and iron(II)] complexes are often susceptible to ligand degradation, which may give the illusion of O2 binding 102 jjjg mechanisms by which this reaction occurs remain essentially unknown. Iron-porphyrin systems are rather more robust. Nonetheless, there are now several well-characterized copper(I) systems that reversibly bind dioxy-at least at low temperature. One that has been structurally character-... 

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