Porphyrin oxygen

Myoglobin contains a heme prosthetic group, which binds oxygen. Heme consists of a single atom of Fe2+ coordinated within a porphyrin. Oxygen binds to myoglobin reversibly this... 

Table 9.7. Volume of one mole of porphyrin-oxygen collision at different oxygen concentrations and at 20°C.

The presence of the protein is of high importance because without it the oxygen atom close to the iron atom would immediately oxidize Fe in heme to Fe and the oxygen molecule would then not bind to iron. In addition, the iron ion in the complex with porphyrin, oxygen and histidine forms an octahedral coordination, a sterically favored structure. 

N—Fe(IV)Por complexes. Oxo iron(IV) porphyrin cation radical complexes, [O—Fe(IV)Por ], are important intermediates in oxygen atom transfer reactions. Compound I of the enzymes catalase and peroxidase have this formulation, as does the active intermediate in the catalytic cycle of cytochrome P Q. Similar intermediates are invoked in the extensively investigated hydroxylations and epoxidations of hydrocarbon substrates cataly2ed by iron porphyrins in the presence of such oxidizing agents as iodosylbenzene, NaOCl, peroxides, and air. 

Porphyrins, derivatives of porphin, are best known for their role as oxygen earriers in hemoglobin. 

The synthesis of porphyrins from bilanones is free of all symmetry restraints.77 The oxo function is necessary to stabilize the bilane system by its electron-withdrawing effect. The synthesis of porphyrins from the parent bilane without the oxo function and with /3-substituents is possible,54 but the method gives rise to preparative problems due to the sensitivity of these compounds to oxidation, to electrophiles and to acids. Nature circumvents these problems in the cellular environment by exclusion of oxygen, when porphyrinogens, the precursors of porphyrins, are produced from bilanes55 in the course of their biosynthesis. 

Another interesting oxidative formation of chlorins from porphyrins is the singlet-oxygen addition to protoporphyrin19 (see Section 1.1.2.2.). 

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