Porphyrin centered electron transfer

Chelation itself is sometimes useful in directing the course of synthesis. This is called the template effect (37). The presence of a suitable metal ion facihtates the preparation of the crown ethers, porphyrins, and similar heteroatom macrocycHc compounds. Coordination of the heteroatoms about the metal orients the end groups of the reactants for ring closure. The product is the chelate from which the metal may be removed by a suitable method. In other catalytic effects, reactive centers may be brought into close proximity, charge or bond strain effects may be created, or electron transfers may be made possible. 

Fukuzumi, S., Ohkubo, K., Wenbo, E., Ou, Z., Shao, J., Kadish, K.M., Hutchison, J.A., Ghiggino, K.P., Sintic, P.J. and Crossley, M.J. (2003) Metal-centered photoinduced electron transfer reduction of a gold(III) porphyrin cation linked with a zinc porphyrin to produce a long-lived charge-separated state in nonpolar solvents. Journal of the American Chemical Society, 125, 14984-14985. 

Achieving fast electron transfer to enzyme active sites need not be complicated. As mentioned above, many redox enzymes incorporate a relay of electron transfer centers that facilitate fast electron transfer between the protein surface and the buried active site. These may be iron-sulfur clusters, heme porphyrin centers, or mononuclear... 

C.H. Brubaker, Michigan State University In the case of the cytochromes, it has been proposed that electron transfer from the iron porphyrin may involve the pi system of the porphyrin and even nearby aromatic rings. Do you think that a similar thing may happen in the case of the reaction between these copper(I) p lastocyanins and the chromium(III) You seem to favor the idea that the important factor is that the Cr(III) be at a site that is reasonably close to the copper center. 

Cytochromes, catalases, and peroxidases all contain iron-heme centers. Nitrite and sulfite reductases, involved in N-O and S-O reductive cleavage reactions to NH3 and HS-, contain iron-heme centers coupled to [Fe ] iron-sulfur clusters. Photosynthetic reaction center complexes contain porphyrins that are implicated in the photoinitiated electron transfers carried out by the complexes. 

The intramolecular electron transfer kg, subsequent to the rapid reduction, must occur because the Ru(III)-Fe(II) pairing is the stable one. It is easily monitored using absorbance changes which occur with reduction at the Fe(III) heme center. Both laser-produced Ru(bpy)3 and radicals such as CO (from pulse radiolysis (Prob. 15)) are very effective one-electron reductants for this task (Sec. 3.5).In another approach," the Fe in a heme protein is replaced by Zn. The resultant Zn porphyrin (ZnP) can be electronically excited to a triplet state, ZnP which is relatively long-lived (x = 15 ms) and is a good reducing agent E° = —0.62 V). Its decay via the usual pathways (compare (1.32)) is accelerated by electron transfer to another metal (natural or artificial) site in the protein e. g.. 

Nitroprusside is capable of ligating redox partners through a cyano ligand to participate in inner-sphere electron-transfer reactions. With hemoglobin an electron is transferred in this fashion from the porphyrin center to yield [Fe(CN)4(NO)] , methemoglobin and NO (see below) [92]. 

Nowadays, most reaction center models carry suitable antenna pigments and acceptor groups and in effect are photosystem models. A typical example for a state-of-the-art system that incorporates many aspects of a photosystem consisted of a boron dipyrrin covalently linked to a zinc(II) porphyrin, which carried a suitably modified C60 derivative as axial ligand. Selective excitation of the boron dipyrrin as antenna pigment resulted in energy transfer to a zinc(II) porphyrin followed by electron transfer to the acceptor109. 

P-450 mechanism are summarized in Figure 1. Dioxygen binds to an iron(II) porphyrin center and is reduced by two electrons to a peroxo species. Subsequent 0—0 heterolysis transforms it into a formally iron (V)-oxo species, which is proposed to be responsible for the oxygen transfer chemistry. 

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