Metalloporphyrins-dioxygen

By determining the rate constants for the various steps in Scheme 8, it has been demonstrated [78] that electron or proton transfer to the metalloporphyrin-dioxygen moiety weakens the 0-0 bond, thereby... 

Durand RR Jr, Bencosme CS, Collman JP, Anson FC. 1983. Mechanistic aspects of the catalytic reduction of dioxygen by cofacial metalloporphyrins. J Am Chem Soc 105 2710. 

Ni CL, Abdalmuhdi I, Chang CK, Anson FC. 1987. Behavior of four anthracene-linked dimeric metalloporphyrins as electrocatalysts for the reduction of dioxygen. J Phys Chem 91 1158. 

An interesting, pH-dependent mechanistic changeover was reported in the H2A-02-[Fem(TPPS)] (TPPS = 5,10,15,20-tetrakis(p-sulfonatophe-nyl)porphyrinate) system in aqueous solution (23). This water-soluble metalloporphyrin exists as a monomer under slightly acidic conditions and reacts with ascorbic acid on the time-scale of several hours to produce [Fen(TPPS)] in the absence of dioxygen. The formation of the [Fem(TPPS +)] radical was also reported at pH 5, but it is not clear how... 

The use of models that mimic a protein active site is normally prompted by the desire to eliminate any influence of the polypeptide backbone surrounding the active site in real biological molecules, which may obscure its physico-chemical properties. The first attempts to synthesize metal complexes similar to the active site of haemoproteins, through the use of simple metalloporphyrin derivatives, failed. The failure was due to the fact that these complexes react irreversibly with dioxygen as a consequence of side autooxidative reactions of the type ... 

Karlin, K. D. Zuberbuhler, A. D. Formation, structure, and reactivity of copper dioxygen complexes, Bioinorganic Catalysis , 2nd edn. (Revised and Expanded) Eds. Reedijk, J. Bouwman, E. Marcel Dekker New York, 1999, pp. 469-534. Fukuzumi, S. Imahori, H. Biomimetic electron-transfer chemistry of porphyrins and metalloporphyrins, Electron Transfer in Chemistry , Vol. 2 Ed. Balzani, V. Wiley-VCH Verlag GmbH Weinheim, 2001, pp. 927-975. 

Porphyrins and metalloporphyrins have been the subject of numerous investigations over the past century. This attention is not surprising if one considers that particular derivatives play central roles in essential biological processes such as photosynthesis, dioxygen transport and storage, and electron transfer. 

In the most important series of polymers of this type, the metallotetraphenylporphyrins, a metalloporphyrin ring bears four substituted phenylene groups X, as is shown in 7.19. The metals M in the structure are typically iron, cobalt, or nickel cations, and the substituents on the phenylene groups include -NH2, -NR2, and -OH. These polymers are generally insoluble. Some have been prepared by electro-oxidative polymerizations in the form of electroactive films on electrode surfaces.79 The cobalt-metallated polymer is of particular interest since it is an electrocatalyst for the reduction of dioxygen. Films of poly(trisbipyridine)-metal complexes also have interesting electrochemical properties, in particular electrochromism and electrical conductivity.78 The closely related polymer, poly(2-vinylpyridine), also forms metal complexes, for example with copper(II) chloride.80... 

The data seem to support an alternative mechanism compared with the so-called Lyons mechanism in which O2 is activated forming a metal-oxo (M=0) species.1831 Via reaction of a second metalloporphyrin with a primarily formed superoxo (Me-OO0) or peroxo species, a metal-oxo is formed, reacting eventually with an alkane according to the oxygen rebound mechanism. Alternatively, radicals present in solution, upon reaction with dioxygen, may form alkyl hydroperoxides that are decomposed by metalloporphyrins.[83]... 

No reports have appeared on oxidation of substrates using metalloporphyrins under conditions at which dioxygen complexes are known to exist at ambient temperatures, for example, the 02 adducts of specially designed picket fence (5, 29, 132) and polymer-supported metalloporphyrins (133), (py)Cr(7TP)02 (134), and... 

Lopez MA, Kollman PA (1993) Application of molecular dynamics and free energy perturbation methods to metalloporphyrin-ligand systems II CO and dioxygen bynding to myoglobin, Protein Sci, 2 1975-1986... 

A related series of mixed-metal face to face porphyrin dimers (192) has been studied by Collman et al.506 A motivation for obtaining these species has been their potential use as redox catalysts for such reactions as the four-electron reduction of 02 to H20 via H202. It was hoped that the orientation of two cofacial metalloporphyrins in a manner which permits the concerted interaction of both metals with dioxygen may promote the above redox reaction. Such a result was obtained for the Co11 /Co" dimer which is an effective catalyst for the reduction of dioxygen electrochemic-ally.507 However for most of the mixed-metal dimers, including a Con/Mnn species, the second metal was found to be catalytically inert with the redox behaviour of the dimer being similar to that of the monomeric cobalt porphyrin. However the nature of the second metal ion has some influence on the potential at which the cobalt centre is reduced. 

Heme proteins are one of the largest classes of metalloproteins studied to date see Iron Heme Proteins Dioxygen Transport Storage Iron Heme Proteins, Peroxidases, Catalases <6 Catalase-peroxidases). More than 5% of protein structures in the Protein Data Bank contain at least one heme moiety. It is no wonder that designing heme proteins has been one of the most active areas of research. Since the dominant secondary structure in heme proteins is a-hehces (accounting for 77% of all secondary structures in known heme proteins),a number of Q -helix-containing metalloporphyrin-peptides have been synthesized using either covalent or noncovalent approaches. 

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