Porphyrins biological process models

There have also been significant advances in the imido chemistry of ruthenium and osmium. A variety of imido complexes in oxidation states +8 to +6 have been reported. Notably, osmium (VIII) imido complexes are active intermediates in osmium-catalyzed asymmetric aminohydroxyl-ations of alkenes. Ruthenium(VI) imido complexes with porphyrin ligands can effect stoichiometric and catalytic aziridination of alkenes. With chiral porphyrins, asymmetric aziridination of alkenes has also been achieved. Some of these imido species may also serve as models for biological processes. An imido species has been postulated as an intermediate in the nitrite reductase cycle. " ... 

This paper has discussed the electrochemistry of metal-carbon and metal-metal bonded metalloporphyrins. The field is quite new and the majority of published studies result from work carried out in our own laboratory during the period 1984 to 1987. To date, relatively few examples of a-bonded metal-carbon and metal-metal bonded metalloporphyrins are known, but it is theoretically possible to prepare and characterize representative complexes with almost all elements in the periodic table. The field is rapidly expanding and future synthetic developments in porphyrin chemistry should certainly lead to new complexes possessing different metal-metal or metal-carbon interactions and possibly different electrochemistry. These types of complexes are of interest in modelling some biological processes, in synthetic organometallic chemistry and in the preparation of new poljrmeric materials with conducting properties. 

Knowledge of the electronic structure of metalloporphyrins is useful, because such compounds can serve as models for biological redox systems. In this context one may wish to know whether, during a redox process, the electron is transferred directly from the metal center or whether porphyrin ligands are also involved in the process. Further, an understanding of the influence of the oxidation state of the metal on the porphyrin ring s charge and reactivity is of interest. 

Model heme systems The mechanisms of heme and hemoprotein reactions with small molecules such as O2, CO and NO has attracted considerable experimental attention owing to the importance of such processes in biological systems. Flash photolysis studies [87] have indicated that the photolabilization of L from simple heme complexes and kinetics of the resulting back reaction (Eq. 6.40) can be modeled by the intermediacy of solvent caged contact pair . Equation (6.41) illustrates this mechanism for the thermal back reaction for the photochemically generated intermediates for a ferrous porphyrin (Por)Fe L (For = porphyrin)... 

For the attainment of marvelous electron transfer processes in the natural sequential potential fields, many noncovalentaly-bound donor-acceptor (DA) systems and covalently-bound DA systems " " have been previously reported. Most of them are artificial models of the photosynthesis comprising simple assemblies of the dyad (DA) or triad [donor-spacer-acceptor (DSA)] functional molecules with a chromophore such as a porphyrin. The quantum efficiency of such systems is lower (<25%) compared with the biological systems (=100%), and thus more efforts for constructing more efficient systems are necessary. Some of the covalen-taly-bound DA systems have been designed for the fabrication of molecule-scale devices based on a molecular electron-transport wire and/or highly ordered molecular arrays on the surface. " Most of such studies employed the DA nonconju-gated molecules. 

The charge separation processes in photosynthesis involve photoinduced electron transfer from chlorophyll to quinone derivatives. A popular methodology of biomimetic chemistry for biological active sites is assembly of supposedly essential components via covalent linkage. TTius, a variety of covalently linked porphyrin-quinone derivatives such as 1 and 2 below have been prepared and photoinduced electron transfer therein investigated as models of photosynthetic electron transfer [12]. There are two major difficulties in the covalent approach to the preparation of this type of face-to-face porphyrin-quinone derivatives. The first is synthetic problems associated with the preparation of highly substituted quinone precursors and the porphyrin-quinone double coupling reactions. Second, there is little room for the systematic modification of the electtonic/steric structures of quinones. 

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