Porphyrins iron redox

D. Lexa and J. M. Saveant, Supramolecular Effects in the Redox and Coordination Chemistry of Superstructured Iron Porphyrins, in Redox Chemistry and Interfacial Behaviour of Biological Molecules , eds.G. DryhurstandK. Niki, Plenum, New York, 1988, p. 1. 

In the same way as porphyrin basicity influences the iron redox qualities, the basicity of the extra ligand plays an important role in this connection the greater the donor power the more negative the potential. The relationship between potentials and the p a of various bases added to the central Fe11 ion (e.g. pyridine, nicotine, histidine, etc.) is essentially linear [Barron (3), Falk (56, 57)]. 

A second example illustrates how hydrogen bonding interactions may stabilize the more oxidized form of a redox cofactor. The porphyrin iron in heme proteins is often liganded by a histidine imidazole side chain. The imidazole is frequently... 

A substantial fraetion of the named enzymes are oxido-reduetases, responsible for shuttling electrons along metabolic pathways that reduce earbon dioxide to sugar (in the ease of plants), or reduce oxygen to water (in the case of mammals). The oxido-reduetases that drive these proeesses involve a small set of redox aetive cofactors , that is, small chemical groups that gain or lose eleetrons. These cofactors include iron porphyrins, iron-sulfur clusters and eopper eomplexes as well as organie species that are ET active. 

Porphyrin-based complexes pass their redox sensitivity on to the corresponding polymers. Advanced conjugated main-chain structures with attached dendrons have been introduced [220]. Grafting of porphyrin-zinc main-chain polymers onto carbon nanotubes was reported [221]. An interesting example dealing with a nonelectrochemically switched polymer was recently given. Here, polymers with porphyrin-iron end groups could be switched between linear and cyclic polymers by a dimerization procedure (Fig. 14) [222, 223],... 

Iron Porphyrins. Porphyrias (15—17) are aromatic cycHc compouads that coasist of four pyrrole units linked at the a-positions by methine carbons. The extended TT-systems of these compounds give rise to intense absorption bands in the uv/vis region of the spectmm. The most intense absorption, which is called the Soret band, falls neat 400 nm and has 10. The TT-system is also responsible for the notable ring current effect observed in H-nmr spectra, the preference for planar conformations, the prevalence of electrophilic substitution reactions, and the redox chemistry of these compounds. Porphyrins obtained from natural sources have a variety of peripheral substituents and substitution patterns. Two important types of synthetic porphyrins are the meso-tetraaryl porphyrins, such as 5,10,15,20-tetraphenylporphine [917-23-7] (H2(TPP)) (7) and P-octaalkylporphyrins, such as 2,3,7,8,12,13,17,18-octaethylporphine [2683-82-1] (H2(OEP)) (8). Both types can be prepared by condensation of pyrroles and aldehydes (qv). 

In the third complex of the electron transport chain, reduced coenzyme Q (UQHg) passes its electrons to cytochrome c via a unique redox pathway known as the Q cycle. UQ cytochrome c reductase (UQ-cyt c reductase), as this complex is known, involves three different cytochromes and an Fe-S protein. In the cytochromes of these and similar complexes, the iron atom at the center of the porphyrin ring cycles between the reduced Fe (ferrous) and oxidized Fe (ferric) states. 

The reduction ofsec-, and /-butyl bromide, of tnins-1,2-dibromocyclohexane and other vicinal dibromides by low oxidation state iron porphyrins has been used as a mechanistic probe for investigating specific details of electron transfer I .v. 5n2 mechanisms, redox catalysis v.v chemical catalysis and inner sphere v.v outer sphere electron transfer processes7 The reaction of reduced iron porphyrins with alkyl-containing supporting electrolytes used in electrochemistry has also been observed, in which the electrolyte (tetraalkyl ammonium ions) can act as the source of the R group in electrogenerated Fe(Por)R. ... 

The electron transfer properties of the cytochromes involve cycling of the iron between the +2 and +3 oxidation states (Cytochrome)Fe + e" (Cytochrome)Fe ° = -0.3Vto+ 0.4V Different cytochromes have different side groups attached to the porphyrin ring. These side groups modify the electron density in the delocalized iz system of the porphyrin, which in turn changes the redox potential of the iron cation in the heme. 

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... 

Reed, C.A. Iron(l) and iron(lV) porphyrins. In Kadish, K.M. (ed.) Electrochemical and Spectrochemical Studies of Biological Redox Components, pp. 333-356. American Chemical Society (1982)... 

FIGURE 4.3. Redox and chemical homogeneous catalysis of trans-1,2 dibromocyclohexane. a cyclic voltammetry in DMF of the direct electrochemical reduction at a glassy carbon electrode (top), of redox catalysis by fhiorenone (middle), of chemical catalysis by an iron(I) porphyrin, b catalysis rate constant as a function of the standard potential of the catalyst couple aromatic anion radicals, Fe(I), a Fe(0), Co(I), Ni(I) porphyrins. Adapted from Figures 3 and 4 of reference lb, with permission from the American Chemical Society. 

The direct electrochemical reduction of carbon dioxide requires very negative potentials, more negative than —2V vs. SCE. Redox catalysis, which implies the intermediacy of C02 (E° = —2.2 V vs. SCE), is accordingly rather inefficient.3 With aromatic anion radicals, catalysis is hampered in most cases by a two-electron carboxylation of the aromatic ring. Spectacular chemical catalysis is obtained with electrochemically generated iron(0) porphyrins, but the help of a synergistic effect of Bronsted and Lewis acids is required.4... 

The distorted octahedral species [10] and [11] are the essential part of the cytochromes acting as redox catalysts. In these, a very specific porphyrin redox potential may have been adjusted by an appropriate choice of the axial ligands exerting a cis effect on the porphyrin system transmitted through the iron atom. In cytochrome c, these axial ligands are the imidazole of a histidine and the thioether function of methionine, as in [10], and in the cytochromes a or b5 they are presumably two imidazoles of histidine, as in [77] (7). 

Table 13. Further series (a—h) of iron or ruthenium porphyrins showing cis effects exerted by the axial ligands X or L on the wavelength of the a-band (or (3-band in Series d, e and g) and the chemical shift (6) of the porphyrin meso-protons (Series c, h o-proton in c). The metal II/III-redox potentials (Ej/2) are also given. For abbreviations, see Table 2... 

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