Porphyrin ferric

The only difference between the active oxidizing and a ferric porphyrin hydroxide complex is two electrons (scheme 4). Indeed, the electrochemical oxidation of hydroxy ferric tetra-mesitylporphyrin shows two reversible one-electron oxidations (40), and, in principle, use of water and an electrode should allow development of a system capable of catalytically oxidizing hydrocarbons. 

Heme proteins in their various forms contain mainly the ferrous or ferric porphyrin moieties [6—77] (R some organic side chain of the protein A a small molecule act-ingas a-donor-TT-acceptor ligand, e.g., CO, 02, NO, CH3CN, CH3NC) (7, 20-34). In fact the binding of dioxygen to the pentacoordinate species [6] and [7] — an essential... 

The diaqua and aqua (hydroxo) hemin complexes encapsulated in the micelles [20] are found to be high-spin (peff = 5.7 — S.Sps). Their high-spin nature is further confirmed from the ESR spectra at 4.2 K (Fig. 4). The spectra are characteristic of high-spin ferric porphyrins with a large zero-field-split Ai ground state with Mg = 1/2 lying lowest. The spectra are axially symmetric (gf = 2.05, = 6.0) for the diaqua complex, while for the aqua (hydroxo)... 

For high-spin ferric porphyrin with axial symmetry, the paramagnetic shift, (AH/H) is given by Eq. (2) [3] ... 

Fig. 15. Schematic diagram of the disposition of ferric porphyrin complex inside micelle. (Taken from Ref. 67)...

The reductive nitrosylation of a synthetic iron porphyrin by HNO (193) proceeds with a reported rate constant of 1 x 107 A/-1 s However, this value was estimated based on a HNO dimerization rate constant of 8 x 109 M-1 s-1 (210), which is now considered to be 1000-fold lower [(8 x 106 A/-1 s-1 (106)]. The recalculated constant for the reaction of HNO with the porphyrin (3 x 10s AT V1) is similar to the estimated value of HNO addition to metMb. Synthetic porphyrins generally react 30-fold faster with NO (1 x 109M 1 s-1) than ferrous Mb [for a recent, thorough review see (44)] due to rate-limiting diffusion of NO through the protein. The similarity in rate constants for HNO with metMb and the ferric porphyrin suggests that the rate-limiting step in reductive nitrosylation is likely addition of HNO to the ferric metal, with little influence from the protein structure. 

Hamachi I,Tsukiji S, Shinkai S, Oishi S. Direct observation of the ferric-porphyrin cation radical as an intermediate in the phototriggered oxidation of ferric- to ferryl-heme tethered to Ru(bpy)3 in reconstituted myoglobin. J Am Chem Soc 1999 121 5500-6. 

Mechanism of Autoreduction of Ferric Porphyrins and the Activation of Coordinated Ligands... 

Scattered reports have appeared in the literature indicating that ferric porphyrins can autoreduce in solutions containing certain potential ligands (10, 11, 12). Probably the best known example is the formation of bis (piperidine) tetraphenylporphorinatoiron (II), TPPFe11 (pip) 2, by the addition of piperidine to chlorotetraphenylporphyrinatoiron(III), Reaction 6. Although this reaction was first reported in 1967 (13), the... 

Autocatalytic Nature of the Reduction. The data in Figure 5 show the reduction rate to increase with time, indicating that the reduction is autocatalytic. This is most likely caused by the reduction of the ferric porphyrin by the intermediate radicals generated by Reaction 12, which are expected to be more potent reducing agents than the cyanide ion. Hence any detailed interpretation of the rates will be severely limited. 

Effect of Water. The reduction rate increased as the concentration of water was decreased. For example, removing water ( 50 mM) to levels where it is not observed by NMR increased the reduction rate almost three fold. Cyanide coordinated to ferric porphyrins acts as a hydrogen-bond acceptor towards water (23). Such an interaction would make the coordinated cyanide more difficult to oxidize (24) and hence would decrease the reaction rate. 

Although the mechanistic details of the above systems have not yet been clarified, our results suggest that the autoreduction of ferric porphyrins by a free radical pathway that most likely involves the homolytic... 

Calculations in which the metal orbitals are explicitly considered will be discussed in Section HID. The molecular orbital theory as outlined here has been quite successful in dealing with the spectra of non-ferric porphyrins, with some exceptions. Ferrous porphyrins and haemopro-teins with small unsaturated ligands such as NO and O2 have additional bands (45) and Mn(III) porphyrins exhibit some quite unusual features (46). Distortion of the haem group may also give rise to anomalous effects. We shall discuss these further in Section V. 

The spectra of ferric porphyrins are much more complex than those of the "typical metalloporphyrins discussed in Section II. Fig. 6 shows the spectra of metmyoglobin (MbH20) and ferrimyoglobin cyanide (MbCN). The latter is seen to have a fairly typical metalloporphyrin spectrum, although the a-band is rather weak and there is a new, weak band in the near infra-red. The spectrum of MbH20 is completely different the bands between 17 kK and 19 kK are similar in position to the a- and /3-bands of the cyanide, but there are new bands at around 10 kK, 16 kK, and 20 kK. 

In chemistry, it is well known that O2 can be strongly bound to a ferrons iron porphyrin in solvents without any protein matrix however, the oxygenated states of most simple iron porphyrins are irreversibly converted into /r-oxodimers (eqnation3), PFe(III)-0-PFe(III), via peroxo and ferryl intermediates (eqnation 2). The /x-oxodimer is usually very stable in solvents, so it is sometimes called a thermodynamic sink. In addition, autoxidation of PFe(II)-02 to an inert ferric porphyrin easily occurs under aerobic conditions (equation 4). Thus, it is clear that the heme pockets of myoglobin and hemoglobin play an important role in protecting the 02-bound heme from dimerization and autoxidation. 

DFT calculations of spin densities in low-spin ferric porphyrins have been reported, and in some cases they agree quite well with the predictions of Figure 14, but in others exactly the opposite pattern of large and small spin densities are predicted. Probably the counterintuitive cases are a result of small differences between positive and negative contributions, and additional calculations will need to be done to refine the methods and functionals so that useful predictions of the relationship between axial-ligand plane orientation and spin density distribution can be made. 

A recent study of the interaction of superoxide anion with Fe(II) porphyrins in dimethyl sulphoxide or acetonitrile has suggested the formation of an complex (por-phyrin)Fe02, which is formulated on the basis of infra-red, U.V.-visible, n.m.r., and E.P.R. spectroscopic measurements as (porphyrin)-Fe (high spin)-02 . The E.P.R. spectrum differs from that of other high spin ferric porphyrin complexes but is... 

The kinetics at 430 mn after a laser flash of the photocatalytic system (pH 8.5) in Scheme 11 are biphasic—a fast reaction on a millisecond time-scale because of formation of [(P)Fe ] + was followed by a much slower reaction on a second time-scale because of the conversion of [(P)Fe ] + to compound II, (P)Fe" =0 [168]. In general, ferric porphyrins have ligand-centered one-electron oxidation potentials at which ferric porphyrins are oxidized to ferric porphyrin n radical cations these are higher than oxidation potentials for metal-centered one-electron oxidation to ferryl porphyrins [102, 170]. Despite the smaller (by ca 0.3 eV) driving force for ligand-centered oxidation than for metal-centered oxidation [102, 170], ligand-centered oxidation of HRP occurs before metal-centered oxidation (Scheme 11). This is be-... 

In acidic solutions ferric MP8 is also rapidly oxidized by [Ru(bpy)3] + to a ferric porphyrin n radical cation the rate constant eti = 5.6 x 10 M s which is much larger than the value for HRP [167]. In alkaline solutions, the oxidation product is ferryl MP8 which is represented as a generalized form, (P)Fe =0. In this instance, however, transformation of [(P)Fe ] + to (P)Fe =0 was too fast to be detected [167]. 

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