Porphyrin-containing Systems

Cytochromes and other Porphyrin-containing Systems.— At neutral pH, the haem group of horse-heart ferricytochrome c lies in a crevice in the globular protein the (low-spin) iron atom is in the plane of the porphyrin ring, and its fifth and sixth coordination positions are occupied by the imidazole nitrogen of histidine-18 and the 

The reduction of ferricytochrome c by at neutral pH appears to be a three-step process. In the first step (A =4.5x 10 lmol- s ) a transient complex is formed between the cytochrome and the hydrated electron, in the second (k= 5 X 10 s ) the haem iron is reduced, and in the third (/ = 1.3 x 10 s ) the protein conformation changes from that appropriate for Fe to that appropriate for Fe. The authors favour a specific pathway for the movement of the electron from the surface of the molecule to the haem iron (step 1). No intermediate complexes were observed in the reduction of ferricytochrome c by the superoxide radical ion. At 20 °C the rate constant for the reaction at pH 4.7—6.7 is 1.4 x 10 1 mol s and as the pH increases above 6.7 the rate constant steadily decreases (eventually reaching zero, indicating that the neutral and high-pH forms if ferricytochrome c are un-reactive). The activation enthalpy is 18 kJ mol and it seems that little protein rearrangement is required for the formation of the activated complex. The kinetics have been reported for the reduction by Cr + of 2-hydroxy-5-nitrobenzyltryptophyl cytochrome c and of iV-formyltryptophyl cytochrome c.  

Sutin s kinetic studies on the oxidation of horse-heart ferrocytochrome c by tris-(phen)cobalt(iii) have recently been extended to acid pH. The reaction is first-order with respect to each reactant but the dependence of the rate on [H+] is not simple. Measurements in chloride medium (7=0.13 mol 1 ) over the pH range 1—7 revealed a rate maximum at pH 2.9 (A =6.7x 10 1 mol s at 25 °C). By contrast, the rate constants at pH 1.0 and 5.8 are 3.2 x 10 and 2.1 x 10 1 mol S , respectively. Below pH 1.7, biphasic kinetics are observed, the slower reaction having a rate constant of ca. 2 s (independent of oxidant concentration). The slow process is ascribed to a conformational change in the ferricytochrome c which is produced in 

There have been several reports dealing with the kinetic behaviour of cytochrome c oxidase and peroxidase but in no case is it possible to conclude much about the 

The debate on whether the pJCa of 8.6 for compound II of horseradish peroxidase (HRP-II) represents the ionization of an iron-bound water molecule continues. Dunford adduces further kinetic evidence that it does while Schejter provides additional n.m.r. evidence that there is no water molecule in the inner co-ordination sphere of the iron(m) atom however, the latter interpretation of the data has been questioned by Vuk-Pavlovi6 and Benko, who suggest that the sixth ligand site of the metal contains what they term a sedentary water molecule. The formation of HRP-I from the reaction of native horseradish peroxidase with HgOa h a rate constant of 2.5 x 10 1 mol s at 25 °C (pH 7.10), with an activation energy of 3.5 1.0 kcal mol . The kinetics of cyanide and fluoride binding to turnip peroxidases Pi and P7 have also been measured.  

The decay of oxyferroperoxidase to ferriperoxidase has been studied by rapidly mixing solutions of ferroperoxidase with various amounts of oxygen and following the time-course of appearance of oxyferroperoxidase and its subsequent decay to ferriperoxidase by reaction with ferroperoxidase. The scheme proposed is 

The reduction of ferricytochrome c by hydrated electrons and by several free radicals has been studied by pulse radiolysis. The reduction of oxidized cytochrome c by [Fe(edta)] - follows first-order kinetics for both protein and reductant, with a rate constant of 2.57 x 10 1 mol s at pH 7 and activation enthalpy and entropy of 6.0 kcal mol and —18 cal K mol , respectively. These values are comparable to those for outer-sphere cytochrome c reductions and redox reactions involving simple iron complexes, and are compatible with outer-sphere attack of [Fe(edta)] at the exposed haem edge, although the possibility of adjacent attack through the haem pocket is not ruled out. The rate data at pH 9 are consistent with [Fe(edta)] reduction of two slowly interconverting forms of the protein, native kt = 2.05 X10 1 mol S ) and high-pH kt = 2.67 x 10 1 mol s ) isomers. A possible route for the transfer of the electron from Cr + to ferricytochrome c has been suggested as a result of the chemical analysis of the chromium(m) product. The reduction by Cr + of the native protein and of ferricytochrome c carboxy-methylated at the haem-linked methionine (residue 80) has been studied kinetically. At pH 6.5 the former process is simple and corresponds to a second-order rate constant of 1.21 x 10 1 mol s . The latter, however, is complex - two chromium- 

Cytochrome c peroxidase is an enzyme which catalyses the oxidation of ferrocyto-chrome c by hydrogen peroxide. At low concentrations of ferrocytochrome c the catalysis follows the classical peroxidase mechanism developed for horseradish peroxidase (HRP) by Chance and George. Two oxidized cytochrome c peroxidase intermediates have been identified in the oxidation of ferrocytochrome c and ferro-cyanide, and (at low substrate concentrations) the reaction scheme is as shown in equation (7) (where CcP represents the native enzyme, CcP-I represents compound I 

The binding of several small molecules to cytochrome c peroxidase has been studied kinetically. In the case of fluoride, the data are consistent with the interaction of F with the protonated form of the enzyme (pJta 5.5) or with the interaction of HF with the ionized form of the protein. The latter is the preferred interpretation, giving a pH-independent rate constant for binding of 5.1 x 10 1 mol s. With cyanide the situation is rather more complex the maximum value of the association rate constant, which occurs between pH 6 and 8, is 1.1 x 10 1 mol S .  

The reduction of cytochrome c oxidase by hydrated electrons has been studied in the absence and in the presence of cytochrome c. In the former case the haem group of the enzyme is not readily reduced, confirming a previous suggestion that haem a is not directly exposed to the solvent. In the presence of cytochrome c the latter is first reduced by eaq (A = 4x 10 1 mol s at 22 C and pH 7.2) and then transfers electrons to the cytochrome c oxidase (k — 6 x 10 1 mol s. It was found that two equivalents of cytochrome c are oxidized initially per equivalent of haem a reduced, showing that one electron is accepted by a second group, probably one of the copper atoms in the enzyme. 

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Synthetic porphyrins have been used extensively as model systems for investigating the complex biological functions of natural porphyrin-containing systems. Given the capabilities of porphyrins to bind and release gases and to act as the active center in catalytic reactions in biological systems, porphyrin-... 

Peroxidases and other Porphyrin-containing Systems.—Dunford and Stillman have written a fairly extensive review on the function and mechanism of action of peroxidases which will be of interest to readers of this section. Comparison of the e.s.r. spectra of ferric horseradish peroxidase (HRP) and myoglobin in O-enriched water has provided further evidence against the presence of a water molecule in... 

Several of the porphyrin-containing systems bearing alligator clips did not possess significantly non-linear I(V) characteristics in both the forward and reverse bias modes when studied in the nanopores. But we have yet to test the metal-containing porphyrins. Although our device studies on the porphyrins have not afforded positive results, these observations were specifically found in the nanopore using a selected set of symmetric structures and should not be used to exclude the search for other porphyrin-based molecular electronic devices. Related structures have found efficacy in the studies by Lindsey and... 

There are, however, some crown type compounds which contain no structural feature except the thiophene subunit, and these deserve some comment here. This is especially true since one of these compounds was prepared very early in the history of crown compounds. Ahmed and Meth-Cohn were interested in sulfur analogs of the porphyrin ring system and prepared compound 7 in 1969 by the method shown in Eq. 

The porphyrin ring system (the parent compound 1 is also known as porphin) consists of four pyrrole-type subunits joined by four methine ( = CH-) bridges to give a macrotetracycle. The macrocycle contains 227i-electrons from which 1871-electrons form a delocalized aromatic system according to Huckel s 4n + 2 rule for aromaticity. The aromaticity of the porphyrin determines the characteristic physical and chemical properties of this class of compounds. The aromatic character of porphyrins has been confirmed by determination of their heats of combustion.1"3 X-ray investigations4 of numerous porphyrins have shown the planarity of the nucleus which is a prerequisite for the aromatic character. 

The large number of cytochromes identified contain a variety of porphyrin ring systems. The classification of the cytochromes is complicated because they differ from one organism to the next the redox potential of a given cytochrome is tailored to the specific needs of the electron transfer sequences of the particular system. The cytochromes are one-electron carriers and the electron flow passes from one cytochrome type to another. The terminal member of the chain, cytochrome c oxidase, has the property of reacting directly with oxygen such that, on electron capture, water is formed ... 

INTRODUCTION IRON-CONTAINING PROTEINS WITH PORPHYRIN LIGAND SYSTEMS... 

The preceding sections of Chapter 7 have discussed iron-containing proteins and enzymes having a porphyrin ring system. Section 7.9 presents a short introduction to the many non-heme iron-containing proteins and enzymes. Two of these are iron-sulfur proteins (Section 7.9.2) and iron-oxo proteins (Section 7.9.3). 

The corrole ring system [188], like porphyrin, contains an aromatic 18 ir-electron chromophore as shown by its electronic and... 

In the case of heme-containing systems, it is believed that the activation barrier for 0-0 bond cleavage can be lowei by the complexation of the resulting oxygen atom by the iron porphyrin center, i.e. ... 

Porphyrins are cyclic tetrapyrrole-containing systems phthalocya-nines are related systems containing benzene rings fused to the external bond of the pyrrole units (see Fig. 5). 

Coenzyme B12 is the cofactor form of vitamin B 2, which is unique among all the vitamins in that it contains not only a complex organic molecule but an essential trace element, cobalt. The complex corrin ring system of vitamin B12 (colored blue in Fig. 2), to which cobalt (as Co3+) is coordinated, is chemically related to the porphyrin ring system of heme and heme proteins (see Fig. 5-1). A fifth coordination position of cobalt is filled by dimethylbenzimidazole ribonucleotide (shaded yellow), bound covalently by its 3 -phosphate group to a side chain of the corrin ring, through aminoisopropanol. 

Plants are green because of the chlorophyll they contain. Chlorophyll, however, requires the porphyrin ring system, which contains nitrogen. If a plant is deficient in nitrogen, it will be less able to pro-... 

The sterols that were chosen as substrates contained two double bonds, one at various positions in the side chain and A5 in the steroid nucleus. Whereas the latter double bond was never touched in reactions with the Fe(III) porphyrin vesicle system 183 in the presence of PhIO, the side chain double bonds of desmosterol 186 and fucosterol 187 were epoxidized to 188 and 189 in 32% and 22% yield, respectively (Fig. 31). In contrast, stigmasterol 190 was not reactive, since the double bond cannot approach the reactive iron-oxo intermediate. 

NMR studies of porphyrin-containing iron(III) complexes are very many owing to their importance in biological systems. The porphyrin systems are tetradentate dianionic ligands and essentially planar, as reported in Fig. 5.7, and allows easy access of monodentate ligands to both the axial coordination positions [15]. 

Note that porphyrin-containing catalytic systems possess a valuable property—high activity in long-chain alkane oxidation. 

There is no doubt that these works promote wider investigations in the branch of heterogeneous catalysis of gas- and liquid-phase reactions using porphyrin-containing heterogeneous catalytic systems. 

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