Ferrous complex

The two non-proteinaceous substances f err over din 33) and pyrimine 34) are unique in that they preferentially form ferrous complexes. A number of iron-containing antibiotics have been isolated 35) but of these only a few such as albomycin 36) and ferrimycin 37) have been characterized. 

Iron or copper complexes will catalyse Fenton chemistry only if two conditions are met simultaneously, namely that the ferric complex can be reduced and that the ferrous complex has an oxidation potential such that it can transfer an electron to H2O2. However, we must also add that this reasoning supposes that we are under standard conditions and at equilibrium, which is rarely the case for biological systems. A simple example will illustrate the problem whereas under standard conditions reaction (2) has a redox potential of —330 mV (at an O2 concentration of 1 atmosphere), in vivo with [O2] = 3.5 x 10 5 M and [O2 ] = 10 11 M the redox potential is +230 mV (Pierre and Fontecave, 1999). 

It is quite evident that the ferrous complexes of porphyrins, both natural and synthetic, have extremely high affinities towards NO. A series of iron (II) porphyrin nitrosyls have been synthesized and their structural data [11, 27] revealed non-axial symmetry and the bent form of the Fe-N=0 moiety [112-116]. It has been found that the structure of the Fe-N-O unit in model porphyrin complexes is different from those observed in heme proteins [117]. The heme prosthetic group is chemically very similar, hence the conformational diversity was thought to arise from the steric and electronic interaction of NO with the protein residue. In order to resolve this issue femtosecond infrared polarization spectroscopy was used [118]. The results also provided evidence for the first time that a significant fraction (35%) of NO recombines with the heme-Fe(II) within the first 5 ps after the photolysis, making myoglobin an efficient N O scavenger. 

Bec, N., Gorren, A. C., Voelker, C., Mayer, B., Lange, R., Reaction of neuronal nitric-oxide synthase with oxygen at low temperature. Evidence for reductive activation of the oxy-ferrous complex by tetrahydrobiopterin, J. Biol. Chem. 273 (1998), p. 13502-13508... 

Recently, molecular orbital calculations on some iron complexes have been made by Gray and his co-workers (2, 30). These values for ferrous complexes are plotted in Figure 2 as the dashed line [3d 4s (MO)]. Hence, Mossbauer spectroscopy provides estimates for the 4s electron contribution for molecular orbital calculations. This correlation does not hold for high spin complexes such as FeCb" and FeFe ". Bersuker (3, 4) has attempted to relate both S and AEq directly to molecular orbital parameters. Using the equations developed from this approach, Bersuker, Gordanskii, and Makarov (5) have concluded that in tin tetrahalides the role of dx-Px bonding is significant. 

B. Ferrous Complexes Bearing Bis(pyrazol-l-yl)acetato Ligands 109... 

Deoxvhemerythrin. Henrerythrin appears to retain its triply bridged crae structure in the deoxy form. A low (3.9 A) resoluticm difference electron density map of deoxyHr vs. metHr fi- n X-ray diffraction suggests that the iron atoms move slightly further apart in deoxyHr, but remain five and six coordinate, respectively (29). Confirmation of the iron coordination comes from near-IR absorption and circular dichroism spectra (40,46,47). Based on model high-spin ferrous complexes, the six coordinate iron is expected to give two of the three observed transitions near 10000 cm- while the five-coordinate iron accounts for the d-d transition at ca. 5000 cm-. ... 

The effect of bond lengthening in the ferrous complex on the iron d orbitals is primarily an increased localization with smaller changes in their orbital energies and no change in their order. The NO X orbitals, on the other hand, are appreciably lowered in energy accompanied by a decrease in the d and d contributions to these orbitals. 

The ferrous complex of octaethyl porphyrin in SDS micelles has been characterized as four coordinated (S = 1) ferrous heme species and is similar to that observed for the ferrous protoheme complex in CTAB. It is noted that ferrous complexes of natural porphyrins cannot be stabilized in aqueous SDS micelles, and much larger aqueous micelles like CTAB were needed to stabilize various ferrous protohemes. This indicates that the environment around the octaethyl porphyrin complex in aqueous SDS is more hydrophobic than that of the analogous natural heme species, suggesting that the OEP moiety is embedded much deeper inside the micellar hydrophobic cavity than the protoporphyrin analogue. 

The NMR of ferrous complexes of MPll and MP8 in aqueous SDS solutions has been studied the spectra are very broad and ill-resolved [23]. The heme proton resonances appear in the range 15 to 30 ppm and resemble those of ferrous hemoproteins. The similarity in the linewidths and spectral range of the heme protons in these ferrous peptide complexes with the ferrous hemoproteins suggests that the larger size of the heme peptide restricts the mobility of the molecule inside the micelles compared to that in case of the protoporphyrin complex in micellar solutions, where the spectrum is better resolved. 

The micelle-encapsulated six coordinated bis(pyridinato) iron(II) complexes of protoporphyrin and OEP have been reported by addition of pyridine to the four coordinate ferrous complex in aqueous micellar solution. The optical spectrum of [Fe(II)(PP)(Py)2] in micelle (Fig. 10) is identical to S = 0 six-coordinate bis(pyridinato) iron(II) porphyrin complex [3]. The magnetic moment measurements in solution confirm their diamagnetic nature. The HNMR spectra are also characteristic low-spin iron(II) resonances (S = 0) with shifts lying in the diamagnetic region (Table 2). 

As pointed out earlier, the principal requirement for an active catalyst for the heterolytic splitting of hydrogen is the presence of two suitably disposed functional groups—a metal atom to combine with the hydride ion and a base ( B) to act as a proton acceptor. In line with the evidence for the presence of a ferrous complex in hydrogenase, Rittenberg (18) has suggested the following model for the active site of the enzyme. 

Thus, antioxidant effects of nitrite in cured meats appear to be due to the formation of NO. Kanner et al. (1991) also demonstrated antioxidant effects of NO in systems where reactive hydroxyl radicals ( OH) are produced by the iron-catalyzed decomposition of hydrogen peroxide (Fenton reaction). Hydroxyl radical formation was measured as the rate of benzoate hydtoxylation to salicylic acid. Benzoate hydtoxylation catalyzed by cysteine-Fe +, ascorbate - EDTA-Fe, or Fe was significantly decreased by flushing of the reaction mixture with NO. They proposed that NO liganded to ferrous complexes reacted with H2O2 to form nitrous acid, hydroxyl ion, and ferric iron complexes, preventing generation of hydroxyl radicals. 

Both iron (II) and iron (III) form complexes with mercaptoacetic acid, SRSH2 (5, 11). The ferrous complexes, Fe(II) (RS)2-2 and Fe(II) (OH) (RS) , are highly air-sensitive and are rapidly oxidized to the intense red ferric complex, Fe(III)OH(RS)2 2 (5). Under air-free conditions the color of this latter complex is observed to fade at moderate to fast rates because of a redox reaction in which the iron is reduced to the ferrous state and the mercaptoacetate is oxidized to the disulfide. Michaelis and Schubert (9) proposed that the catalysis takes place through the alternate oxidation and reduction of iron ions in a sequence similar to that just described, but Lamfrom and Nielsen (4) were able to show that under mildly acid conditions the rate of oxygen uptake of solutions containing iron and... 

Although, relevant information about ferrous hemeproteins kinetics, dynamics and ligand photodissociation pathways has been obtain, less is known about ferric hemeproteins photophysic processes. Recent studies performed with Hbl-CN and Mb-CN at ultrafast time scale, have suggested that some of the transients intermediaries observed after ferrous complexes ligand photodissociation are observed in ferric Mb and Hbl [7], However, time-resolved infrared data shows that the complex remained six coordinated after photoexcitation. In this work we present ultrafast data on ferric Hbl-NO, HM-N3, HM-H2S and metHbl complexes that suggest a mechanism for the photoinduced reduction of Hbl species. 

The bleaching signal for Hbl-NO, Hbl-fhS, Hbl-N3 and metaquo Hbl were observed at 410 nm, 410 nm, 415 rnn and 407 nm, respectively, and similar to the ferrous complexes, the sharp rice in the probe transmission signal suggests that the ligand photodissociation occurs within the time resolution of our experiment. The transient spectra were well fitted with time constants of 600 fs and 3 ps. Similar time constants have been previously observed [9]. More than 90 % of the signals were recovered after 10 ps of photoexcitation. 

Collman, J. P., Gagne, R. R., Halbert, T. R., Marchon, J. C., Reed, C. A., Reversible oxygen adduct formation in ferrous complexes derived from a picket fence porphyrin - model for oxymyoglobin. J. Am. Chem. Soc. 1973, 95, 7868-7870. 

Optical absorption spectroscopy and magnetic circular dichroism (MCD) have been used to monitor the appearance of EPR-silent ferrous complexes at the expense of the ferric precursor.35,36 38 7 51 52 Other spectroscopic methods such as... 

SCHEME 4.3 Cytochrome P450 and peroxidase pathways to hydroperoxo-ferric intermediate or Compound 0 (5). Ferric cytochrome P450 (1) is reduced to the ferrous state (2), which can hind dioxygen to form oxy-ferrous complex (3). Reduction of this complex results in the formation of peroxo-ferric complex (4), which is protonated to give hydroperoxo-ferric complex (5). The same hydroperoxo-ferric complex is formed in peroxidases and catalases via reaction with hydrogen peroxide. 

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