Adventitious iron

It has been proposed that a major source of oxygen radicals in sickle erythrocytes is mutant hemoglobin HbS. However, although HbS showed an accelerated autoxidation rate under in vitro conditions, its in vivo oxidative activity was not determined. Sheng et al. [401] suggested that the observed oxidation rate of HbS is exaggerated by adventitious iron. Dias-Da-Motta et al. [402] proposed that another source of enhanced superoxide production in sickle cells are monocytes in contrast, there is no difference in superoxide release by sickle... 

Transition-metal-ion-free solutions of ascorbate autoxidize (i.e., react with 02) only slowly (e.g., carefully demetalized with a chelating resin such as Chelex 100 1.25 x 10-4mol dm"3 solutions lose only 0.05% ascorbate/15min (Buettner 1988 for a review giving valuable information how to deal with ascorbate solutions, see Buettner and Jurkiewicz 1995). The stability of ascorbate solutions is dramatically reduced in the presence of EDTA which apparently catalyzes the degradation by chelating adventitious iron ions (Buettner and Jurkiewicz 1995). 

Combined EPR and 57Fe Mossbauer spectroscopic studies of the MoFe protein in various overall oxidation states39 40,42,44 48,49) have provided strong evidence for the presence of six metal clusters two M centers that are the protein-bound form of the FeMo-cofactor (a novel Mo-Fe-S cluster) and four unusual tetranuclear iron clusters referred to as the P clusters. These will be discussed separately below. In addition, Mossbauer spectra of MoFe proteins from Azotobacter vinelandii, Clostridium pas-teurianum, and Klebsiella pneumoniae all show an additional component termed S39 40 42, which accounts for 6% of the total Fe present (2 Fe per molecule) and which has Mossbauer parameters (AEq = 1.35 mm/s d = 0.60 mm/s) different from those expected for likely impurities such as high-spin Fe2+ or Fe3+ ( adventitious iron). At present, it is difficult to decide whether species S is an unusual and persistent impurity or an integral part of the MoFe protein. 

The 3Fe cluster in Fdll from D. gigas has been particularly well studied, as it contains only one type of cluster and is usually free from adventitious iron. The use of Mossbauer spectroscopy shows that the oxidized cluster contains three equivalent iron centres, with an isomer shift appropriate for high-spin Fe" in a tetrahedral environment of sulfur atoms. Addition of one electron to give the reduced form of the cluster results in one iron centre remaining as Fe " and the other two remaining equivalent and having an oxidation state of +2.5. Thus, the valencies... 

Figure 17-5 EPR spectra of bovine calcineurin under different redox conditions. Calcineurin was isolated from bovine brain as described [94] and concentrated to 20mgml for the sample shown in (A). The broad features from 100-150mT represent high spin Fe + bound to calcineurin while the sharp feature at g = 4.3 is contributed by a small amount of adventitious Fe " in the sample. Anaerobic treatment of the enzyme with SOmM ascorbic acid (B) had no effect on the EPR signal of the Fe + ion of calcineurin but led to a slight decrease of the resonance of adventitious iron at g = 4.3. Anaerobic addition of 2.0mM Na2S204 and 50/xM methylviologen (C) completely abolished the EPR signals of all Fe + species. The buffer in all samples was 20 mM Tris-HCl, 0.1 niM EDTA, l.OmM magnesium acetate, l.OmM dithiothreitol, 0.15m KCl, pH 7.5. EPR conditions microwave frequency, 9.233 GHz modulation, 1.0 mT at 100 kHz temperature, 4.0 K microwave power, 0.2 mW.

Similarly, when catalyzed the reaction rate decreases significantly as a function of pH level. The optimum reaction pH level is approximately 9.5 to 10.5. Iron, and especially copper, in the boiler may act as adventitious catalysts. However, as metal transport polymers are frequently employed, iron, copper, or cobalt may be transported away from contact with sulfite, and thus are not available for catalysis. (This may be a serious problem in high-pressure units employing combinations of organic oxygen scavengers and metal ion catalysts.)... 

In the PDB 1 YEW structure, the zinc ion site in pmoC is not identified in pMMO by other spectroscopic techniques such as ICP-AES. The reference 190 authors believe that this zinc ion is either adventitious (perhaps depositing from the crystallization buffer) or in a location usually occupied by another metal ion, possibly another copper ion or an iron ion in vivo. 

Fig. 9. EPR spectra of heme-hemopexin and heme-N-domain. X-band EPR spectra at 4 K of ferri-mesoheme-hemopexin (a) and ferri-mesoheme-N-domain (b) are shown. The concentration of both heme complexes was 0.15 mM in 50 50 (v/v) 10 mM sodium phosphate/150 mM NaCl (pH 7.2) glycerol. The g-value scale is noted at the top and the -values observed are noted in each spectrum. Although both complexes are low-spin (some adventitious high-spin iron is present), the differences in g-values indicate nonidentical heme environments in the two complexes (.114).

Achieving low fusion temperatures was largely adventitious in the earliest times but over the many years empirical additions were discovered. Chief among these were iron oxide, silica and lime added individually and under the right circumstances. Although we know now, soda and potash from the fuel ashes mixing with the ore also played a significant role. 

The reduction and oxidation of radicals are discussed in Chapter. 6.3-6.5. That in the case of radicals derived from charged polymers the special effect of repulsion can play a dramatic role was mentioned above, when the reduction of poly(U)-derived base radicals by thiols was discussed. Beyond the common oxidation and reduction of radicals by transition metal ions, an unexpected effect of very low concentrations of iron ions was observed in the case of poly(acrylic acid) (Ulanski et al. 1996c). Radical-induced chain scission yields were poorly reproducible, but when the glass ware had been washed with EDTA to eliminate traces of transition metal ions, notably iron, from its surface, results became reproducible. In fact, the addition of 1 x 10 6 mol dm3 Fe2+ reduces in a pulse radiolysis experiment the amplitude of conductivity increase (a measure of the yield of chain scission Chap. 13.3) more than tenfold and also causes a significant increase in the rate of the chain-breaking process. In further experiments, this dramatic effect of low iron concentrations was confirmed by measuring the chain scission yields by a different method. At present, the underlying reactions are not yet understood. These data are, however, of some potential relevance to DNA free-radical chemistry, since the presence of adventitious transition metal ions is difficult to avoid. 

Trituration of the crude cw-azide with hexane at ambient temperature caused detonation to occur. Isolation of low molecular weight carbonyl azides should be avoided, or extreme precautions taken. A similar incident with the analogous cyclobutane diazide was reported [1], During use as an intermediate in preparation of the 1,2-diamine, the explosive diazide was never isolated or handled free of solvent [2]. The trans isomer (0.25 mole) was prepared by the latter technique, but as a 45 w/v% solution in toluene, rather than as the recommended 10% solution. When a magnetic spin-bar was introduced, the solution detonated. Initiation may have involved friction from adventitious presence of iron filings adhering to the magnetic bar [3],... 

EA, NM, ET, WK, and IL), all but ET are relatively high either in carbon (>1%) or iron (>10%), or both. Whether this pattern of results is chemically significant or merely adventitious cannot be stated definitely at this time. In contrast, those ashes on which all PAHs exhibit photoreactivity (TX and AR) are low both in carbon and iron. ET is the only ash which deviates from this general trend. 

Lippard and co-workers (58, 59) proposed a route (Scheme 4) for the production of 37 from 36 based on the stoichiometry of iron complex to dioxygen. Binding of dioxygen to 36 might yield a reactive superoxo or peroxo adduct that might oxidatively react with solvent, ligand, or adventitious protons to yield 37 directly, using one mole of 02 per mole of 36. 

The core of horse and human ferritin has been identified as crystalline ferrihydrite, 5Fe203 9H2O (49, 137), with some adventitious phosphate bound at the surface of the core. The proportion of phosphate to iron is very low. By contrast, the cores of bacterial ferritin are... 

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