Iron instability

When microscopically examined, this group of precipitates lack defined shape and generally assumes color reflective of the wine. Precipitates in this category include protein, phenolics (and complexes of the two), copper, and iron instabilities (casses) as well as paraffin used to coat corks. 

Metal instabilities ( casse ) are relatively rare today but when encountered, the metals involved are generally copper or iron. Copper casse is present as an initially white and later a reddish-brown precipitate in bottled or other wines stored under low-oxygen conditions. An iron instability may be present as either ferric phosphate ( white casse) or ferric tannate ( blue casse). Even though a ferric phosphate instability is described as white casse, it may assume various shades of a blue amorphous precipi-... 

All the anhydrous - -3 and +2 halides of iron are readily obtained, except for iron(III) iodide, where the oxidizing properties of Fe and the reducing properties of 1 lead to thermodynamic instability. It has, however, been prepared in mg quantities by the following reaction, with air and moisture rigorously excluded,... 

Ion transfer instability through protective films, 272 Iron... 

In the Galaxy, we know 93 (3 Cephei (Stankov Handler 2004) and about 100 SPB-type stars (De Cat et al. 2004). They fall within the instability strips predicted by the theory. The K-mechanism driving pulsations in (3 Cephei and SPB stars strongly depends on the abundance of the iron-group ions in the driving zone at temperatures around 2 x 105 K (Dziembowski Pamyatnykh 1993, Dziembowski et al. 1993). Theoretical models predict that pulsations of (3 Cephei and SPB-type vanish for Z = 0.01 and Z = 0.006, respectively (Pamyatnykh 1999). 

AGB stars constitute excellent laboratories to test the theory of stellar evolution and nucleosynthesis. Their particular internal structure allows two important processes to occur in them. First is the so-called 3(,ldredge-up (3DUP), a mixing mechanism in which the convective envelope penetrates the interior of the star after each thermal instability in the He-shell (thermal pulse, TP). The other is the activation of the s-process synthesis from alpha captures on 13C or/and 22Ne nuclei that generate the necessary neutrons which are subsequently captured by iron-peak nuclei. The repeated operation of TPs and the 3DUP episodes enriches the stellar envelope in newly synthesized elements and transforms the star into a carbon star, if the quantity of carbon added into the envelope is sufficient to increase the C/O ratio above unity. In that way, the atmosphere becomes enriched with the ashes of the above nucleosynthesis processes which can then be detected spectroscopically. 

The total yield of hydrocarbon for the reaction between Cp2Fe2(C0) and LAH was v 40% (0.4 mmole of hydrocarbon product per mmole of iron starting material). LiCpFe(C0)2 and Cp2Fe2(C0) were both in solution after the reaction as shown by infrared spectra even when a 30-fold excess of LAH was used. We believe that HCpFe(C0)2 was formed initially, but because of its thermal instability, decomposed to H2 and Cp2Fe2(C0). Support for this... 

Nalidixic acid and sodium nalidixate form a strong colored complex with iron III. The maximum of absorbance of the complex is 410 nm and Beer s Law is obeyed from 10 to 250 yg of nalidixic acid per ml(33)5 and from 0.43 to 17.05 mg iron III per ml.(34) Nalidixic acid complexes through the oxo-group on C-4 and the carboxylic acid group on C-3. Three moles of nalidixic acid complex with one mole of iron III. The instability constant of the complex was calculated to be 2.11 x 10 8 by Dick and Murgu.(34)... 

Fridovich recently summarized important aspects concerning the accurate detection and measurement of superoxide. He indicates that univalent reduction of O2 to superoxide is a facile process, but the instability of superoxide in aqueous solutions hinders its detection and measurement. To measure intracellular superoxide, he favors use of the rapid inactivation of [4Fe-4S]-containing dehydratases (such as aconitase) by oxidation of their iron-sulfur clusters. See Oxygen, Oxides Oxygen Radicals... 

One type of the constituent metallocenters in the MoFe protein has the properties of a somewhat independent structural entity. This component, referred to as the FeMo cofactor (FeMo-co), was first identified by Shah and Brill (1977) as the stable metallocluster extracted from acid-denatured MoFe protein. The FeMo-co was able to fully activate a defective protein in the extracts of mutant strain UW45, a protein which subsequently was shown to contain the P clusters but not the EPR-active center. The isolated cofactor accounted for the total S = t system observed by EPR and Mdssbauer spectroscopies of the holo-MoFe protein (Rawlings et al., 1978). Elemental analysis indicated a composition of Mo Fee-8 Se-g for the cofactor, which, if there are two FeMo-co s per a2 2> accounts for all the molybdenum and approximately half the iron in active enzyme (Nelson etai, 1983). Although FeMo-co has been extensively studied [reviewed in Burgess (1990)] the structure remains enigmatic. To date, all attempts to crystallize the cofactor have failed. This is possibly due to the instability and resultant heterogeneity of the cofactor when removed from the protein. Also, there is a paucity of appropriate models for spectral comparison (see Coucouvanis, 1991, for a recent discussion). Final resolution of this elusive structure may require its determination as a component of the holoprotein. 

Posner and coworkers proposed that the highly electrophilic epoxide could not be isolated due to its inherent instability but was a potent alkylating agent responsible for parasite death. However, Wu and coworkers have isolated a small quantity (1-2% yield) of the epoxide 16 in their iron(II) degradations of artemisinin using iron(II) sulphate in aqueous acetonitrile s. These authors conclude that it is not the active killing species since any external nucleophiles would have to compete with the in-built nucleophile (the OH moiety). Moreover, Avery and coworkers also concluded that the epoxide could not... 

The potassium atoms are attached to the ring itself and not to iron, so that isomerism becomes possible depending on the positions assumed by these atoms. The meta- and para-salts are more evenly balanced and may be expected to show greater stability the ortho-salt, is the double salt, 4KCN.Fe(CN)2, and in view of the instability of ferrous cyanide this complex may not be stable. Also, either the meta- or the para- compound may be easily transformed into the nitro-prussidc. 

From the reaction of iron atoms with a solution of 1,5-COD in methylcyclohexane, brown crystals of Fe(l,5-COD)2 can be isolated in 40% yield (81). The thermal instability (rdec = -30°C) of this substance presents special problems in isolation of a pure product however, subsequent reactions can often be performed on solutions freed from iron metal by low-temperature filtration. The yellow-brown cobalt system, Co(l,5-COD)2, may be prepared similarly (134). 

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