Iron Initiators

Low activity has been reported for several simple iron-based initiators including oxides,827 porphyrins,860 carboxylates861-863 and alkoxides, 64-866 However, the ferric cluster [Feslqs-0)(OEt)13], (284), is a highly active initiator for the polymerization of LA 867 97% conversion of 450 equivalents is achieved in just 21 min at 70 °C in toluene. Polydispersities are typically between 1.15 and 1.30, even at monomer loadings of 1,000 equivalents. 

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Deming TJ (1999) Cobalt and iron initiators for the controlled polymerization of alpha-amino acid-N-carboxyanhydrides. Macromolecules 32 4500-4502... 

Carvediol is a vasodilator with beta-adrenergic antagonist activity. It has cardioprotective activity in animal models. The antioxidant effect of carvediol was compared with five other beta blockers in iron-initiated lipid peroxidation, where it inhibited TBARs formation and protected membrane-bound tocopherol in rat brain homogenate (Yue et al., 1992a). The ortJ <)-substituted phenoxylethyl-amine is responsible for the improved antioxidant activity. 

In addition to the well-known iron effects on peroxidative processes, there are also other mechanisms of iron-initiated free radical damage, one of them, the effect of iron ions on calcium metabolism. It has been shown that an increase in free cytosolic calcium may affect cellular redox balance. Stoyanovsky and Cederbaum [174] showed that in the presence of NADPH or ascorbic acid iron ions induced calcium release from liver microsomes. Calcium release occurred only under aerobic conditions and was inhibited by antioxidants Trolox C, glutathione, and ascorbate. It was suggested that the activation of calcium releasing channels by the redox cycling of iron ions may be an important factor in the stimulation of various hepatic disorders in humans with iron overload. 

The mechanism of iron-initiated superoxide-dependent lipid peroxidation has been extensively studied by Aust and his coworkers [15-18]. It was found that superoxide produced by xanthine oxidase initiated lipid peroxidation, but this reaction was not inhibited by hydroxyl radical scavengers and, therefore the formation of hydroxyl radicals was unimportant. Lipid peroxidation depended on the Fe3+/Fe2+ ratio, with 50 50 as the optimal value [19]. Superoxide supposedly stimulated peroxidation both by reducing ferric ions and oxidizing ferrous ions. As superoxide is able to release iron from ferritin, superoxide-promoted lipid peroxidation can probably proceed under in vivo conditions [16,20]. 

Thus, the mechanism of MT antioxidant activity might be connected with the possible antioxidant effect of zinc. Zinc is a nontransition metal and therefore, its participation in redox processes is not really expected. The simplest mechanism of zinc antioxidant activity is the competition with transition metal ions capable of initiating free radical-mediated processes. For example, it has recently been shown [342] that zinc inhibited copper- and iron-initiated liposomal peroxidation but had no effect on peroxidative processes initiated by free radicals and peroxynitrite. These findings contradict the earlier results obtained by Coassin et al. [343] who found no inhibitory effects of zinc on microsomal lipid peroxidation in contrast to the inhibitory effects of manganese and cobalt. Yeomans et al. [344] showed that the zinc-histidine complex is able to inhibit copper-induced LDL oxidation, but the antioxidant effect of this complex obviously depended on histidine and not zinc because zinc sulfate was ineffective. We proposed another mode of possible antioxidant effect of zinc [345], It has been found that Zn and Mg aspartates inhibited oxygen radical production by xanthine oxidase, NADPH oxidase, and human blood leukocytes. The antioxidant effect of these salts supposedly was a consequence of the acceleration of spontaneous superoxide dismutation due to increasing medium acidity. 

In specific cases, the operating costs for the removal of manganese (initial concentration of 6 ppm) and iron (initial concentration of 15 ppm) from groundwater to levels of 0.04 and 0.3 ppm, respectively, have been estimated at 0.40 per 1000 gal of gronndwater. These operating costs assnme 2 hr per shift of operating labor (D165121, p. 9). 

Scheme 9.40 Iron-initiated ring expansion reaction via carbenium ions.

In addition to the ring expansion of cyclobutane derivatives, 1-cyclopentylcyclo-pentanol derivatives and spiro compounds (Scheme 9.41) could be utilized in the iron-initiated transformation utilizing anhydrous iron(III) chloride on silica, with good yields [99]. 

The uptake of iron from transferrin by reticulocytes is a time-, temperature-, and energy-dependent process in which integrity of both protein and cells is required (64, 65). Synthetic iron chelates, once thought to be effective iron donors (66), appear to depend on membrane-bound transferrin as an intermediate agent cells depleted of the protein by preincubation and washing no longer accept iron from such complexes (67). When such cells are reincubated with transferrin, their capacity to accept iron initially bound to synthetic chelators is largely restored. 

Fig. 53 pe-pH diagram for the system iron (initial solution 10 mmol Fe + lOmmol Cl) Variation of pE, pH in steps of 1 (above) and 0.5 (below, higher raster resolution, numbers as indicated in the figure above)... 

Under the aforementioned assumptions, Bringas et al." developed a suitable EPT mathematical model able of describing the separation and concentration kinetics of zinc and iron initially present in real spent passivation baths. Using the kinetic and equilibrium parameters reported in Table 29.2 and experimental data, the EPT mathematical model was successfully validated leading to the conclusion that 88% of the simulated concentration values of zinc and iron in the spent passivation bath fell within the range... 

The potential (P in Fig. 6.1) at which passivity of iron initiates (passivating potential) approximates, but is not the same as, the Flade potential because of IR drop through the insulating layer first formed and because the pH of the electrolyte at the base of pores in this layer differs from that in the bulk of solution (concentration polarization). These effects are absent on decay of passivity. 

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