Ferric complexing abilities

In the LI CAM series lipophilicity, hence ability to cross cell membranes, was introduced by substitution of alkyl groups on the terminal nitrogens262. Rastetter et al.256 have also produced a linear polycatecholamide. However, unlike the LICAM series their siderophore is a chiral analogue, synthesized from L-asparagine, for which a formation constant of io46-5 1-2 has been calculated for its ferric complex. In contrast the formation constant for ferric enterobactinate is 1052 263. ... 

The cytotoxicity of BLM is believed to result from its ability to bind iron, activate oxygen, and form an activated BLM (Fe-114) (556) which cleaves DNA and possibly RNA (557). The ability of the Fe(II)-BLM complex to bind to oxygen and produce oxygenated BLM species such as 02-Fe(III)-BLM or 02-Fe(II)-BLM may be due to the presence of delocalized 77-electrons around the iron and the strong iron-pyrimidine 77-back-bonding (558, 559). Oxygenated BLM accepts an additional electron to form activated low-spin ferric-peroxide-BLM (Oi -Fe(III)-BLM) (558, 559). The structural features of Fe-BLM responsible for DNA (or RNA) degradation remain unclear (560). Bleo-... 

Another system under investigation is the iron/ chromium redox flow battery (Fe/Cr RFB) developed by NASA. The performance requirements of the membrane for Fe/Cr RFB are severe. The membrane must readily permit the passage of chloride ions, but should not allow any mixing of the chromium and iron ions. An anionic permselective membrane CDIL-AA5-LC-397, developed by Ionics, Inc., performed well in this system. ° It was prepared by a free radical polymerization of vinylbenzyl chloride and dimethylaminoethyl methacrylate in a 1 1 molar ratio. One major issue with the anionic membranes was its increase in resistance during the time it was exposed to a ferric chloride solution. The resistance increase termed fouling is related to the ability of the ferric ion to form ferric chloride complexes, which are not electrically repelled by the anionic membrane. An experiment by Arnold and Assink indicated that... 

The donor types D3, D4, and D6 of Keilin and Nicholls (37) all reduce compound I of Type A enzymes directly to the ferric state in a two-electron process without detectable intermediates. Each of these donors is probably also able to bind in the heme pocket of the free enzyme. Alcohols (type D3) form complexes with free ferric Type A enzymes whose apparent affinities parallel the effectiveness of the same alcohols as compound I donors (39). Formate (type D3) reacts with mammalian ferric enzyme at a rate identical to the rate with which it reduces compound I to free enz5mie (22). Its oxidation by compound I may thus share an initial step analogous to its complex formation with ferric enzyme. Formate also catalyzes the reduction of compound II to ferric enzyme by endogenous donors in the enz5mie (40, 41). Both compound I and compound II may thus share with the free enzyme the ability to ligate formate in the heme pocket. Nitrite, which is oxidized to nitrate by a two-electron reaction with compoimd I (type D4), also forms a characteristic complex with free enzyme (42). In both cases the reaction involves the donor in its protonated (HNO2) form. 

Nitric oxide binds in the end on position to ferrous hemes, with the nitrogen atom facing the iron (Scheidt and Piciulo, 1976 Jameson et d., 1981 Peng and Ibers, 1976). It binds more tightly to ferrous iron, but has a greater ability than CO to bind to iron in the ferric state, producing a diamagnetic complex. Ferrous-NO complexes are reasonably stable compared to ferrous-02 complexes, but in many cases they can be oxidized or reduced. Nitric oxide is much more likely than CO to react at a metal site rather than to bind reversibly. 

Another approach for the determination of PVs, as described in the Alternate Protocol, is a spectrophotometric method based on the ability of peroxides to oxidize iron(II) to iron(III). The ferric ion forms a complex with xylenol orange, whose concentration can be determined spectrophotometrically. The ferrous oxidation/xylenol orange (FOX) method is very rapid, requires little sample, and can determine PVs as low as 0.1 meq active oxygen/kg oil. 

ALTERNATE PROTOCOL DETERMINATION OF PEROXIDE VALUE BY MEASUREMENT OF IRON OXIDATION The ferrous oxidation/xylenol orange (FOX) method is based on the ability of lipid peroxides to oxidize ferrous ions at low pH. The resulting oxidation is quantitated by using a dye that complexes with the generated ferric ions to produce a color that can be measured spectrophotometrically. Peroxide values (PVs) as low as 0.1 meq active oxygen/kg sample can be determined with this method, providing a distinct advantage over iodometric titration. 

Iron is a nonamphoteric, transition element with the ability to exist in two oxidation states—Fe2+ (ferrous) and Fe3+ (ferric). A positive reaction to alkaline ferric chloride is an indication of the presence of hydroxyl groups with which Fe2+ forms colored complexes. Stable copper and iron chelates... 

In conclusion, any attempt to reconcile toxicological and chemical data on superoxide should take into account two important prerequisites first, it is essential to distinguish the toxicity of intra- versus extracellular superoxide second, the one-electron oxidizing properties of HO2 should not be overlooked and one should exercise a certain amount of caution before adopting the common viewpoint that the key toxicity of superoxide comes from its ability to recycle ferrous iron complexes from their ferric homologs. 

Virtually all microorganisms—with the exception of certain lactobacilli— require iron as cofactor of many metabolic enzymes and regulatory proteins because of its ability to exist in two stable oxidation states. Although iron is one of the most abundant elements in the environment, it is often a limiting factor for bacterial growth. This is so because of the formation of insoluble ferric hydroxide complexes under aerobic conditions at neutral pH, which impose severe restrictions on the availability of the element. Consequently, bacteria have evolved specialized high-affinity transport systems in order to acquire sufficient amounts of this essential element. 

Most bacteria have the ability to produce and secrete molecules—called siderophores—to fulfill their iron requirements. Siderophores are special iron-chelating agents that facilitate iron solubilization and uptake. They are water-soluble, low-molecular weight molecules that bind ferric ions strongly. The ability of bacteria to utilize siderophores is associated with the presence of transport systems that can recognize and mediate uptake of the ferric-siderophore complexes into the cell. These iron-acquisition systems are regulated in response to iron availability, and their action thus increases under iron limitation conditions. 

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