Iron complexes with magnesium ions

The porphyrins are cyclic compounds formed from four pyrrole rings linked by methane bridges in a ring system (White et al., 1959 Harper, 1963). A characteristic property of porphyrins is the formation of complexes with metal ions such as iron and magnesium. 

Partial hydolysis of phytate to myo-inositoltetrakis- and -triphosphate is desirable from a nutritional physiological point of view. In comparison with phytate, these less phosphory-lated myo-inositols do not form such stable complexes with cations. Consequently, the absorption of zinc, iron, calcium and magnesium ions is not... 

Problems can be encountered if the deposits contain calcium or magnesium salts that could precipitate CaF2 or MgF2- Another advantage of the use of HF or HF-containing solutions is the partial control of ferric iron corrosion (see Chapter 4). Fluoride ions form very stable complexes with ferric ions ... 

H. 8-Hydroxyquinaldine (XI). The reactions of 8-hydroxyquinaldine are, in general, similar to 8-hydroxyquinoline described under (C) above, but unlike the latter it does not produce an insoluble complex with aluminium. In acetic acid-acetate solution precipitates are formed with bismuth, cadmium, copper, iron(II) and iron(III), chromium, manganese, nickel, silver, zinc, titanium (Ti02 + ), molybdate, tungstate, and vanadate. The same ions are precipitated in ammoniacal solution with the exception of molybdate, tungstate, and vanadate, but with the addition of lead, calcium, strontium, and magnesium aluminium is not precipitated, but tartrate must be added to prevent the separation of aluminium hydroxide. 

Tables 2.12 and 2.13 list the logarithm of the stability constants for the complexes of these chelating agents with various metal ions. Note that with the exception of Chel-138, calcium and magnesium form rather stable complexes with these chelating agents Fe3+ forms the most stable chelate of any metal listed. Generally, ferric iron is followed by Cu2+, Zn2+, Mn2+, Fe2+, Ca2+, and Mg2+. The weak acid properties of these chelating agents must be considered in any evaluation of their behavior. Because they are weak acids, the hydrogen ion tends to compete with the metal ions for association with the active groups.

Tetracyclines and Metals. Tetracyclines can combine with metal ions, such as calcium, magnesium, aluminum, and iron, in the GI tract to form complexes that are poorly absorbed. Thus, the simultaneous administration of certain drugs (e.g., antacids, iron preparations, products containing calcium salts) by patients on tetracycline therapy could result in a significant decrease in the amount of antibiotic absorbed. When two drugs are recognized as having a potential to interact, there is sometimes a tendency to believe that one of them should be discontinued. In the case of the tetracycline antacid interactions, problems can be... 

These compounds cause 50% inhibition of a partially purified reductase from Novikoff rat tumor at 10 8 to 10-7 M (150,151). Approximately the same degree of inhibition was observed with other mammalian reductases (152, 153), but the non-heme iron containing reductase from E. coli was not affected. The inhibition of the mammalian reductases is only partially reversible (154). Since these compounds are strong metal chelators complexation of iron is probably involved in the mechanism of inhibition however excess Fe2+ does not reverse the inhibition, and other evidence indicates that these compounds do not act solely by chelating free iron from solution thus depriving the enzyme of a cofactor (150, 151). Kinetic studies indicate no competition with respect to nucleoside diphosphate substrate, nucleotide effector, or magnesium ions, but partial competition for the dithiol substrate was observed. 

In soils and sediments, complexation can increase organic phosphorus stabilization, especially with iron (III) and calcium ions and their minerals (Harrison, 1987 House and Denison, 2002). The interaction with iron (III) was reported to transform a large part of the labile and moderately labile organic phosphorus forms supplied with manure to paddy soils into more resistant organic phosphorus, possibly because inositol phosphates initially bound to calcium or magnesium were transformed into iron-bound compounds (Zhang et aL, 1994). In the presence of calcium, myo-inositol hexakisphosphate can form two soluble calcium complexes with one or two calcium ions (Ca - or Ca2-phytate), but when three calcium ions are involved (Cag-phytate), the complex precipitates at all pH values (Graf, 1983). This enhances the interaction of myo-... 

Kosaku Tadeka and his coworkers achieved major new results in exploring the structure of the cornflower dye (called protocyanin complex) in 2005 four metal ions (one iron, one magnesium, and two calcium ions) were shown to form a complex with 6-6 molecules of other anthocyanin and flavone pigments. In the vacuoles of rose petals, no metals were found. 

Phospholipids can form salts, for example with metal ions, due to the presence of one free hydroxyl group of the bound phosphoric acid in phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol and two free hydroxyl groups in phosphatidic acid. Mostly, calcium and magnesium ions are involved in these reactions, but the resulting complexes have hydrophobic character, as in metal soaps. Cations of heavy metals (copper, manganese and iron) bound in phospholipids catalyse autoxidation significantly less than free metal ions, phospholipids can thus become synergists of antioxidants. 

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