Ascorbic acid iron complexes

In vitro experiments (38) showed that iron could form soluble chelates with ascorbic acid at the pH of the normal intestinal lumen. Soluble ascorbic acid-iron chelates formed at an acidic pH remained in solution even after the alkalinization of the medium (39). Intraintestinal installation of the pH-adjusted chelates into the rat enhanced the absorption of iron. The authors also suggest that the ascorbic acid normally present in mammalian bile has a physiologically important role in the absorption of nonheme iron from the diet. Whether the whole ascorbic acid-iron chelate is taken up intact into the mucosal cell under these conditions has not been established. Iron is, at the same time, more soluble, reduced, and more absorbable at intestinal pH in the presence of ascorbic acid those factors, with or without direct mucosal uptake of ascorbic acid-iron complexes, explain the contribution of ascorbic acid to the enhancement of iron availability. 

It is of interest to note that the activating effect of ascorbic acid on papain occurs only if ferrous ions are present otherwise the effect of the vitamin is depressant (Maschmann and Helmert, 1934). There is evidence suggesting that the ascorbic acid-iron complex activates by first reducing dithiol compounds associated with the enzyme, and that these thiol compounds in turn activate the enzyme (Purr, 1935). Similar reactions may be involved in the activation of arginase by the ascorbic acid-iron complex (Purr, 1933). 

In vinyl compound polymerization of vinyl acetate, alcohol, bromide, chloride, or carbonate, ascorbic acid can be a component of the polymerization mixture (733-749). Activators for the polymerization have been acriflavine (734), other photosensitive dye compounds (737,738), hydrogen peroxides (740,741,742), potassium peroxydisulfate (743), ferrous sulfate, and acyl sulfonyl peroxides (747). Nagabhooshanam and Santappa (748) reported on dye sensitized photopolymerization of vinyl monomers in the presence of ascorbic acid-sodium hydrogen orthophosphate complex. Another combination is vinyl chloride with cyclo-hexanesulfonyl acetyl peroxide with ascorbic acid, iron sulfate, and an alcohol (749). Use of low temperature conditions in emulsion polymerization, with ascorbic acid, is mentioned (750,751). Clarity of color is important and impact-resistant, clear, moldable polyvinyl chloride can be prepared with ascorbic acid as an acid catalyst (752) in the formulation. 

Probably the most extensively applied masking agent is cyanide ion. In alkaline solution, cyanide forms strong cyano complexes with the following ions and masks their action toward EDTA Ag, Cd, Co(ll), Cu(ll), Fe(ll), Hg(ll), Ni, Pd(ll), Pt(ll), Tl(lll), and Zn. The alkaline earths, Mn(ll), Pb, and the rare earths are virtually unaffected hence, these latter ions may be titrated with EDTA with the former ions masked by cyanide. Iron(lll) is also masked by cyanide. However, as the hexacy-anoferrate(lll) ion oxidizes many indicators, ascorbic acid is added to form hexacyanoferrate(ll) ion. Moreover, since the addition of cyanide to an acidic solution results in the formation of deadly... 

Masking by oxidation or reduction of a metal ion to a state which does not react with EDTA is occasionally of value. For example, Fe(III) (log K- y 24.23) in acidic media may be reduced to Fe(II) (log K-yyy = 14.33) by ascorbic acid in this state iron does not interfere in the titration of some trivalent and tetravalent ions in strong acidic medium (pH 0 to 2). Similarly, Hg(II) can be reduced to the metal. In favorable conditions, Cr(III) may be oxidized by alkaline peroxide to chromate which does not complex with EDTA. 

Ascorbic acid also forms soluble chelate complexes with iron (142—145). It seems ascorbic acid has no effect on high iron levels found in people with iron overload (146). It is well known, in fact, that ascorbic acid in the presence of iron can exhibit either prooxidant or antioxidant effects, depending on the concentration used (147). The combination of citric acid and ascorbic acid may enhance the iron load in aging populations. Iron overload may be the most important common etiologic factor in the development of heart disease, cancer, diabetes, osteoporosis, arthritis, and possibly other disorders. The synergistic combination of citric acid and ascorbic acid needs further study, particularly because the iron overload produced may be correctable (147). 

Ascorbic acid, for example, is oxidized to dehydroascorbic acid with reduction of the iron(III) ions. The Fe(II) ions so produced react with 2,2 -bipyridine with formation of a colored complex. 

Camire (2002) showed that texturization does not seem to have a great effect on mineral retention and bioavailability. Others have reported increased retention of ascorbic acid in rice- and maize-based snacks (Hazell and Johnson, 1989 Plunkett and Ainsworth, 2007), increased iron diffusibility and absorption of iron-complexed protein (Poltronieri et al, 2000 Watzke, 1998), and no difference in iron and zinc absorption in human subjects fed textured bran-flour (Fairweather-Tait et al, 1989). 

The same model was applied to the oxidation of the H2A form of ascorbic acid. In this case iron(III) was found to be a somewhat more active catalyst than copper(II). The difference could be explained by assuming that Fe(III) forms a more stable complex with H2A than does Cu(II) because of the higher charge of the metal ion. 

The formation of the [M(HA)](" 1>+ complex was confirmed in independent pH-metric experiments in the case of copper(II). These studies also provided evidence that ascorbic acid is coordinated to the metal center in its monoprotonated form. Because of relatively fast redox reactions between iron(III) and ascorbic acid, similar studies to confirm the formation of [Fe(HA)]2+ were not feasible. However, indirect kinetic evidence also supported the formation of the [M(HA)](" 1>+ complex in both systems (6). 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

The iron absorption is enhanced when soluble monomeric complexes are formed (in the presence of e.g. proteins, aminoacids, ascorbic acid, EDTA, citric acid etc.) [13]. 

The RQ flex test kit (Merck) which uses specific test strips is useful for the semi-quantitative determination of several analytes. D(+) ascorbic acid can be determined in fortified food products with an accuracy of 85-115% (unpublished data), however the procedure cannot be applied to coloured food products. Added iron salts may be extracted from food products with dilute sulphuric acid and adjusted to pH2 with NaOH solution. Fe3+ is reduced to Fe2+ with ascorbic acid. Fe2+ reacts with Ferrospectral to form a red-violet complex. An internal calibration is provided on a barcode which is read by the RQ-flex reflectometer prior to any measurements. This avoids the need to calibrate the instrument with standard solutions. 

An extractive spectrophotometric procedure based on the complexation of reduced Iron(II) with 5-Chloro-7-iodo-8-hydroxyquinoline (CIHQ) for the estimation of micro amounts of vitamin C. The resulting brown colored complex was extracted into chloroform to give a reddish brown extract which shows an absorption band at 485 nm. This chelate was formed immediately and the apparent molar absorptivity and Sandell s sensitivity for vitamin C was found to be 8.5 x 105 dm3 mol"1 cm 1 and 2.072xl0 4g cm 2. Linear relationship between absorbance and concentration of ascorbic acid is observed up to 0.8 pg ml"1. Interference studies of different substances including sugars, vitamins and amino acids, metal ions and organic acids were carried out. The utility of the method was tested by analysing some of the marketed products of vitamin C... 

Thus, antioxidant effects of nitrite in cured meats appear to be due to the formation of NO. Kanner et al. (1991) also demonstrated antioxidant effects of NO in systems where reactive hydroxyl radicals ( OH) are produced by the iron-catalyzed decomposition of hydrogen peroxide (Fenton reaction). Hydroxyl radical formation was measured as the rate of benzoate hydtoxylation to salicylic acid. Benzoate hydtoxylation catalyzed by cysteine-Fe +, ascorbate - EDTA-Fe, or Fe was significantly decreased by flushing of the reaction mixture with NO. They proposed that NO liganded to ferrous complexes reacted with H2O2 to form nitrous acid, hydroxyl ion, and ferric iron complexes, preventing generation of hydroxyl radicals. 

To enhance iron excretion, intensive chelation therapy is used. The most successful drug is desferrioxamine B, a powerful Fe3+-chelator produced by the microbe Streptomyces pilosus,6 The formation constant for the Fe(III) complex, called ferrioxamine B, is 103afi. Used in conjunction with ascorbic acid—vitamin C, a reducing agent that reduces Fe3+ to the more soluble Fe2+— desferrioxamine clears several grams of iron per year from an overloaded patient. The ferrioxamine complex is excreted in the urine. 

Chlorins (2) are undoubtedly the most important dihydroporphyrins, since the chlorin chromophore is found in chlorophylls and some bacteriochlorophylls and, as the magnesium complex, is the catalyst in photosynthesis. The method of choice for formation of trans-chlorins involves reduction of iron porphyrins with sodium in boiling isopentyl alcohol (57JCS3461), but methods involving photochemical reduction of tin(IV) porphyrins, isomerization of phlorins, reduction of metalloporphyrins with sodium anthracenide followed by protonolysis, heating with sodium ethoxide, and photoreductions of zinc(II) porphyrins in the presence of ascorbic acid have also been employed. The best method for formation of c/s-chlorins (note that all natural chlorophylls possess the trans arrangement) appears to... 

Whilst these acids are of minor importance in man, another multifunctional acid, ascorbic add (vitamin C 6), has an apparent role in human iron metabolism as one of the postulated factors aiding iron uptake. Ascorbic add reduces Fe111 to Fe" with probable complexation of the latter. It thus converts Fe"1 to a more soluble form. Ascorbic acid can also reduce Cu" to Cu1 (equation 1), and there is accumulating evidence of a link between the metabolism of ascorbic acid and that of Cu.34... 

Absorption of iron by the individual varies with age, iron status, the amount and chemical form of the iron ingested, and with conditions in the gastrointestinal tract, only about 5—15% of iron in the diet being normally absorbed. Ferrous iron, as the sulphate, gluconate, fumarate or lactate or as ferrous ammonium sulphate, is appreciably taken up into the bloodstream from the duodenum, especially in the presence of ascorbic acid, a reducing agent. Little difference was found in the extent of their absorption between ferrous sulphate and the various chelates, but ferric ammonium citrate or polysaccharide complexes were only very poorly absorbed22)... 

Fe2+ to generate an enzyme-bound iron-oxygen complex, and ascorbic acid (vitamin C) to subsequently reduce this complex. 

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