Chelates of ascorbic acid

Martell, A.E. (1982). Chelates of ascorbic acid formation and catalytic properties. In Ascorbic Acid Chemistry, Metabolism and Uses (eds. P. Seib and B. Tolbert) Advanced Chemistry Series No. 200, p. 153, American Chemical Society, Washington DC. 

The oxidation of ascorbic acid by Cu(II) ion is somewhat less rapid than the rate of oxidation by Fe(III), but is considered to proceed by the same type of mechanism. As may be seen from that data in Table I, the monoprotonated metal chelates of ascorbic acid are generally quite weak and tend to be extensively dissociated in solution. Moreover, as electrons are withdrawn from the ligand to give first the radical anion and finally the neutral dehydroascorbic acid, the aflBnities of these oxidized forms for metal ions are further decreased with each oxidation step. Therefore,... 

The stoichiometric redox reactions of ascorbic acid with oxidizing metal ions and metal chelates, of the type illustrated in Scheme 1, are also involved in the mechanisms of oxidation of ascorbic acid by various oxidants since they function as very efficient catalysts for such reactions. Further details concerning electron transfer processes in the metal chelates of ascorbic acid will be presented in the following discussion of the role of simple metal ascorbate chelates and of mixed ligand ascorbate chelates in the oxidation of ascorbic acid by molecular oxygen. 

A. E. Martell Chelates of Ascorbic Acid. Formation and Catalytic Properties (Ascorbic Acid, Chemistry, Metabolism and Uses, Eds. P. A. Seib and B. M. Tolbert), pp. 153—178. ACS Symposium Series 200 (1982). 

Many reactions catalyzed by the addition of simple metal ions involve chelation of the metal. The familiar autocatalysis of the oxidation of oxalate by permanganate results from the chelation of the oxalate and Mn (III) from the permanganate. Oxidation of ascorbic acid [50-81-7] C HgO, is catalyzed by copper (12). The stabilization of preparations containing ascorbic acid by the addition of a chelant appears to be negative catalysis of the oxidation but results from the sequestration of the copper. Many such inhibitions are the result of sequestration. Catalysis by chelation of metal ions with a reactant is usually accomphshed by polarization of the molecule, faciUtation of electron transfer by the metal, or orientation of reactants. 

The calcium form of EDTA instead of free EDTA is used in many food preparations to stabilize against such deleterious effects as rancidity, loss of ascorbic acid, loss of flavor, development of cloudiness, and discoloration. The causative metal ions are sequestered by displacing calcium from the chelate, and possible problems, such as depletion of body calcium from ingestion of any excess of the free chelant, had it been used, are avoided. 

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... 

Khan, M. M.T., and Martel, A. E. (1967), Metal ion and metal chelate catalyzed oxidation of ascorbic acid by molecular oxygen. I. Cupric and ferric ion catalyzed oxidation, J. Am. Chem. Soc., 89,4176-4185. 

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)... 

One of the best examples of this type of process is seen in the copper(u)-catalysed oxidation of ascorbic acid (vitamin C). Co-ordination of ascorbic acid (in the enediolate form) to copper(n) is a very favourable process and results in the formation of complex 9.2 containing a five-membered chelate ring (Fig. 9-2). 

Chelation, in osazone formation, III, 43 Chemical constitution, physiological activity and, of ascorbic acid analogs,... 

Studies on the antioxidant properties of anthocyanins on human low-density lipoprotein (LDL) and lecithin liposome systems in vitro showed that the inhibition of oxidation increased dose-dependently with antioxidant concentration. The oxidation was catalyzed by copper in the LDL system and the effects of the anthocyanins were explained by several antioxidant mechanisms including hydrogen donation, metal chelation and protein binding [33]. Anthocyanins also prevented the oxidation of ascorbic acid (vitamin C), through chelate formation with the metal ions, and finally by the formation of an ascorbic (copigment)-metal-anthocyanin complex [49]. 

Catalysis of the Autoxidation of Ascorbic Acid by Metal Ions and Metal Chelates... 

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