Ascorbic acid metal complexing properties

Well-characterized transition metal complexes of vitamin C (ascorbic acid) are rare, and a select number of these exhibit anticancer properties. Hollis et al. have described the first examples to be fully characterized by X-ray crystallography.316 Treatment of [Pt(H20)2L2]2+ (L = NH3, MeNH2 L2 = en, 1,2-chxn) with ascorbic acid gives either the mono- or bisascorbate species of the type cis-[PtL2(C2,Os-ascorbate)] (e.g., (125)) or m-[PtL2(C2-ascorbate)(03-ascorbate)], respectively. Interestingly, in both types of complexes, platinum(II) forms a bond with the C-2 atom of ascorbic... 

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]. 

Some antioxidants possess antimicrobial properties, such as propyl gallate and butylated hydroxy anisole, which are somewhat effective against bacteria. Butylated hydroxy toluene has demonstrated some antiviral activity. Compatibility of antioxidants with the drug, packaging system and the body should be studied carefully. For example, tocopherols may be absorbed onto plastics ascorbic acid is incompatible with alkalis, heavy metals, and oxidizing materials such as phenylephrine, and sodium nitrite and propyl gallate forms complexes with metal ions such as sodium, potassium and iron. 

This section will be largely concerned with the reactions of L-ascorbic acid with metal ions and metal ion complexes. However, it would be inappropriate to discuss this subject without a consideration of the properties of the compound which impinge upon inorganic chemistry. It is therefore important to include in this section a discussion of the rather complex redox chemistry of vitamin C and some information on the detection and properties of the species which are frequently cited as intermediates in many redox reactions, namely the ascorbate radicals. 

ECPs including coordination complexes are also able to show electrocatalytic properties e.g., toward the oxidation of nitric oxide in the case of porphyrin functionalized polypyrroles containing various metallic centres [275], for the oxygen or hydrogen peroxide reduction in the case of cobalt-salen PEDOT [276] or iron-containing polysalen [245], or for oxidation of ascorbic acid in the case of osmium bipyridyl functionalized PPy [277]. 

Rutin and several other flavonoid glycosides exhibit antioxidant properties and affect the flexibility and permeability of blood capillaries. Rutin (formerly vitamin P) is thus used in pharmaceutical preparations and food supplements. Together with other substances called bioflavonoids (see Section 5.15), rutin increases levels of ascorbic acid in various animal organs, either by protection against oxidation catalysed by metal ions, or by increased ascorbic acid utilisation in the body. Natural sources of ascorbic acid containing flavonoids (such as rosehips with a considerable amount of rutin) are thus more effective than synthetic vitamin C. A complex of rutin with iron causes dark discoloration of asparagus in tins, and the stannous complex of rutin is yeUow. 

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