Halogenation with ascorbic acid

Recently the halogenation of ascorbic acid in acidic media has been reported by Kiss and Berg (71) and independently by Pedersen and coworkers (72, 73). The treatment of ascorbic acid with halogen acids in acetic or formic acids as solvent affords 5-halo-5-acyloxy derivatives... 

Chemical Properties. The most significant chemical property of L-ascorbic acid is its reversible oxidation to dehydro-L-ascorbic acid. Dehydro-L-ascorbic acid has been prepared by uv irradiation and by oxidation with air and charcoal, halogens, ferric chloride, hydrogen peroxide, 2,6-dichlorophenolindophenol, neutral potassium permanganate, selenium oxide, and many other compounds. Dehydro-L-ascorbic acid has been reduced to L-ascorbic acid by hydrogen iodide, hydrogen sulfide, 1,4-dithiothreitol (l,4-dimercapto-2,3-butanediol), and the like (33). 

The most important point during sample preparation is to prevent oxidation of ascorbic acid. Indeed, it is easily oxidized by an alkaline pH, heavy metal ions (Cu and Fe ), the presence of halogens compounds, and hydrogen peroxide. The most suitable solvent for this purpose is metaphosphoric acid, which inhibits L-ascorbic oxidase and metal catalysis, and it causes the precipitation of proteins. However, it can cause serious analytical interactions with silica-based column, e.g., C18 or amino bonded-phases [542] and it could co-elute with AA. 

A reaction, that does not involve the enediol takes place on halogenation. Treatment of 138 with hydrogen bromide-acetic acid affords 5-0-acetyl-6-bromo-6-deoxy-L-ascorbic acid (166).360 Compound 166 was used for the synthesis of 6-substituted derivatives of 138, via the 5,6-anhydro derivative.361... 

In certain systems ascorbic acid was so effective in lowering the valence state of metals that it was used in analytical chemistry (8). Ascorbic acid was used with gold, lead, bismuth, tellurium, copper, phosphorus, uranium, halogens, mercury, and cobalt. 

Halogenated hydrocarbons in aqueous samples are stable at low temperature and reduced pH. Acidification with HCl prevents degradation and allows prolonged storage of samples. Sodium bisulfite and ascorbic acid are also effective preservatives. Samples preserved with either the bisulfite or ascorbic acid were stable over a 112-day experimental period (Maskarinec et al. 1990). 

L-Ascorbic acid reacts as a Michael donor with acrolein or methyl vinyl ketone the products can be converted to cyclic acetals as shown in Scheme 5- The analogue (15) of ascorbic acid has been made as indicated in Scheme 6 the halogen at C-3... 

Denaturation of the DNA The main drawback of the denaturation of the DNA is that the harshness of the treatment can limit the ability to detect phenotypic markers of interest. Experimenters have developed all sorts of alternatives to deal with this particular problem. Among these is precisely the new non-halogenated thymidine analog, EdU. Salic and Mitchison [22] describe the development of EdU on sections of mouse brain already mounted on glass slides. After the removal of paraffin, sections were stained with 10 pM Alexa 568 azide for 10-30 min, after previous incubation for 10-30 min with 100 mM Tris + 0.5-1 mM CUSO4 and 50-100 mM ascorbic acid (added last). In their case, they counterstained the tissue with Hoechst stain, yet another of the bis-benzimide blue fluorescent dyes used to stain DNA. 

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