Ascorbic acid vitamin reagents

Fumed silica A-200 (Pilot plant at the Institute of Surface Chemistry, Kalush, Ukraine specific surface area Ascorbic acid (vitamin C) and all-rac-a-Tocopheryl acetate (vitamin E acetate) were used as adsorbates. Folin-Ciocalteu s phenol reagent (Merck) was used to measure the total polyphenolic index. Silica samples with different degree of surface silylation were obtained using gas-phase chemical modification of highly disperse silica (A-200) surface by trimethylchlorosilane.6... 

Other redox reagents include dinucleotides such as FAD (flavine adenine dinucleotide), lipoic acid, which we will meet when we discuss the chemistry of thiamine, and ascorbic acid (vitamin C)> which you met in Chapter 49. Ascorbic acid can form a stable enolate anion that can transfer a hydride ion to a suitable oxidant. 

USE Analytical reagent in the determination of ascorbic acid (vitamin C) which reduces the dye to a colorless hydroxy compd. 

Antioxidant activity of silica nanocomposites with immobilized vitamin C was tested using the polyphenolic activity index.8 After adsorption of ascorbic acid on the silica surface and centrifugation, the excess solution was removed to obtain the suspension of a fixed volume (2 ml). Distilled water, sodium carbonate solution, and Folin-Ciocalteu s phenol reagent were subsequently added to suspensions and to the reference Vitamin C solution. The suspensions were then stored for 30 min, and the optical density of supernatant was measured at X = 750 nm. The reference solution of ascorbic acid was used to compare antioxidant activity of vitamin-containing nanocomposites with the activity of dissolved vitamin C. 

The complexity of the redox processes involving vitamin C have not prevented numerous studies of the oxidation of L-ascorbic acid and some on the oxidation and reduction of dehydroascorbic acid and other oxidation products. One of the reasons for the great interest in this subject is that the role of vitamin C in living systems is certainly connected to its oxidation-reduction behaviour and this may ultimately be the key to understanding the biological mechanisms of the actions of vitamin C. In this. section we will examine the oxidation and reduction of vitamin C by a wide variety of reagents, with emphasis on the behaviour of L-ascorbic acid since this is the most studied compound in this respect. 

Vitamin C Vitamin C activity resides in two naturally occurring compounds ascorbic acid and its oxidation product, dehydroascorbic acid. In human tissues ascorbic acid predominates. Ascorbic acid is labile in most samples, oxidizing to dehydroascorbic acid and then degrading to 2,3-diketogluconic acid. Various reagents can be used to prevent this oxidation in plasma or whole blood samples. Extraction with 5% metaphosphoric or trichloroacetic acid is the usual initial preparation. Only ascorbic acid may be detected by UV spectrophotometry at 245-265 nm, the absorption maxima of dehydroascorbic acid being 210 nm. A similar problem exists with electrochemical detection where ascorbic acid oxidizes at +0.7V with carbon electrodes. Fluorescent derivatives may be formed with 2-4-din-itrophenylhydrazine or o-phenyldiamine. These derivatives can be assayed by reversed-phase HPLC. 

Figure 9 Effect of water/methanol ratio with tetrabutylammonium phosphate reagent on retention times of water-soluble vitamins on xBondapak C18 ascorbic acid (O), niacin ( ), folic acid ( ), pyridoxin ( ), riboflavin (A), vitamin Bjj (A), niacinamide (V), thiamine (V), x not eluted from column. (From Ref. 103.)... 

In the visual titration procedure a dilute solution of the dye is used to titrate the ascorbic acid in an acid extract of the tissue or vitamin source. A pink color that persists for 15 seconds after the addition of one drop of the indophenol reagent is taken as the end point of the titration. The application of this reagent is based upon the assumption that ascorbic add is the prindpal, or peihaps only, substance in add extracts of the vitamin sources that reduces indophenol at a pH of 1-1, which is essentially true for a condderable number of plant and animal tissue extracts. 

Early studies showed that vitamin C has properties t5 ical of an organic acid, but since these could be destroyed by the removal of two hydrogen atoms on oxidation by iodine (with the formation of dehydroascorbic acid, CX) they clearly were not due to the presence of a carboxyl group. This, coupled with the extreme instability of ascorbic acid to alkaline reagents... 

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