Vitamin colour reactions

Some conversion into the anhydrovitamin (112) occurs during silica gel t.l.c. of retinyl palmitate in non-polar solvents. Some new colour reactions of vitamin A are reported to be better than the Carr-Price reaction. The kinetics and mechanism of acid-catalysed isomerization of retinyl acetate into the trans-retro-derivative (113) have been studied. Oppenauer oxidation of kitol (39) results in specific cyclopentanol-cyclopentanone oxidation. ... 

Miscellaneous.—The sensitivity of the Zimmermann colour reaction for 17-oxo-steroids (alkaline m-dinitrobenzene) and the stability of the colour vary according to the base used. Assay methods for the active hydroxylated derivatives of vitamin D have been reviewed. ... 

Needles. + HjO from AoOEt-MeOH. M.p. l52-3°. [a] ) — 109 in CHCI3. Absorption spectrum and colour reactions similar to ergo-sterol.. Irradiation —> vitamin EL. 

A semi-quantitative determination can be made by comparison with a series of standards on the same layer, before or after carrying out a colour reaction. A flawless quantitative determination after elution has not so far been achieved as a result of the extreme instabflity of the vitamin A compounds, especially on active adsorbents. Success may perhaps be gained by conversion to the more stable anhydro-form and following then with TLC. The radioactive measurement of isotopes has been carried out frequently [68, 98]. 

In the modified colour reaction for small amounts of vitamin D (e. g., extracts with about 400 lU per 2.5 ml), the solutions for a single determination are pipetted into test tubes as described above but are treated with only 2.0 ml reagent. After briefly mixing, the contents are introduced into 1 cm cuvettes and after precisely 20 sec the extinction measured with a spectrophotometer at 500 nm against the blank solution. 

Fig. e. Quantitative determination of a-tocopherol in water-soluble vitamin A + E concentrate, after hydrolysis [8,130]. Solvent cyclohexane-ether (80 + 20). a-T zone located in UV (254 nm) in part B, scraped ofT, eluted and submitted to colour reaction. 1 a-T standard 2 extract of the hydrolysed sample solution A Eefer-ence chromatogram, sprayed with A SbCls-reagent ... 

Tests for Vitamin A.—In addition to the biological tests by-feeding, the vitamin may be detected and estimated (1) spectroscopically, by observing the absorption coefficient at 328 mp. in the solution and (2) chemically, by the antimony trichloride colour reaction (p. 204) (3) by measuring the time required for dark-adaptation in order that the subject may recognise a dimly-lit diagram. 

OO A chemist adds a few drops of deep violet-red iodine solution to a vitamin C tablet. The iodine solution quickly becomes colourless. Then the chemist adds a solution that contains chlorine, CI2. The chemist observes that the violet-red colour of the iodine reappears. Explain the chemist s observations, in terms of redox reactions. 

Riboflavin (vitamin B2 6.18) consists of an isoalloxazine ring linked to an alcohol derived from ribose. The ribose side chain of riboflavin can be modified by the formation of a phosphoester (forming flavin mononucleotide, FMN, 6.19). FMN can be joined to adenine monophosphate to form flavin adenine dinucleotide (FAD, 6.20). FMN and FAD act as co-enzymes by accepting or donating two hydrogen atoms and thus are involved in redox reactions. Flavoprotein enzymes are involved in many metabolic pathways. Riboflavin is a yellow-green fluorescent compound and, in addition to its role as a vitamin, it is responsible for the colour of milk serum (Chapter 11). 

Possible interference by scraped-off silica gel has been tested on a large number of equally-sized parallel strips by elution and treatment with colour reagent it was found always to be zero. The chloroform used in various operations is stabilised with 1% ethanol but need not be purified. The colour intensity from vitamin is about 3—4% higher than that from an equal amount of vitamin Dg. The calibration curves are linear for amounts up to 500 lU of both active substances. Our standard for each reaction has thus the suitable value of 200 IXJ. Pure vitamin D is totally and selectively inactivated by the inhibitor so that it need not be added to the standard. The otherwise usual correction for absorption of contaminants, carried out at 550 nm, has been shown to be superfluous the contaminants which generally interfere have been taken into consideration or eliminated. 

Chlorine-tofidine (Rgt. No. 42) has proved to be a universal and sensitive reagent [8]. B, Bg, Bg, B g, nicotinic acid, nicotiaamide, pantothenic acid, folic acid, biotin and rutin appear as grey blue spots on a white background after a short while, B g turns violet and Bg greenish. Vitamin C yields no colour. The reaction depends however on the amount of substance, intensity of spraying, thickness and moisture content of the layer and the time at which the layer is inspected. 

Like other vicinal dicarbonyl derivatives of sugars, dehydroascorbic acid is involved in the MaiUard reaction. Dehydroascorbic acid (or its bicyclic hydrate. Figure 5.27) is a y-lactone that is readily hydrolysed under a base catalysis to its parent unstable compound that undergoes a series of irreversible reactions. These reactions result in loss of vitamin C and the formation of coloured products, and the discoloration of fruit and vegetable products. 

Sulfur dioxide (bisulfites) also reacts with thiamine yielding inactive compounds (see Section 5.6.6), forms adducts with riboflavin, nicotinamide, vitamin K, inhibits ascorbic acid oxidation (see Section 5.14.6.1.6) and reacts with ascorbic acid degradation products, reduces o-quinones produced in enzymatic browning reactions back to 1,2-diphenols (see Section 9.12.4), causes decolourisation of fruit anthocyanins (see Section 9.4.1.5.7) and reacts with synthetic azo dyes to form coloured or colourless products (see Section 11.4.1.3.2). Sulfur dioxide also reacts with pyrimidine bases in vitro, specifically with cytosine and 5-methylcytosine. Important reactions of sulfur dioxide are shown in Figure 11.4. 

The reaction of vitamin A with antimony trichloride in chloroform to produce a blue colour. It can be used for the estimation of vitamin A in serum... 

The blue colour produced by the reaction of vitamin A and antimony trichloride fades rapidly. Mixing and measurement must take place within ten seconds. The satisfactory use of null-point instruments is precluded by the time required to make adjustments. 

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