Riboflavin decomposition

Hais, I. M., German, J., Lankasov, and Skranova, M. (1973). Recoveries and reproducibility in the thin-layer chromatographic fluorimetric determination of flavins by the elution method. VIII. Chromatography of riboflavin decomposition products. J. Chromatogr. 66 311-319. 

The extraction and cleanup steps for riboflavin are similar to those proposed for thiamin. Special care should be taken to protect samples and standards from UV hght and alkaline conditions. Strict control in the oxidation process is also necessary to avoid riboflavin decomposition. Although riboflavin is classified as a water-soluble vitamin, some problems can arise when dissolving the vitamin in water, and this must be taken into consideration when preparing the standard solutions. 

Riboflavin forms fine yellow to orange-yeUow needles with a bitter taste from 2 N acetic acid, alcohol, water, or pyridine. It melts with decomposition at 278—279°C (darkens at ca 240°C). The solubihty of riboflavin in water is 10—13 mg/100 mL at 25—27.5°C, and in absolute ethanol 4.5 mg/100 mL at 27.5°C it is slightly soluble in amyl alcohol, cyclohexanol, benzyl alcohol, amyl acetate, and phenol, but insoluble in ether, chloroform, acetone, and benzene. It is very soluble in dilute alkah, but these solutions are unstable. Various polymorphic crystalline forms of riboflavin exhibit variations in physical properties. In aqueous nicotinamide solution at pH 5, solubihty increases from 0.1 to 2.5% as the nicotinamide concentration increases from 5 to 50% (9). 

Photochemical decomposition of riboflavin in neutral or acid solution gives lumichrome (3), 7,8-dimethyl all oxazine, which was synthesized and characterized by Karrer and his co-workers in 1934 (11). In alkaline solution, the irradiation product is lumiflavin (4), 7,8,10-trimethyhsoalloxazine its uv—vis absorption spectmm resembles that of riboflavin. It was prepared and characterized in 1933 (5). Another photodecomposition product of riboflavin is 7,8-dimethy1-10-foTmylmethy1isoa11oxazine (12). 

Riboflavin occurs as a yellow to orange-yellow, crystalline powder. When dry, it is not affected by diffused light, but when in solution, light induces deterioration. It melts at about 280° with decomposition, and its saturated solution is neutral to litmus. One gram dissolves in 3000 to about 20,000 mL of water, the variations being due to differences in the internal crystalline structure. It is less soluble in alcohol than in water. It is insoluble in ether and in chloroform, but it is very soluble in dilute solutions of alkalies. 

Drugs whose decomposition rates are influenced by the pH include doxorubicin hydrochloride (47), minoxidil (48), menadione sodium bisulfite (67), metronidazole (68), riboflavin (69), colchicine (70), tetracycline hydrochloride (71), phenobarbital (72), dacarbazine (73), furosemide (74), daunorubicin hydrochloride (75), and demeclo-cycline hydrochloride (76). Figures 3 to 6 illustrate how the pH of the solution can influence the degradation rate of some of the reported photolabile drugs. 

In a study of the first-order decomposition of riboflavin in 0.05 mol dm NaOH using accelerated storage techniques, the temperature was programmed to rise from 12.5 to 55°C using a programme constant, b, of 2.171x10 K b The initial concentration, Cq, of riboflavin was 10 mol dm , and the concentration remaining at time t was as follows ... 

Figute 4.18 Example 4.10 accelerated storage plot for the decomposition of riboflavin in 0.05 mol dm NaOH using data from reference 29. 

When dry, riboflavin is not affected appreciably by diffused light it deteriorates, however, in solution in the presence of light. This deterioration is very rapid in the presence of alkalies, producing lumiflavine. This deterioration may be retarded by buffering on the acid side, but under acid conditions, light can produce lumichrome. Neither of the.se decomposition products possesses biological activity. 

Disposable transport-facilitating moieties are also used to enhance the absorption of the water-soluble vitamins used as food additives, such as thiamine, ascorbic acid, and riboflavine. The vitamin derivatives obtained are poorly water-soluble and therefore are less extracted during the preparation of the food, which also gives some protection against oxidative decomposition. The increased lipophilicity enhances absorption from the intestinal tract (Fig. 32)151 155>. 

Several vitamins are known to be photolabile, and the photochemical stability of these compounds is influenced by TPN composition. The photochemical stability depends on composition of the amino acid solutions as well as the presence of lipids in the preparations (i.e., the formation of emulsions). Photochemical decomposition of the hpophihc vitamin A is reduced in admixtures containing lipids, possibly due to diffusion of the vitamin into the lipophilic phase. On the other hand, the hydrophilic vitamin riboflavin is protected by emulsification, probably because the opaque emulsion will reduce the optical transmission of the preparation to some extent (Smith et al., 1988). However, emulsification protects neither the water-soluble vitamin C nor the lipohilic vitamins A and K1 from photochemical degradation, which illustrates the complexity of photochemical reactions in heterogeneous media (Smith et. al., 1988 Billionrey et al., 1993). 

The most thoroughly studied of the phenoxy acids is 2,4-D. Its photochemical decomposition by hydrolysis and oxidation leads, through various intermediate products, to chlorine-free polyquinoidal humic adds. The cleavage of the phenyl— carbon bond, leading to the 2,4-dichlorbphenol intermediate, is sensitised by riboflavin. 1,2,4-Trihydroxybenzene formed by hydroxy substitution is oxidised by air to 2-hydroxybenzoquinone, which is then polymerised (Crosby and Tutass,... 

Isolated as the barium salt, CjtHj.BaNjOjjPj, small yellow spheres clustered like grapes. Absorption max 366, 445 nm. The absorption curve is practically identical with that of riboflavine. There is some stronger absorption between 450 and 510 nm resulting in aq sol ns which are more reddish and less green than those of riboflavine. The appearance of a strong greenish fluorescence indicates decomposition and loss of catalytic activity. 

Ishimitsu, S., S. Fujimoto, and A. Ohara. 1985. The photochemical decomposition and hydroxylation of phenylalanine in the presence of riboflavin. Chem. Pharm. Bull. 33 1552-1556. 

Vitamin B2 Food contains three B2 vitamers, riboflavin and its two coenzyme forms, flavin mononucleotide and flavin adenine dinucleotide, which are the predominant vitamers in foods and are usually bound to proteins. Their analysis usually takes place after extraction with dilute mineral acids with or without enzymatic hydrolysis of the coenzymes (which is necessary to convert all forms to riboflavin and to quantify them as total riboflavin). The extracts may be purified using SPE with Cig cartridges. All the operations performed prior to analysis need to be done under subdued lighting to avoid decomposition of riboflavin upon exposure to light. RP chromatography with Cig columns is used along with fluorescence detection (excitation, 440 nm emission, 520 nm). 

The yellow flavins and also derivatives and degradation products can be detected specifically and in small amounts in quartz lamp radiation (cf. Table 48). Riboflavin, riboflavin-5 -phosphate and lumiflavin show brilliant yellow fluorescence in long-wave UV light lumichrome fluoresces blue and other products of photochemical decomposition likewise yellow and also blue, green and violet [80]. 

Riboflavin crystallizes from a variet of solvents as fine orange needles. The decomposition point is about 280° but values found in the literature may differ several degrees from this. It is very soluble in alkali and in 36% hydrochloric acid in the cold, and in 18% hydrochloric acid when heated. The vitamin is unstable in alkali while relatively stable toward acid. The water solution is yellow in color and shows an intense yellowish-green fluorescence (maximum 565 m/r) which is useful for quantitative determination. 

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