Riboflavine compound

FIGURE 10.1 The structural formula of riboflavin and partial structures of riboflavin compounds. The latter show only those portions of the molecule that differ from riboflavin. 1 — Riboflavin (RF), 2 — flavin mononucleotide or 5 -riboflavin monophosphate (FMN or 5 -FMN), 3 — flavin adenine dinucleotide (FAD). 

More conveniently, compound (13) was directly condensed with barbituric acid (14) in acetic acid (28) or in the presence of an acid catalyst in an organic solvent (29). The same a2o dye intermediate (13) and alloxantin give riboflavin in the presence of palladium on charcoal in alcohoHc hydrochloric acid under nitrogen. This reaction may involve the reduction of the a2o group to the (9-phenylenediamine by the alloxantin, which is dehydrogenated to alloxan (see Urea) (30). 

Since many essential nutrients (e.g., monosaccharides, amino acids, and vitamins) are water-soluble, they have low oil/water partition coefficients, which would suggest poor absorption from the GIT. However, to ensure adequate uptake of these materials from food, the intestine has developed specialized absorption mechanisms that depend on membrane participation and require the compound to have a specific chemical structure. Since these processes are discussed in Chapter 4, we will not dwell on them here. This carrier transport mechanism is illustrated in Fig. 9C. Absorption by a specialized carrier mechanism (from the rat intestine) has been shown to exist for several agents used in cancer chemotherapy (5-fluorouracil and 5-bromouracil) [37,38], which may be considered false nutrients in that their chemical structures are very similar to essential nutrients for which the intestine has a specialized transport mechanism. It would be instructive to examine some studies concerned with riboflavin and ascorbic acid absorption in humans, as these illustrate how one may treat urine data to explore the mechanism of absorption. If a compound is... 

Since long retention times are often applied in the anaerobic phase of the SBR, it can be concluded that reduction of many azo dyes is a relatively a slow process. Reactor studies indicate that, however, by using redox mediators, which are compounds that accelerate electron transfer from a primary electron donor (co-substrate) to a terminal electron acceptor (azo dye), azo dye reduction can be increased [39,40]. By this way, higher decolorization rates can be achieved in SBRs operated with a low hydraulic retention time [41,42]. Flavin enzyme cofactors, such as flavin adenide dinucleotide, flavin adenide mononucleotide, and riboflavin, as well as several quinone compounds, such as anthraquinone-2,6-disulfonate, anthraquinone-2,6-disulfonate, and lawsone, have been found as redox mediators [43—46]. 

Reduced flavins (FADH2, FMNH2, and riboflavin) generated by flavin-dependent reductases have been hypothesized to reduce azo dyes in a nonspecific chemical reaction, and flavin reductases have been revealed to be indeed anaerobic azoreductases. Other reduced enzyme cofactors, for example, NADH, NADH, NADPH, and an NADPH-generating system, have also been reported to reduce azo dyes. Except for enzyme cofactors, different artificial redox mediating compounds, especially such as quinines, are important redox mediators of azo dye anaerobic reduction (Table 1). 

Various dyes can be used as photosensitizers, including methylene blue, riboflavine, and hematoporphyrin derivative. The selection of the photosensitizer should be in favor of a compound that exclusively leads to Reaction (b), so that a clear interpretation of the results is possible. 

The second maximum is riboflavin-independent (Fig. 1). In this case, luminol obviously plays a double role it is the chemiluminogenous detection compound for free radicals and photosensitizer as well. It is a remarkable characteristic of this system that the signal intensity decreases only very slowly, giving an opportunity for detection of nonenzymatic antioxidants. 

The types of compounds that can be analyzed by fluorometry are rather limited. Benzene ring systems, such as the vitamins riboflavin (Figure 8.13) and thiamine, are especially highly fluorescent compounds and are analyzed in foods and pharmaceutical preparations by fluorometry. Metals can be analyzed by fluorometry if they are able to form complex ions by reaction with a ligand having a benzene ring system. 

The infrared technique has been described in numerous publications and recent reviews were published by Davies and Giangiacomo (2000), Ismail et al. (1997) and Wetzel (1998). Very few applications have been described for analysis of additives in food products. One interesting application is for controlling vitamin concentrations in vitamin premixes used for fortification of food products by attenuated total reflectance (ATR) accessory with Fourier transform infrared (FTIR) (Wojciechowski et al., 1998). Four vitamins were analysed - Bi (thiamin), B2 (riboflavin), B6 (vitamin B6 compounds) and Niacin (nicotinic acid) - in about 10 minutes. The partial least squares technique was used for calibration of the equipment. The precision of measurements was in the range 4-8%, similar to those obtained for the four vitamins by the reference HPLC method. 

HPLC with fluorescence detection was employed for the analysis of riboflavin (RF), flavin mononucleotide (FMN) and flavin-adenin dinucleotide (FAD) in beer, wine and other beverages. The investigation was motivated by the finding that these compounds are responsible for the so-called taste of light which develops in beverages exposed to light. Samples were filtered and injected in to the analytical column without any other pretreatment. Separations were carried out in an ODS column (200 X 2.1mm i.d. particle size 5 pm). Solvents A and B were 0.05 M phosphate buffer (pH 3) and ACN, respectively. The... 

Larson et al. (1992) studied the photosensitizing ability of 2, 3, 4, 5 -tetraacetylriboflavin to various organic compounds. An aqueous solution containing aniline was subjected to a medium-pressure mercury arc lamp (X >290 nm). The investigators reported that 2, 3, 4, 5 -tetraacetylribofiavin was superior to another photosensitizer, namely riboflavin, in degrading aniline. Direct photolysis of aniline without any photosensitizer present resulted in a half-life of 23 h. In the presence of riboflavin and 2, 3, 4, 5 -tetraacetylribofiavin, the half-lives were 1 min and 45 sec, respectively. Photoproducts identified in both reactions were azobenzene, phenazine, and azoxybenzene. 

An aqueous solution of amitrole can decompose in the following free radical systems Fenton s reagent, UV irradiation, and riboflavin-sensitized photodecomposition (Plimmer et al, 1967). Amitrole-5- C reacted with Fenton s reagent to give radiolabeled carbon dioxide, unlabeled urea, and unlabeled cyanamide. Significant degradation of amitrole was observed when an aqueous solution was irradiated by a sunlamp (L = 280-310 nm). In addition to ring compounds, it was postulated that other products may have formed from the polymerization of amitrole free radicals (Plimmer et al., 1967). 

Photolytic. When a dilute aqueous solution (1-10 mg/L) of bromacil was exposed to sunlight for 4 months, the TV-dealkylated photoproduct, 5-bromo-6-methyluracil, formed in small quantities. This compound is less stable than bromacil and upon further irradiation, the de-brominated product, 6-methyluracil was formed (Moilanen and Crosby, 1974). Acher and Dunkelblum (1979) studied the dye-sensitized photolysis of aerated aqueous solutions of bromacil using sunlight as the irradiation source. After 1 h, a mixture of diastereoisomers of 3-5ec-butyl-5-acetyl-5-hydroxyhydantoin formed in an 83% yield. In a subsequent study, another minor intermediate was identified as a 5,5 -photoproduct of 3-5ec-butyl-6-methyluracil. In this study, the rate of photooxidation increased with pH. The most effective sensitizers were riboflavin (10 ppm) and methylene blue (2-5 ppm) (Acher and Saltzman, 1980). Direct photodegradation of bromacil is not significant (Acher and Dunkelblum, 1979 Ishihara, 1963). 

Photolytic. When propachlor in an aqueous ethanolic solution was irradiated with UV light (>, = 290 nm) for 5 h, 80% decomposed to the following cyclic photoproducts W-isopropyloxindole, W-isopropyl-3 hydroxyoxindole, and a spiro compound. Irradiation of propachlor in an aqueous solution containing riboflavin as a sensitizer resulted in completed degradation of the parent compound. 3-Hydroxypropachlor was the only compound identified in trace amounts which formed via ring hydroxylation (Rejtb et al, 1984). Hydrolyzes under alkaline conditions forming W-isopropylaniline (Sittig, 1985) which is also a product of microbial metabolism (Novick et al., 1986). 

The carbon dioxide anion-radical was used for one-electron reductions of nitrobenzene diazo-nium cations, nitrobenzene itself, quinones, aliphatic nitro compounds, acetaldehyde, acetone and other carbonyl compounds, maleimide, riboflavin, and certain dyes (Morkovnik and Okhlobystin 1979). The double bonds in maleate and fumarate are reduced by CO2. The reduced products, on being protonated, give rise to succinate (Schutz and Meyerstein 2006). The carbon dioxide anion-radical reduces organic complexes of Co and Ru into appropriate complexes of the metals(II) (Morkovnik and Okhlobystin 1979). In particular, after the electron transfer from this anion radical to the pentammino-p-nitrobenzoato-cobalt(III) complex, the Co(III) complex with thep-nitrophenyl anion-radical fragment is initially formed. The intermediate complex transforms into the final Co(II) complex with the p-nitrobenzoate ligand. 

A number of nitrogen heterocyclic, aromatic compounds, riboflavin 26, folic acid 27a and biopterin 27b, isolated from natural sources, are related in structure to natural redox enzyme cofactors. The electrochemistry of these and related compounds has been studied extensively. 

Where patients are at risk of Wernicke s encephalopathy - for example, because of chronic alcohol abuse, hyperemesis gravidarum, or malnutrition - they should be given thiamine. In many countries no intravenous preparation of thiamine alone is available, and the compound preparations that are available are prone to cause anaphylactoid reactions, so they should be given by slow infusion, and with adequate facilities for resuscitation. A high potency preparation (Pabrinex ) that contains thiamine 250 mg in 10 ml with ascorbic acid, nicotinamide, pyridoxine and riboflavin, can be given by intravenous infusion over 10 min. 

As aromatic compounds have been exhausted as building blocks for life science products, A-heterocyclic structures prevail nowadays. They are found in many natural products, such as chlorophyll hemoglobin and the vitamins biotin (H), folic acid, niacin (PP), pyridoxine HCl (Be), riboflavine (B2), and thiamine (Bi). In life sciences 9 of the top 10 proprietary drugs and 5 of the top 10 agrochemicals contain A-heterocycIic moieties (see Tables 11.4 and 11.7). Even modern pigments, such as diphenylpyrazolopyrazoles, quinacri-dones, and engineering plastics, such as polybenzimidazoles, polyimides, and triazine resins, exhibit an A-heterocydic structure. 

---