Riboflavin photolysis

Saidi, B., Warthesen, J.J. 1995. Effect of heat and homogenization on riboflavin photolysis in milk. Int. Dairy J. 5, 635-645. 

Reactivity of Bovine Whey Proteins, Peptides, and Amino Acids toward Triplet Riboflavin as Studied by Laser Flash Photolysis. Journal of Agricultural and Food Chemistry, Vol. 52, No. 21, (October 2004), pp. 6602-6606, ISSN 0021-8561. 

Chlorinated dibenzo ip-dioxins are contaminants of phenol-based pesticides and may enter the environment where they are subject to the action of sunlight. Rate measurements showed that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is more rapidly photolyzed in methanol than octachlorodi-benzo-p-dioxin. Initially TCDD yields 2,3,7-trichlorodiben-zo-p-dioxin, and subsequent reductive dechlorination is accompanied by ring fission. Pure dibenzo-p-dioxin gave polymeric material and some 2,2 -dihydroxybiphenyl on irradiation. Riboflavin-sensitized photolysis of the potential precursors of dioxins, 2,4-dichlorophenol and 2,4,5-trichloro-phenol, in water gave no detectable dioxins. The products identified were chlorinated phenoxyphenols and dihydroxy-biphenyls. In contrast, aqueous alkaline solutions of purified pentachlorophenol gave traces of octachlorodibenzo-p-dioxin on irradiation. 

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. 

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. Acher et al. (1981) studied the dye-sensitized photolysis of terbacil in aerated aqueous solutions over a wide pH range. After a 2-h exposure to sunlight, terbacil in aqueous solution (pH range 3.0-9.2) in the presence of methylene blue (3 ppm) or riboflavin (10 ppm) decomposed to 3-ter/-5-butyl-5-acetyl-5-hydroxyhydantoin. Deacylation was observed under alkaline conditions (pH 8.0 or 9.2) affording 3-/er/-5-hydroxyhydantoin. In neutral or acidic conditions (pH 6.8 or 3.0) containing riboflavin, a mono-Wdealkylated terbacil dimer and an unidentified water-soluble product formed. Product formation, the relative amounts of products formed, and the rate of photolysis were all dependent upon pH, sensitizer, temperature, and time (Acher et ah, 1981). 

The ionisation state of molecules in the solution state appears to be an important variable in photodegradation mechanisms. A recent pubhcation on riboflavin oral liquid preparations shows that the formulation is most photostable at pHs between 5 and 6, where the non-ionised form predominates [78]. The rate of photolysis increase 80-fold at pH 10.0, owing to increased redox potential. Conversely, at pH 3.0, the increased photolysis is associated with the excited singlet state, in addition to the triplet state. 

Riboflavin (CCLV) is photosensitive on irradiation in alkaline solution, it is converted into lumiflavin (CCLVI)302 and in neutral or acid solution, lumichrome (CCLVII) is produced.147 The photolysis of 9-(2 -hydroxyethyl)isoalloxazine (CCLVIII) is also a general aeid- and base-catalyzed reaction. 

Riboflavin is stable to heat but is extremely sensitive to light, a fact of some nutritional significance. Do not leave bottled milk in the sunshine (see Fig. 15-8) Many products of photolysis are formed (Fig. 3-5). Among them is lumichrome. 

The riboflavin triplet reacts with dGMP acid by ET (k = 6.6 x 109 dm3 mol-1 s 1), and evidence for the formation of the (deprotonated) Gua radical cation has been obtained by laser flash photolysis (Lu et al. 2000). The photosensitized reactions of dGuo by TRP is thought to follow two pathways, the formation of Z has been attributed to an ET reaction (Type I), and the reaction of singlet dioxygen [ChCAg) Type II] leads to 4-OH-8-oxo-G and 8-oxo-G (Ravanat et al. 1998). The effect of D20 and azide on the 4-OH-8-oxo-G yields shows that this... 

Photolysis of riboflavin leads to the formation of lumiflavin in alkaline solution and lumichrome in acidic or neutral solution (see Figure 7.2). Because lumiflavin is chloroform extractable, photolysis in alkaline solution, followed by chloroform extraction and fluorimetric determination, is the basis of commonly used chemical methods of assaying riboflavin. The photolysis proceeds by way of intermediate formation of cytotoxic riboflavin radicals, and the addition of riboflavin and exposure to light has been suggested as a means of inactivating vimses and bacteria in blood products (Goodrich, 2000). 

Photolysis of riboflavin occurs in vivo during phototherapy for neonatal hyperbilirubinemia (Section 7.4.4). There is no evidence that normal exposure to sunlight results in significant photolysis of riboflavin, although it is possible that some of the lumichromes found in urine may arise in this way. 

As shown in Table 7.1, the total riboflavin concentration in plasma is very much lower than in most tissues. About 50% of plasma riboflavin is free riboflavin, which is the main transport form, with 44% as FAD and the remainder as riboflavin phosphate. The vitamin is largely protein bound in plasma free riboflavin binds to both albumin and a- and /3-globulins, and both riboflavin and the coenzymes also bind to immunoglobulins. The products of photolysis of riboflavin bind to albumin with considerably higher affinity than riboflavin itself this albumin binding may represent a mechanism to prevent tissue... 

Intestinal bacterial cleavage of the ribityl side chain results in the formation of 10-hydroxyethylflavin (an oxidation product of lumifiavin), lumichrome, and 7- and 8-carboxy-lumichromes, which are also excreted in the urine. Some of the lumichromes detected in urine may result from photolysis of riboflavin in the circulation. 

Neonatal hyperbilirubinemia is normally treated by phototherapy. The peak wavelength for photolysis of bilirubin is 450 nm, the same as that for photolysis of riboflavin (Section 7.1). Infants undergoing phototherapy show biochemical evidence of riboflavin depletion, with a significant increase in the EGR activation coefficient. Provision of additional riboflavin to maintain plasma concentrations enhances the photolysis of bilirubin, apparently as a result of reactive oxygen radicals generated by the products of photolysis of riboflavin. 

However, even relatively low concentrations of riboflavin can cause damage to DNA under conditions of photolysis, with damage to deoxy-guanosine in isolated DNA, and activation of DNA repair mechanisms in cells in culmre. it is therefore not common practice to use riboflavin supplements as an adjunct... 

Martin CB, Shi X, Tsao M-L, Karweik D, Brooke J, Hadad CM, Platz MS. (2002) The photochemistry of riboflavin tetraacetate and nucleosides. A study using density functional theory, laser flash photolysis, fluorescence, UV-Vis and time resolved infrared spectroscopy. J Phys Chem B 106 10263-10271. 

Amino-3-(4 -nitrophenyl)propan-l,3-diol in its optically active form has been selectively photooxidised to the corresponding ketone,and a study of the oxidation of podophyllotoxin (27) by sodium persulfate using laser flash photolysis has enabled rate constants to be determined for the formation and decay of transients. Photoinduced electrochemical oxidation of benzyl alcohol to benz-aldehyde has been achieved with 100% product selectivity and 100% current efficiency using visible radiation in the presence of riboflavin 2, 3, 4, 5 -tetraace-tate. ... 

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