Riboflavin, complexes

Flavin mononucleotide (FMN)-adenosine and flavin adenine dinucleotide (FAD)-adenosine complexes show quenched triplet lifetimes compared to FMN alone, which is cited as evidence of intramolecular com-plexation between the flavins and adenosine by Shiga and Piette [142]. Adenosine phosphates also form complexes with FAD [143]. The com-plexation between a flavin and adenosine is identical to the intermolecular complexing of adenosine and flavin moieties, in the latter case enforced by hydrophobic bonding [144-146]. Rath and McCormick [147] have examined the riboflavin complexes of a series of purine ribose derivatives... 

H2O 5 -Bromo-5 -deoxyadenosine-riboflavine complex, trihydrate RIBBAD 30 459... 

In aqueous solution, riboflavin has absorption at ca 220—225, 226, 371, 444 and 475 nm. Neutral aqueous solutions of riboflavin have a greenish yellow color and an intense yellowish green fluorescence with a maximum at ca 530 nm and a quantum yield of = 0.25 at pH 2.6 (10). Fluorescence disappears upon the addition of acid or alkah. The fluorescence is used in quantitative deterrninations. The optical activity of riboflavin in neutral and acid solutions is [a]=+56.5-59.5° (0.5%, dil HCl). In an alkaline solution, it depends upon the concentration, eg, [a] J =—112-122° (50 mg in 2 mL 0.1 Ai alcohohc NaOH diluted to 10 mL with water). Borate-containing solutions are strongly dextrorotatory, because borate complexes with the ribityl side chain of riboflavin = +340° (pH 12). 

Flavin mononucleotide was first isolated from the yellow en2yme in yeast by Warburg and Christian in 1932 (4). The yellow en2yme was spHt into the protein and the yellow prosthetic group (coen2yme) by dialysis under acidic conditions. Flavin mononucleotide was isolated as its crystalline calcium salt and shown to be riboflavin-5Lphosphate its stmeture was confirmed by chemical synthesis by Kuhn and Rudy (94). It is commercially available as the monosodium salt dihydrate [6184-17 /, with a water solubiUty of more than 200 times that of riboflavin. It has wide appHcation in multivitamin and B-complex solutions, where it does not require the solubili2ers needed for riboflavin. 

Introduction of the cobalt atom into the corrin ring is preceeded by conversion of hydrogenobyrinic acid to the diamide (34). The resultant cobalt(II) complex (35) is reduced to the cobalt(I) complex (36) prior to adenosylation to adenosylcobyrinic acid i7,i -diamide (37). Four of the six remaining carboxyhc acids are converted to primary amides (adenosylcobyric acid) (38) and the other amidated with (R)-l-amino-2-propanol to provide adenosylcobinamide (39). Completion of the nucleotide loop involves conversion to the monophosphate followed by reaction with guanosyl triphosphate to give diphosphate (40). Reaction with a-ribazole 5 -phosphate, derived biosyntheticaHy in several steps from riboflavin, and dephosphorylation completes the synthesis. 

Thiobacillus ferrooxidans function. 6, 651 Rhus vernicifera stellacyanin structure, 6,651 Riboflavin 5 -phosphate zinc complexes, 5,958 Ribonucleotide reductases cobalt, 6,642 iron, 6,634... 

Four of the B vitamins are essential in the citric acid cycle and therefore in energy-yielding metabolism (1) riboflavin, in the form of flavin adenine dinucleotide (FAD), a cofactor in the a-ketoglutarate dehydrogenase complex and in succinate dehydrogenase (2) niacin, in the form of nicotinamide adenine dinucleotide (NAD),... 

The water-soluble vitamins comprise the B complex and vitamin C and function as enzyme cofactors. Fofic acid acts as a carrier of one-carbon units. Deficiency of a single vitamin of the B complex is rare, since poor diets are most often associated with multiple deficiency states. Nevertheless, specific syndromes are characteristic of deficiencies of individual vitamins, eg, beriberi (thiamin) cheilosis, glossitis, seborrhea (riboflavin) pellagra (niacin) peripheral neuritis (pyridoxine) megaloblastic anemia, methyhnalonic aciduria, and pernicious anemia (vitamin Bjj) and megaloblastic anemia (folic acid). Vitamin C deficiency leads to scurvy. 

Rice bran is the richest natural source of B-complex vitamins. Considerable amounts of thiamin (Bl), riboflavin (B2), niacin (B3), pantothenic acid (B5) and pyridoxin (B6) are available in rice bran (Table 17.1). Thiamin (Bl) is central to carbohydrate metabolism and kreb s cycle function. Niacin (B3) also plays a key role in carbohydrate metabolism for the synthesis of GTF (Glucose Tolerance Factor). As a pre-cursor to NAD (nicotinamide adenine dinucleotide-oxidized form), it is an important metabolite concerned with intracellular energy production. It prevents the depletion of NAD in the pancreatic beta cells. It also promotes healthy cholesterol levels not only by decreasing LDL-C but also by improving HDL-C. It is the safest nutritional approach to normalizing cholesterol levels. Pyridoxine (B6) helps to regulate blood glucose levels, prevents peripheral neuropathy in diabetics and improves the immune function. 

Water-soluble vitamins removed by hemodialysis (HD) contribute to malnutrition and vitamin deficiency syndromes. Patients receiving HD often require replacement of water-soluble vitamins to prevent adverse effects. The vitamins that may require replacement are ascorbic acid, thiamine, biotin, folic acid, riboflavin, and pyridoxine. Patients receiving HD should receive a multivitamin B complex with vitamin C supplement, but should not take supplements that include fat-soluble vitamins, such as vitamins A, E, or K, which can accumulate in patients with renal failure. 

Incorporation of a flavin electron donor and a thymine dimer acceptor into DNA double strands was achieved as depicted in Scheme 5 using a complex phosphoramidite/H-phosphonate/phosphoramidite DNA synthesis protocol. For the preparation of a flavin-base, which fits well into a DNA double strand structure, riboflavin was reacted with benzaldehyde-dimethylacetale to rigidify the ribityl-chain as a part of a 1,3-dioxane substructure [49]. The benzacetal-protected flavin was finally converted into the 5 -dimethoxytri-tyl-protected-3 -H-phosphonate ready for the incorporation into DNA using machine assisted DNA synthesis (Scheme 5a). For the cyclobutane pyrimidine dimer acceptor, a formacetal-linked thymine dimer phosphoramidite was prepared, which was found to be accessible in large quantities [50]. Both the flavin base and the formacetal-linked thymidine dimer, were finally incorporated into DNA strands like 7-12 (Scheme 5c). As depicted in... 

Electron transfer from a macrocycle (82), based on cyclen, complex to coordinated riboflavin proceeds via an inner sphere electron transfer pathway. The riboflavin coordinates through the imide and the relevance to the interception of biological electron transfer pathways is discussed.709... 

The water-soluble vitamins generally function as cofactors for metabolism enzymes such as those involved in the production of energy from carbohydrates and fats. Their members consist of vitamin C and vitamin B complex which include thiamine, riboflavin (vitamin B2), nicotinic acid, pyridoxine, pantothenic acid, folic acid, cobalamin (vitamin B12), inositol, and biotin. A number of recent publications have demonstrated that vitamin carriers can transport various types of water-soluble vitamins, but the carrier-mediated systems seem negligible for the membrane transport of fat-soluble vitamins such as vitamin A, D, E, and K. 

Dependence of tt complex binding constants upon nucleoside ionization potentials for ( ) uridine, ( ) thymidine, (A) cytidine, (o) adenosine, (O) guanosine, and (A) N,N,-dimethyladenosine. Panel A shows association constants for the binding of nucleosides to riboflavin. Panel B shows association constants for the self association of nucleosides. (Reproduced from Ref. 82. Copyright 1981, American Chemical Society.)... 

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. 

Riboflavin is also known as vitamin B2. It contains a complex isoalloxazine ring that humans are unable to synthesize. The complex ring is hooked onto a live-carbon sugar derivative, ribitol, closely related to the ribose that occurs in RNA. The RDA for adult males is 1.3 mg/day and for adult females 1.1 mg/day. Values decrease with increasing age but increase in pregnancy and lactation. Organ meats, milk, bread products, and fortified cereals are substantial sources of riboflavin. 

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. 

Riboflavin, folic acid and lumiflavin each bind one Ru(bpy)2 unit. The MLCT spectra of the complexes are pH dependent, and an a-iminocarbonyl chelating mode for each ligand is proposed. Coordination results in a lowering of the values ( 9 to 5) of the N(3)-H protons." ... 

Vitamin B2 is a complex of several vitamins riboflavin, folate, nicotinate, and pantothenic acid. 

In foods vitamin B2 occurs free or combined both as FAD and FMN and complexed with proteins. Riboflavin is widely distributed in foodstnffs, but there are very few rich sources. Only yeast and liver contain more than 2mg/100g. Other good sources are milk, the white of eggs, fish roe, kidney, and leafy vegetables. Since riboflavin is continuously excreted in the urine, deficiency is qnite common when dietary intake is insufficient. The symptoms of deficiency are cracked and red lips, inflammation of the lining of the month and tongue, mouth ulcers, cracks at the comer of the mouth, and sore throat. Overdose of oral intake present low toxicity, probably explained by the limited capacity of the intestinal absorption mechanism [417]. 

Thiamine, riboflavin, nicotinamide, pyridoxine, and folic acid can be determined together by using DAD, but pantothenic acid and biotin do not have adequate sensitivity for UV detection in complex matrices. 

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