7,8-Dimethyl-isoalloxazine

As shown in Figure 7.1, riboflavin consists of a tricyclic dimethyl-isoalloxazine ring conjugated to the sugar alcohol ribitol. The metabolically active coenzymes are riboflavin 5 -phosphate and flavin adenine dinucleotide (FAD). In some enzymes the prosthetic group is riboflavin, bound covalently at the catalytic site. 

Flavins (El) catalyze many different bioreactions of physiological importance [7-9]. Riboflavin, flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD) have the 7,8-dimethyl isoalloxazine ring in common but differ in the side chain attached to NIO. With their five redox states, fully oxidized, one-electron reduced semiquinoid (F1H and F1 ), and fully reduced hydroquinone (FIH2 and F1H ), flavins are involved in one-electron and two-electron transfer reactions [10]. 

Flavin a compound containing the 6,7-dimethyl-isoalloxazine ring system (Fig.). Biologically important F. are Flavin adenine dinucleotide (see), Flavin mononucleotide (see) and Riboflavin (see under Vitamins). See Flavin enzymes. 

The binding of a flavin prosthetic group to hepatic monamine oxidase has recently been reported to involve a thiol . FAD is linked as a thioether formed between a cysteine and a methyl substituent on the dimethyl isoalloxazine. While binding is generally conceived to be a reversible process and these cases must be viewed as an extreme, they do provide clear examples of the general concept. 

Riboflavin (vitamin B2) consists of a dimethyl-isoalloxazine nucleus combined with ribitol. Its structure is shown here ... 

Yellow (hence the name) prosthetic groups,//av/w adenine dinucleotide (FAD) and the somewhat less common flavin mononucleotide (FMN) are linked to protein in flavoproteins (Fig. 29), The nomenclature is not quite accurate since 6,7-dimethyl-isoalloxazine, which is responsible for the color, is not linked to ribose, as in the nucleotides, but to the corresponding sugar alcohol ribitol. In both FAD and FMN the 6,7-dimethyl-... 

We may continue with the question what is meant at all by the term flavin Is it synonymous with isoalloxazine 101) or with 7,8-dimethyl-isoalloxazine 139)1 If the term flavin is meant to comprise the 7,8-methyl groups as seems most reasonable, any flavin modifled in such a position must be termed 8-X-8- or-flavin, in contrast to additions to the same position, which must lead to 8-X-l,8-dihydroflavin. 

Isay reaction, 2, 79-80 3, 259 Ismelin 7, 656 Isoalloxazine oxidation states, 1, 252 Isoaminopterin synthesis, 3, 327 Isoarsindolines, 1, 543-544 Isoarsinoline, tetrahydro-synthesis, 1, 552-553 stability, 1, 552 Isoascorbic acid structure, 4, 552 ( )-IsoavenacioIide synthesis, 1, 416 Isoazapenem synthesis, 1, 465 Isobacteriochlorin synthesis, 4, 419 Isobacteriochlorin, dimethyl-biosynthesis, 1, 105 Isobacteriochlorin, methyl-biosynthesis, 1, 105 Isobacteriochlorin, trimethyl-biosynthesis, 1, 105 Isobarbituric acid Mannich reaction, 3, 71 synthesis, 3, 133... 

Vitamin B2 or riboflavin is chemically defined as 7,8-dimethyl-10-(lY-D-tibityl)isoalloxazine. Figure 1 shows the oxidized and reduced form of the vitamin. The ending flavin (from the latin word flavus—yellow) refers to its yellowish color. 

Riboflavin (vitamin Bj) is chemically specified as a 7,8-dimethyl-10-(T-D-ribityl) isoalloxazine (Eignre 19.22). It is a precnrsor of certain essential coenzymes, such as flavin mononucleotide (FMN) and flavin-adenine dinucleotide (FAD) in these forms vitamin Bj is involved in redox reactions, such as hydroxylations, oxidative carboxylations, dioxygenations, and the reduction of oxygen to hydrogen peroxide. It is also involved in the biosynthesis of niacin-containing coenzymes from tryptophan. 

Riboflavin (vitamin B2) 6,7-dimethyl-9-(D-l-ribityl)isoalloxazine (63), was discovered as a coloring matter in milk in 1879, but its importance was not then realized. Deficiency causes lesions of the eye and of the angle of the mouth. Riboflavin is phosphorylated by adenosine triphosphate (ATP) to give riboflavin 5 -phosphate (flavinadenine mononucleotide, FMN) and then flavinadenine dinucleotide (FAD) (64 R = riboflavin). These function as prosthetic groups in a number of flavoproteins which are dehydrogenation catalysts by virtue of the oxidation-reduction properties of the isoalloxazine system. 

Pyrazines. In the thirties, the attention on pyrazines was focused on its industrial role in dyes, photographic emulsions and chemotherapy. Its importance in life processes was indicated in its derivative, vitamin B2 (riboflavin, 6,7-dimethyl-9-(l -D-ribityl isoalloxazine). Later,in the midsixties, it was identified in foods and its contributions to the unique flavor and aroma of raw and processed foods attracted the attention of flavor chemists Pyrazine derivatives contribute to the roasting, toasting, nutty, chocolaty, coffee, earthy, caramel, maple-like, bread-like, and bell pepper notes in foods. The reader is referred to the reviews on Krems and Spoerri (88) on the chemistry of pyrazines, and the review of pyrazines in foods by Maga and Sizer (89, 90) Table XVI summaries sensory description and threshold of selected pyrazines. 

The isoalloxazine nucleus of the flavins [3-(R or H)-7,8-dimethyl-lO-R -isoalloxazines] may exist in the fully reduced (1,5-di-hydro-), the radical (semiquinone), and the fully oxidized (quinone) states. Because of acid-base equilibria, each of these oxidation states... 

Synonyms Vitamin B2 Beflavine Flavaxin Flax-ain Lactoflavin Isoalloxazine 7,8-dimethyl-lO-(D-ribo-2,3,4,5-tetrahydroxyphentyl)... 

CPR is composed of two domains an N-terminal membrane-binding domain and a hydrophilic C-terminal catalytic domain. The C-terminal domain can be cleaved from the N-terminal domain by limited proteolysis. The C-terminal domain consists of an FMN-binding section similar to flavodoxin connected by a linker section to an FAD- and NADPH-binding section similar to ferredoxin-NADP" " reductase. The two flavins are only 4 A apart at their 7,8-dimethyl edges, and the planes of the isoalloxazines form an 150° angle (Figure 6), suggesting that electrons are passed directly from one flavin to other." ... 

D-ribo-2,3,4,5-tetrahydroxypentyl)isoalloxazine and 7,8-dimethyl-10-ribityhsoalloxazine its formula is C17H20N4O6. Riboflavin has a molar mass of 376.37 grams (13.3 ounces). It is heat-stabile but easily degraded by light. Riboflavin was referred to as vitamin G in the early part of the twentieth century because it was recognized as a dietary factor needed for growth. Riboflavin was first isolated in 1879, and its chemical structure was determined in 1933. 

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