Tricyclic isoalloxazine ring

These yellow redox systems all contain the conjugated tricyclic isoalloxazine ring system. Riboflavin, vitamin B2, is an isoalloxazine which is phosphorylated and then adenylated to two active redox coenzyme forms - FMN (flavin adenine mononucleotide, R = phosphorylated ribose) and FAD (flavin adenine dinucleotide, adenylated FMN). 

Havoenzymes catalyze a large number of diverse redox interconversions reflecting the chemical versatility of the tricyclic isoalloxazine ring system of the flavin coenzymes, which act as a two-electron/one-electron switch as well as a molecular oxygen activator (Ghisla and Massey, 1989). Therefore flavin coenzymes serve ... 

Since flavin semiquinones are tricyclic heteronuclear aromatic systems, the spin density due to the unpaired electron can be distributed at a number of sites on the isoalloxazine ring. Knowledge of the location and the extent of spin density at these locations on the ring system is important for the determination of sites that may participate... 

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. 

The TAPA (I) molecule is characterized by a tricyclic aromatic system having electron acceptor properties and a chiral handle attached to the central ring. A biological molecule, such as riboflavin (Vitamin B2, II), has certain analogous features, i.e., an isoalloxazine ring known to act as an electron acceptor and an asymmetric substituent (D-ribityl) in the middle ring. Riboflavin can therefore be expected to show stereoselective interactions similar to (I). In fact, as seen in Fig. 3, helicenes are well resolved on Vitamin B2. 

Chemically, riboflavin may be described as a tricyclic, nitrogen-containing isoalloxazine ring-system with a ribitol side chain. Its systematic name is 7,8-dimethyl-10-(l-D-ribityl)-benzo[g]pteridin-2,4-dion, with an empirical formula of C17H20N4O6 and molecular weight of 376.36 g/mol. 

Based on their unique tricyclic isoalloxazine structure (benzene, pyrimidine, and pyrazine ring), the reactivity of flavin compounds in chemical and biochemical reactions is very complex (for review see 2,4), and only some reactions that are important for the analysis of flavin compounds can be mentioned in this text. Riboflavin and FMN are very sensitive to UV light (5). The effective photolysis of riboflavin is a complex, pH-dependent reaction, which results in the formation of 60% to 70% lumiflavin in basic solutions or lumichrome in neutral or slightly acid solutions as the main component (Fig. 2). In contrast to riboflavin, lumiflavin can be easily extracted from biological samples with chloroform and measured photometrically (at 450 nm). The lumiflavin method has been widely used for analytical riboflavin determination in various sources. When protected from light, riboflavin is rather stable in neutral and slightly acidic solutions. 

Isoalloxazine. Designation of the tricyclic heteronuclear organic ring found in flavins. 

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