Isoalloxazine system, tricyclic

The functional end of the flavin coenzymes FMN and FAD is the tricyclic isoalloxazine system, with the numbering system shown in structure I, the air-stable, yellow, oxidized form. The other two functionally important redox states are the one-electron-reduced semiquinone, II (pKa = 8.4 for dissociation at N(5)), and the two-electron-reduced, colorless dihydroflavin, III. In the dihydro form N(5), C(4a), C(la), andN(l) form a diaminoethylene system and it was anticipated that nitrogen at the 5 and 1 positions would be key to coenzymatic function. 

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... 

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 ... 

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. 

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