alloxazine

Alloxazine is a heterocyclic compound that forms the central tricyclic core of the flavin family of cofactors. In common biochemical terminology the nucleus is the isoalloxazine ring system, the oxidized form of which is found in the ubiquitous cofactors flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN). The ring system is based on a benzene ring fused to two nitrogen-containing rings, giving a planar, highly conjugated heteroaromatic structure.

In biochemistry, the isoalloxazine ring is redox-active and can undergo two-electron and one-electron reductions. The oxidized form can be reduced to semiquinone and hydroquinone states, enabling flavin-dependent enzymes to transfer electrons in oxidative metabolism, detoxification, and other catalytic processes. The fluorescence and photochemistry of the isoalloxazine nucleus also make it a useful probe in biochemical studies and a component in light-activated systems.

Alloxazine serves as a scaffold that is formed and modified in nature as part of the riboflavin biosynthetic pathway. Lumazine derivatives are precursors in the synthesis of riboflavin, and the final product contains the isoalloxazine ring as its chemically active core. Synthetic derivatives of the alloxazine/isoalloxazine framework are studied for redox properties, photosensitization, and as fluorescent probes in enzymology and medicinal chemistry.

Overall, the alloxazine/isoalloxazine nucleus is a key chemical motif for understanding flavin-based catalysis and electron transfer in living systems.