isoalloxazines

Isoalloxazine is a tricyclic heteroaromatic ring system that forms the core of the flavin family of cofactors. It is the central chromophore and redox-active component of flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), as well as of riboflavin (vitamin B2). The isoalloxazine nucleus is retained in these molecules, providing the chemical basis for their biological roles.

Structurally, the isoalloxazine ring system consists of three fused rings: a benzene ring fused to a pyrazine ring fused to a pyrimidine-like ring. It is highly conjugated and contains several nitrogen atoms, contributing to planarity, UV–visible absorption, and, in many contexts, fluorescence. The redox properties of the isoalloxazine core enable reversible one- and two-electron transfers.

Biologically, flavoproteins exploit the isoalloxazine moiety to catalyze a wide range of redox reactions. In cells, riboflavin is converted to FMN and FAD, which carry the isoalloxazine ring in different phosphorylation states. The flavin ring can exist in oxidized, one-electron reduced semiquinone, and two-electron reduced hydroquinone forms, allowing versatile electron transfer in metabolic pathways.

Derivatives and related species include photochemical products such as lumichrome and lumiflavin, which retain the isoalloxazine core but differ in substitutions or side chains. The isoalloxazine chromophore is a focus of study in bioinorganic chemistry, enzymology, and photophysics due to its distinct redox behavior and light-absorbing properties.