Selected ionisations and tautomeric forms of reduced flavin. (A) and (B) are the anionic, respectively the neutral forms of reduced flavin encountered at the active center of flavoproteins. The pK interconnecting (A) and (B) is ≈ 7 in the free state , but can vary strongly at the active center of proteins. Protonation of (A) at N(5) to yield (C) can not be observed in the free state but this chromophore is obtained by twofold alkylation at N(5) . The pK's linking species (B), (C) and (D) have been estimated from the properties of appropriate model compounds . See text for further details.