Expanding Reaction Horizons: Evidence of the 5‐Deazaflavin Radical Through Photochemically Induced Dynamic Nuclear Polarization.

Deazaflavins are important analogues of the naturally occurring flavins: riboflavin, flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD). The use of 5‐deazaflavin as a replacement coenzyme in a number of flavoproteins has proven particularly valuable in unraveling and manipulating their reaction mechanisms. It was frequently reported that one‐electron‐transfer reactions in flavoproteins are impeded with 5‐deazaflavin as the cofactor. Based on these findings, it was concluded that the 5‐deazaflavin radical is significantly less stable compared to the respective flavin semiquinone and quickly re‐oxidizes or undergoes disproportionation. The long‐standing paradigm of 5‐deazaflavin being solely a two‐electron/hydride acceptor/donor—"a nicotinamide in flavin clothing"—needs to be re‐evaluated now with the indirect observation of a one‐electron‐reduced (paramagnetic) species using photochemically induced dynamic nuclear polarization (photo‐CIDNP) 1H nuclear magnetic resonance (NMR) under biologically relevant conditions. [ABSTRACT FROM AUTHOR]