Unraveling the ¹³ C Hyperfine Structure of the 5‐Deazaflavin Radical in Solution Using Photo‐CIDNP Spectroscopy

Despite structural similarity to flavins, 5‐deazaflavins were long thought to act only as two‐electron transfer agents. Recently, photo‐chemically induced dynamic nuclear polarization (photo‐CIDNP) spectroscopy provided conclusive evidence for the 5‐deazaflavin mononucleotide (5‐deazaFMN) radical under physiological conditions. Here, we comprehensively characterize its hyperfine structure, completing the findings of the previous study. A full characterization of the 5‐deazaflavin radical is invaluable in light of the re‐evaluation of its biological role and potential applications in catalysis. Using 13 C isotopologues of 5‐deazaFMN and 7,8‐didemethyl‐5‐deazaFMN, we probe the 13 C hyperfine interaction of the 5‐deazaFMN radical using photo‐CIDNP. Linear fits with predictions from density functional theory calculations reveal similar hyperfine structures for both protonation states. Comparison with the FMN radical highlights the impact of structural modifications on electronic properties.