Investigation of 6,7,8-trimethyllumazine and its radicals by NMR and photo-CIDNP spectroscopy

6,7,8-Trimethyllumazine (TML) is a structural analog of the natural cofactor 6,7-dimethyl-8-ribityllumazine. Under basic conditions, TML undergoes a distinctive disproportionation reaction upon photoexcitation. The transiently formed radical pair can be investigated by photo-chemically induced dynamic nuclear polarization (photo-CIDNP) spectroscopy. In this contribution, the structure of the TML anion is analyzed systematically using NMR spectroscopy. Furthermore, the transiently formed TML radicals are investigated and their hyperfine structures elucidated by ¹H and ¹³C photo-CIDNP spectroscopy. Experimental photo-CIDNP intensities are compared with isotropic hyperfine coupling constants from density functional theory (DFT) calculations. The results confirm the formation of an oxidized TML˙ radical and a reduced TMLH˙^- radical, the latter potentially protonated at N1. Comparative analysis reveals a substantially different hyperfine structure of the formed radical species which is rationalized based on calculations of spin density distributions. The results provide important insights into photo-induced one-electron transfer reactions of 6,7-dimethyllumazines and their potential role in redox processes in biological systems. The detection and characterization of the oxidized TML˙ radical is of special interest as this oxidation state has not been satisfactorily described in the literature so far. Thus this contribution advances the understanding of the mechanism of formation and the structure of lumazine radicals.