Fluorescence Properties of Flavin Semiquinone Radicals in Nitronate Monooxygenase

Fluorescent cofactors like flavins can be exploited to probe their local environment with spatial and temporal resolution. Although the fluorescence properties of the oxidized and two-electron-reduced states of flavins have been studied extensively, this is not the case for the one-electron-reduced state. Both the neutral and anionic semiquinones have proven particularly challenging to examine, as they are unstable in solution and are transient, short-lived species in many catalytic cycles. Here, we report that the nitronate monooxygenase (NMO) from Pseudomonas aeruginosa PAO1 is capable of stabilizing both semiquinone forms anaerobically for hours, thus enabling us to study their spectroscopy in a constant protein environment. We found that in the active site of NMO, the anionic semiquinone exhibits no fluorescence, whereas the neutral semiquinone radical shows a relatively strong fluorescence, with a behavior that violates the Kasha-Vavilov rule. These fluorescence properties are discussed in the context of time-dependent density functional theory calculations, which reveal low-lying dark states in both systems.

Experimental and Theoretical Study on the Cycloreversion of a Nucleobase-Derived Azetidine by Photoinduced Electron Transfer

Azetidines are interesting compounds in medicine and chemistry as bioactive scaffolds and synthetic intermediates. However, photochemical processes involved in the generation and fate of azetidine-derived radical ions have scarcely been reported. In this context, the photoreduction of this four-membered heterocycle might be relevant in connection with the DNA (6-4) photoproduct obtained from photolyase. Herein, a stable azabipyrimidinic azetidine (AZT_m ), obtained from cycloaddition between thymine and 6-azauracil units, is considered to be an interesting model of the proposed azetidine-like intermediate. Hence, its photoreduction and photo-oxidation are thoroughly investigated through a multifaceted approach, including spectroscopic, analytical, and electrochemical studies, complemented by CASPT2 and DFT calculations. Both injection and removal of an electron result in the formation of radical ions, which evolve towards repaired thymine and azauracil units. Whereas photoreduction energetics are similar to those of the cyclobutane thymine dimers, photo-oxidation is clearly more favorable in the azetidine. Ring opening occurs with relatively low activation barriers (<13 kcal mol^-1 ) and the process is clearly exergonic for photoreduction. In general, a good correlation has been observed between the experimental results and theoretical calculations, which has allowed a synergic understanding of the phenomenon.