To avoid this issue, nitrone 38, a close analog of nitronate 5, was used in the UV-Vis spectra experiments because 38 can also readily react with 1a (Fig. ), mimicking the radical initiation step of the model XEC reaction. Intriguingly, a further enhancement of the CT complex spectra was observed when nitrone 38 was added to a sample containing 1a and the reduced CsER, indicating a quaternary CT complex was formed to enable efficient radical formation and prolongation. No CT complex was observed using free FMNhq with 1a and 38, suggesting the CT complex is formed within the enzyme active site (Supplementary Fig. ).