Extended Data Fig. 6: Radical stability and isotope labelling.

a, Superimposed UV–vis absorption spectra at time points between 0 and 400 min. Inset, absorbance at 383 nm at 0, 4, 129, 140, 210 and 400 min. Experiments were repeated three times. b, X-band spectra of the radical observed in collected cells grown in minimal medium supplemented with deuterated amino acids. EDTA (0.5 mM) was added before induction. From top: non-labelled tyrosine, β,β-d₂ tyrosine, 3,5-d₂ tyrosine, indole-d₅ tryptophan and d₅ glycine. The doublet signal collapses to a singlet when β,β-deuterated tyrosine is incorporated in the protein. Furthermore, the additional coupling to the remaining 3 or 5 proton in the 3,5-d₂ tyrosine grown cells disappears, which is also in line with the radical being tyrosine-derived. Finally, the exclusion of tryptophan and glycine as source for the observed radical is evident from the two lower traces in the figure, which are identical to the top spectrum. Five independent cultures were grown, each including one of the indicated deuterated amino acids. Spectra were recorded at 100 K in a nitrogen-flow system. The spectra have been normalized to the same double integrals; that is, the same number of spins in the cavity.