Extended Data Fig. 5. Mass spectra of the nitrile product when AetD reaction was performed with ¹³C-labelled isotopologues of 5,7-dibromo-L-tryptophan.

a, AetD + 5,7-dibromo-L-tryptophan. b, AetD + [2-¹³C₁]-5,7-dibromo-L-tryptophan. c, AetD + [3-¹³C₁]-5,7-dibromo-L-tryptophan. d, AetD + [1,2,3-¹³C₃]-5,7-dibromo-L-tryptophan. A 1 Da increase in the mass of the nitrile product was observed only when [3-¹³C₁]-5,7-dibromo-L-tryptophan and [1,2,3-¹³C₃]-5,7-dibromo-L-tryptophan were used as substrates, confirming the β-carbon C-3 is retained in the product.