L hercynine

L-glutathione (GSH) was obtained from Sigma Aldrich (G4251), L-ergothioneine from Santa Cruz Biotechnology (sc-200814), L-cysteine from Fisher Biotech (BP376-100), L-hercynine from Toronto Research Chemicals (H288900), L-histidine from Sigma Aldrich (H8000), choline chloride from Sigma Aldrich (C1879), glycine-betaine from Sigma Aldrich (B3501), ectoine from Sigma Aldrich (81619), proline-betaine from VWR (TCS0358), L-carnitine from Sigma Aldrich (C0283), and dimethypropiothetin hydrochloride from Sigma Aldrich (80828). These chemicals were used without further purification. Other chemicals include IPTG from GoldBio (2481C100), TCEP from Chem-Impex (00194), dithiothreitol from Chem-Impex (00127), Tris-HCl from MP Biomedicals (816100), HEPES from Chem-Impex (00174), EDTA from VWR Chemicals (BDH9232), NaCl from VWR Chemicals (BDH9286), Imidazole from Chem-Impex (00418), Sypro Orange from Sigma (S5692), and monobromobimane (mBBr) from Sigma (B4380). BHI broth was obtained from BD (37500, lot 1159859) while Luria broth was obtained from Fisher Bioreagent (BP9723). Milli-Q water was used to make all solutions.

High performance liquid chromatography-grade methanol, water and acetonitrile were purchased from J.T. Baker (Phillipsburg, NJ, USA). Formic acid (mass-spectrometry grade) was obtained from Fluka Analytical (Buchs, Switzerland) and ergothioneine standard was from AbaChemScene (Monmouth Junction, NJ, USA). L-Histidine standard was purchased from Sigma-Aldrich (St. Louis, MO, USA). L-Hercynine, L-Hercynine-d3 and ergothioneine-d9 were purchased from Toronto Research Chemicals (Toronto, ON, Canada) and DL-histidine-d3 was procured from Cayman (Ann Arbor, MI, USA). Deuterium-labeled standards, DL-histidine-d3, ergothioneine-d9 and L-Hercynine-d3 were used as internal standards in MS analysis. The reproducibility of the MS analysis was confirmed using quality control (QC) samples prepared in mobile phase A; the concentration of the internal standard for QC1 was 75 ng mL⁻¹ and that for QC2 was 125 ng mL⁻¹ (Supplementary Materials: Table S1).