Glutamic acid

Barley seeds were purchased from Jiangsu Yanjiang Institute of Agricultural Sciences, China, in July 2019. Ethyl acetate, linol, methanol, glutamic acid, and sodium hypochlorite were purchased from Sinopharm Chemical Reagent Company (Shanghai, China). methanol (chromatographic grade), glacial acetic acid, acetonitrile, phenolic acids, and flavonoids were purchased from Sigma-Aldrich Chemical Co. (Shanghai, China). Trichloroacetic acid (TCA), 2-thiobarbituric acid (TBA), 3-mercaptopropionic acid (3-MP), 2,2′-diphenyl-1-picrylhydrazyl (DPPH), 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), ATP, nicotinamide adenine dinucleotide phosphate (NADPH), ethylene diamine tetraacetic acid (EDTA), β-mercaptoethanol, coenzyme A (CoA), L-phenylalanine, trolox, GABA, dimethylaminobenzenesulfonyl chloride (derivatizing agent), and polyvinyl pyrrolidone (PVP) were purchased from Maclean Biochemical Technology Co., Ltd. (Shanghai, China). Hydrochloric acid and acetone were purchased from Nanjing Chemical Reagent Co., Ltd. (Nanjing, China).

Eight amino acids including tyrosine, tryptophan, threonine, glutamic acid, aspartic acid, histidine, phenylalanine, and arginine (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) were added into a 30% methanol aqueous solution of azalomycin F (100 μM) to obtain a mixed solution by vortexing, according to the molar concentration ratio of azalomycin F to each amino acid of 1:8. Simultaneously, the 30% methanol aqueous solution that only contained 100 μM azalomycin F was prepared as a blank control for background subtraction. After being incubated in a water bath at 37 °C for 30 min, these solutions were analyzed by a TripTOF 5600+ mass spectrometry (AB Sciex, Framingham, MA, USA) with an electrospray ion resource, and the detailed parameters are shown in Table S3 for negative ionization modes.