2 mercaptobenzothiazole 2 mbt

Fresh litchi fruit was collected from the Yongfuda litchi orchard (Haikou, Hainan, China) in June 2022. Haikou is located on Hainan Island in China. It has a typical tropical marine climate and annual sunshine duration of over 2,000 h. The climate is humid, the temperature rises fast, and the average annual precipitation is approximately 260 mm. The Yongfuda litchi orchard is located in a volcanic rock soil planting area. Once harvested, the peel and flesh of the litchi were immediately removed, and the litchi seeds were flash-frozen with liquid nitrogen by slow immersion to prevent seed shattering and endogenous compound changes. The commonly used MALDI matrix, 2-mercaptobenzothiazole (2-MBT), was obtained from Sigma-Aldrich (St. Louis, MO, USA). Amino acid and oligopeptide standards, including His, Gly-Gly-Leu (tripeptide), Ala-His-Lys (tripeptide), Leu-Leu-Tyr (tripeptide), and Arg-Gly-Asp-dTyr-Lys (pentapeptide), were purchased from Bankpeptide Biological Technology Co., Ltd. (Hefei, Anhui, China). Trifluoroacetic acid (TFA) and liquid chromatography–mass spectrometry (LC-MS)-grade methanol and ethanol were obtained from Merck & Co., Inc. (Darmstadt, Germany). Ultrapure water in the whole process of the experiments was prepared using a Millipore Milli-Q system (Bedford, MA, USA). All other reagents and chemicals were purchased from Merck, unless otherwise noted.