Peptide-EBX (Ac-KLAFG) (2.6 mg, 2.7 μmol, 1.5 equiv.) was weighed on the analytical balance in an Eppendorf tube. Afterwards, 18 μL of a solution of L-Glutathione (100 mM in water) (1.8 μmol, 1.0 equiv.) The reaction mixture was then diluted with 347 μL Tris buffer (pH 9.0, 100 mM), overall concentration: 5 mM, and placed on a shaker and shaken for 3 h at RT without protecting from atmosphere and light. At the end of the reaction, 10 μL of the crude was diluted with 60 μL MeCN/water 1:1 and injected in RP-HPLC. HPLC ratio (210 nm) of the product: > 95%. The desired products were isolated by Prep-RP-HPLC.