Peptide-EBX (5 μmol, 1 equiv.) was weighed on the analytical balance and dissolved in 200 μL non-degassed DMA. Afterwards, 55 μL of a solution of Cys containing peptides (100 mM in DMF, for Glutathione 100 mM in DMF/H2O 1:1) (5.5 μmol, 1.1 equiv.), 10 μL DIPEA (100 mM in DMF, 10 μmol, 2 equiv.) were added. The reaction was stirred for 1 h at RT without protecting from atmosphere and light. At the end of the reaction, 10 μL of the crude was diluted with 90 μL of MeCN/water 1:1 and injected in RP-HPLC. The desired products were isolated by Prep-RP-HPLC. HPLC-UV ratio was determined by taking the ration of Aprod/Atotal where Aprod = area in mAU of the product peak and Atotal = area in mAU of the combined peptide containing species.