RP-HPLC (214 nm; 30 min 5–60% acetonitrile gradient) analysis of the first labelling step with biotin-R NGL H, conducted in the presence of Ni 2+ as shown in a . A high concentration of NiSO 4 was used as Ni 2+ functions to quench both the N terminus of the acyl acceptor peptide and the GLH leaving group released from the acyl donor substrate. In the upper chromatogram the biotin-R NGL H peak extends beyond the height of the y -axis—the scale for the upper and lower panels is the same.
