Considering the susceptibility of proteins to facile denaturing, we next evaluated the applicability of the MgCl2-mediated sulfenylation to the Trp-selective labeling of proteins using an ionic liquid as solvent. We subjected the six Trp-containing 129-residue lysozyme protein 12 to the reaction. The conjugation of protein 12 (0.1 mM) and the Cys(Acm)(O) peptide 1 (1.5 mM) in 5% H2O/BMPy•OTf in the presence of 30 mM MgCl2, and 0.1% TFA at 37 °C for 24 h gave a mixture of nonlabelled parent lysozyme 12 and labelled materials 13, which could not be separated by HPLC (Figures and).