After incubation with compound 4, PfCLK3 was digested with trypsin. The resulting peptides were then analysed by electrospray ionisation mass spectrometry and compared with the list of expected peptides using Protein Prospector. This yielded four peptides modified by the expected monoisotopic mass of compound 4 minus Cl (Fig. ), with one principal peptide being >10-fold greater in abundance, YSVVCELVGK. This peptide corresponds to residues 364-373 (Fig. ). This ion was then further fragmented by CID to reveal a single modification on Cys368 (Fig. Chloroacetamide 4 was also evaluated using a thermal shift assay. In the presence of an excess of 4, full-length PfCLK3 was highly thermodynamically stable, with a 20 ℃ shift in Tm compared to the DMSO control (Table ). This large shift demonstrates an impressive stabilisation effect of chloroacetamide 4's binding to PfCLK3.