Covalent adduct formation was investigated by intact protein mass spectrometry. Apo protein kinase domain was compared with samples which had been incubated with compounds 2, 3 and 4 at varying pHs. These experiments were performed on PfCLK3 kinase domain (334-699) given the full-length recombinant protein did not ionize using ESI TOF analysis. At physiological pH 7.4, no covalent adduct formation was observed for compound 2 after 4 hours. When the pH was raised to pH 9, adduct formation was observed. This implies a lack of warhead reactivity, with basic conditions required to deprotonate Cys368, increasing nucleophilicity and driving product formation. Cysteine reactivity is governed by the side chain pKa, which can vary from 3.5 to 12 depending on specific protein microenvironments. Cys368 can therefore be considered to be relatively weakly nucleophilic. The more reactive also substituted acrylamide 3 demonstrated no adduct formation at pH 7.4, while the most reactive chloroacetamide 4 demonstrated 100% singular adduct formation (Fig. ).