on binding to PADI4 in the COLDER assays it is converted to Cit4 which no longer binds and hence inhibition is not observed. To assess whether warhead 2 was essential for inhibition, we additionally decided to synthesise the H-amidine analogue, Me4. Me4 had a comparable affinity to Arg4, however unlike Arg4 it also acted as a weak inhibitor of PADI4, consistent with our hypothesis that lack of PADI4 inhibition by Arg4 is due to it being turned over as a substrate (Figure ). The reduction in affinity and PADI4 inhibition of Me4 relative to the Famidine parent cP4_4 suggests that the fluorine atom forms important interactions within the active site of PADI4 that are crucial for binding, as well as acting as the leaving group.