Cisplatin binding to the Nδ and N∊ atoms of His15. (a) 100 K data collection at 1.8 Å resolution, (b) 300 K data set 1 at 2.0 Å resolution. The 2Fo − Fc map (in purple) is at the 1.5 r.m.s. cutoff level and the anomalous difference density map (in orange) is at the 3σ cutoff level. The Nδ and N∊ atoms in the imidazole ring are labelled, together with the Pt and Cl atoms of the bound cisplatin moiety. The Cl occupancies calculated from SHELXTL are 73 and 51% for Cl1 and Cl2 in the 100 K data set and 78 and 68% for Cl1 and Cl2 in the RT data set 1. Cl3 and Cl4 have occupancies of 25 and 36%, respectively, in the 100 K data set.