Section 26.1.2.4.1. The mercuri-iodide (K₂HgI₄) derivative

After trying several levels of substitution, RJP used the K₂HgI₄ salt at a molar concentration eight to ten times that of lysozyme. Based on the fact that lysozyme contains two methionine residues per molecule, and in keeping with a suggestion of Bluhm et al. (1958), RJP was expecting to see two heavy-atom sites,¹ but the hk0 difference-Patterson map was interpretable in terms of a single site of substitution. This site, however, was very close to the crystallographic twofold axis that runs along a diagonal in the [001] projection of the unit cell, and it proved necessary to correct the details of the first interpretation when phase information became available from other derivatives. It then appeared that there was one (or ) on the twofold axis between two protein molecules, but that it was best modelled by two closely spaced sites to allow for the elongated shape of the group (see Table 26.1.2.1). Several other heavy-atom salts, including K₂HgBr₄, K₂PtBr₄ and K₂AuCl₄, gave derivatives in which the heavy atom was attached to the same site as K₂HgI₄, and consequently seemed not to provide useful additional phase information.