Figure 4.

Generation of a truly apo structure by sequential dialysis against a solution containing glycerol and then only water. (a) Electron-density maps at 1.2 Å resolution for the apo structure after thorough dialysis, with lactose superimposed for comparison, showing only density for three water molecules. A 2m|F _o| − m|F _c| omit map contoured at 0.7σ is shown covering the lactose-binding site, His158 and Glu184 (grey mesh), as well as an |F _o| − m|F _c| map contoured at 3.5σ (green mesh). (b) Equivalent maps for the room-temperature apo X-ray structure (Saraboji et al., 2012 ▸) at 1.25 Å resolution contoured at the same levels. Residual electron density for parts of lactose can still be seen. (c) Equivalent electron-density maps at 0.86 Å resolution for the previous apo structure at 100 K. Full analysis of the apo structure (neutron and X-ray) will be presented elsewhere.