Figure 3.

SAD-based structure determination in P2₁. (a) Electron density calculated from density-modified SAD phases around the CDR loops of the final Nb36-C85-2 model. (b) As in (a) but displaying the final 2mF _o − DF _c map. (a) and (b) are contoured at 1.2σ. (c) Close-up of the double conformation of a Cys side chain bound to a Hg atom together with a map calculated from the anomalous differences and phases derived from the final model with Hg atoms omitted. The two sites were modelled with occupancies of 0.6 and 0.4, respectively, and the map is contoured at 6σ. (d) Anomalous map calculated as in (c) around two Cys85 side chains each bound to one Hg atom. The S atom of one of these cysteines apparently also forms a nonbonded interaction (dotted blue line) with the neighbouring Hg. The map is contoured at 16σ and comparison with (c) justifies that both Hg sites are modelled with full occupancy. (e) Difference mF _o − DF _c map from the final model contoured at 4σ (green) and −4σ (red) of the same two Hg atoms and cysteines further supporting this interpretation.