Figure 4.

The atomic model and the calculated Coulomb potential map of insulin at one of the zinc ion sites for different assigned zinc charge states: (a) neutral, (b) +0.5, (c) +0.75, (d) +1.0, (e) +2.0. The (2F _obs − F _calc) map is contoured at the 1σ level and is depicted in blue. The residual (F _obs − F _calc) map is contoured at the +3σ level (depicted in green) and −3σ level (depicted in red). The zinc ion is coordinated by three histidines. Green and red arrows point to residual Coulomb potential map peaks that appeared when improper charge states are used. The +0.75 charge yields the optimal residual map.