Figure 3.

2|F_o| − |F_c| electron density map at Glu-H173 in the charge-separated state (D⨥Q_AQ_B⨪) of the RC from Rb. sphaeroides with Cd²⁺ (a) and without Cd²⁺ (b). The structure shown in b is similar to the one reported by Stowell et al. (7). Electron density contours (purple) are displayed at 1σ above the mean level of the map with the Glu-H173 side chain excluded for the calculation of phases. In the structure without Cd²⁺, the electron density of the side chain of Glu-H173 is weaker, indicating greater mobility of the side chain, compared with the structure in the presence of Cd²⁺.