Figure 5.

Structure of the X state accumulating in subunit B under illumination at a low bromide ion concentration. (a) The |F_{On_0.01M}| – |F_{On_0.1M}| difference map, contoured at 3.4 σ (positive, purple; negative, sky blue) and 6 σ (negative, dark blue), is overlaid on the models of X (cyan) and N (yellow) trapped in subunit B. Here, F_{On_0.01M} and F_{On_0.1M} are the diffraction amplitudes from the illuminated crystals that had been soaked in 1.5 M Na-citrate solutions containing 0.01 and 0.1 M, respectively, NaBr. The former crystal (crystal id: 1107 in Table S1) was rapidly cooled to 83 K after illumination at 240 K; the other crystal (crystal id: 974 in Table S1) was slowly cooled from 240 to 100 K in red light. The occupancy of the resting state in subunit B was suggested to be similar (∼35%) in these illuminated crystals; namely, the contribution of the resting state to the |F_{On_0.01M}| − |F_{On_0.1M}| map was nearly zero. (b) The 2Fo-Fc map that was derived from the quantity 2.2|F_{On_0.01M}| – 1.2|F_{On_0.1M}| is contoured at 1.3 σ (blue) and overlaid on the models of X (cyan) and the resting state (purple) in subunit B. This electron density map represents the structural feature that would be expected for a mixture of X (65%) and the resting state (25%) in subunit B. Although the two crystals illuminated at different bromide ion concentrations (0.01 and 0.1 M NaBr) contained different amounts (∼30% and 65%, respectively) of N in subunit B, the contribution of this N state to the 2Fo-Fc map was cancelled out when the quantity 2.2|F_{On_0.01M}| – 1.2|F_{On_0.1M}| was used. (c) The structures of X (cyan), N (yellow), and the resting state (salmon) are compared. To see this figure in color, go online.