Figure 2.

Light-induced structural changes in the trimeric pHR-bromide complex. |F_light| – |F_dark| difference maps observed for the C2 crystal that had been soaked in a solution containing 0.01 M (a and b) or 0.1 M NaBr (c). The crystal was cooled in the dark to 100 K at a cooling rate of 2.3 K/s after illumination with red light at 240 K (a), cooled rapidly to 83 K after illumination at 240 K (b), or slowly cooled in red light from 240 to 100 K (c). Each |F_light| – |F_dark| difference map was evaluated by comparing the diffraction amplitudes (|F_light|) observed for an illuminated crystal with those (|F_dark|) observed for an unilluminated crystal, contoured at 3.2 σ (positive densities in purple, negative in green), and overlaid on the structural model of the resting state (a–c). In the middle and lower panels, the difference map around the retinal chromophore in subunit B (d–f) or subunit C (g–i) were contoured at 3 σ (positive densities in purple, negative in sky blue) and 8 σ (positive densities in red, negative in dark blue) and overlaid on the structural models of the resting state (pink) and the major reaction state (green or cyan) trapped in subunit B or subunit C. To see this figure in color, go online.