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Structural results from intermediate trapping studies of bacteriorhodopsin. Four structures of resting (Belrhali et al., 1999) (purple, Protein Data Bank entry 1qhj), early (Edman et al., 1999) (blue, 1qkp), intermediate (Royant et al., 2000) (green, 1eop) and late (Luecke et al., 1999) (yellow, 1c8s) conformations are shown. These intermediate conformations were trapped by illuminating crystals at 110 K, 170 K and during thawing, respectively. A clear evolution of the retinal can be observed for these structures, which moves towards the cytoplasm as the temperature is raised. Moreover, significant displacements of Trp-182, Asp-85 and Arg-82 are also observed, as are rearrangements of water molecules recorded in the corresponding Protein Data Bank entries (not shown for reasons of clarity).