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Figure 2
Determination of phototriggered-state occupancy x by ensemble refinement. A model of the dark state, obtained from refinement against pure dark-state data, is combined with a model of the pure photolyzed state obtained using extrapolated structure factors. In this ensemble model, the occupancy of the photolyzed state is set to x and the occupancy of the dark state to (1 − x). Such ensemble models are prepared using a range of values for x and each model is refined against data from partially photolyzed crystals. The residual difference density in the active site is used to evaluate whether the chosen value for x is correct. For instance, negative difference density at the position of the substrate carboxyl­ate indicates that the value of x is too low, whereas positive density indicates that the value of x is too high. If the value of x is chosen correctly, no residual density should be present.

IUCrJ
Volume 13| Part 5| September 2026|
ISSN: 2052-2525
https://doi.org/10.1107/S2052252526005403