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Time-resolved wide-angle X-ray scattering of the haemoglobin:carbon monoxide complex. (a) Sketch of the experimental set-up at the dedicated time-resolved beamline at the European Synchrotron Radiation Facility. Polychromatic X-ray pulses are generated in an undulator and a rotating chopper (triangle) is used to isolate a chosen pulse train. Protein samples are held within a glass capillary (red) and are excited by laser pulses (green) incident perpendicular to the X-ray beam. Concentric diffusive X-ray scattering rings are collected on a charge-couple device (CCD) detector. (b) Integration in rings and subtraction of ‘laser off’ images from ‘laser on’ images yields difference scattering curves, which are the fingerprint of the structural rearrangements in the protein. Difference scattering recorded from haemoglobin in complex with carbon monoxide 100 µs after photoexcitation (red) are compared with ‘static’ differences between haemoglobin with carbon monoxide bound and deoxyhaemoglobin (black). (c) Surface representation of the expected time-dependent structural changes in haemoglobin. Regions of the proteins that are involved in the changes are coloured red. Reproduced by permission from Macmillan Publishers: Nature Methods (Cammarata et al., 2008), copyright (2008).