The ligand photodissociation of sperm whale carboxymyoglobin (MbCO) at low temperature (15K-100K) under extended illumination has been studied by X-ray Absorption Near Edge Structure (XANES) spectroscopy using the dispersive technique. XANES simulations through the multiple scattering (MS) approach allow one to interpret the spectroscopic data in structural terms, and to investigate the Fe site structure configurations of the states that follow the CO photodissociation as a function of temperature. The Fe site in the photoproduct is unbound, with an overall structure similar to the deoxy-form (Mb) of the protein. The Fe site structure changes from T < 30K(Mb*) to T > 50K (Mb**), revealing the existence of a slower unbound state Mb**. A model is proposed which includes the faster state (Mb*) as a planar porphyrin ring with a displacement of Fe from the heme plane of less than 0.3 A, and the slower state (Mb**) with a domed heme.
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