INTERMEDIATE STATES IN LIGAND PHOTODISSOCIATION OF CARBOXYMYOGLOBIN STUDIED BY DISPERSIVE-X-RAY ABSORPTION

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