Polarized X-ray absorption spectroscopy of the low-temperature photoproduct of carbonmonoxy-myoglobin

• Published in: Journal of Synchrotron Radiation
• Main Authors: Richard Kahn, Jean Vicat, Yvonne Soldo, Alessandro Arcovito, Stefano Della Longa, Beatrice Vallone, Agostina Congiu Castellano, Jean Louis Hazemann
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 1999
• Subjects: Instrumentation; Nuclear and High Energy Physics; Radiation; Sperm whale
• Online Access: https://www.openaccessrepository.it/record/102045; https://doi.org/10.1107/s0909049599010845

Visible light can break the Fe—CO bond in Fe(II) carbonmonoxy-myoglobin (MbCO) giving an unligated product (Mb*) that is almost stable at T < 30 K. Fe K-edge polarized X-ray absorption spectra (P-XAS) of the photoproduct (T = 20 K) of an oriented single crystal (0.2 × 0.2 × 0.3 mm) of sperm whale MbCO (space group P21) have been collected. By rotating the crystal the X-ray photon polarization vector has been oriented almost parallel (with an angle α = 23°) or perpendicular (α = 86°) to the heme normal of each myoglobin molecule. The crystal was continuously illuminated by a white-light source during the data collection. The polarized data give novel information on the Fe-heme electronic/structural rearrangement following photolysis. The XANES (X-ray absorption near-edge structure) spectrum polarized in the direction close to the Fe—CO bond changes dramatically after photolysis, exhibiting a shift of ∼2 eV, due to electronic relaxation of empty states of pz symmetry, while more subtle changes are observed in the spectrum polarized along the heme plane, sensitive to the heme-plane geometry. Changes in the pre-edge region can be interpreted to provide insight into the electronic structure of the highest occupied and lowest unoccupied molecular orbitals (HOMO–LUMO) in the MbCO → Mb* photochemical reaction at low temperature.