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

• Published in: Journal of Synchrotron Radiation
• Main Author: Arcovito, Alessandro
• Other Authors: Della Longa, S, Vallone, B, Castellano, A, Kahn, R, Vicat, J, Soldo, Y, Hazemann, J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 1999
• Subjects: XANES; Low temperature photoproduct; Settore BIO/10 - BIOCHIMICA; Sperm whale
• Online Access: http://hdl.handle.net/10807/11820; 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 x 0.2 x 0.3 mm) of sperm whale MbCO (space group P2(1)) have been collected. By rotating the crystal the X-ray photon polarization vector has been oriented almost parallel (with an angle (alpha = 23 degrees) or perpendicular (alpha = 86 degrees) 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 similar to 2 eV, due to electronic relaxation of empty states of p(z) 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.