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

Visible light can break the Fe-CO bond in Fe(II) carbonmonoxy-myoglobin (Mb CO) 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 Mb CO (space group P2(1)) have been collected. By rotating the crystal the X-r ay photon polarization vector has been oriented almost parallel (with an an gle (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 giv e novel information on the Fe-heme electronic/structural rearrangement foll owing photolysis. The XANES (X-ray absorption near-edge structure) spectrum polarized in the direction close to the Fe-CO bond changes dramatically af ter photolysis, exhibiting a shift of similar to 2 eV, due to electronic re laxation of empty states of p(z) symmetry, while more subtle changes are ob served in the spectrum polarized along the heme plane, sensitive to the hem e-plane geometry. Changes in the pre-edge region can be interpreted to prov ide insight into the electronic structure of the highest occupied and lowes t unoccupied molecular orbitals (HOMO-LUMO) in the MbCO --> Mb* photochemic al reaction at low temperature.