Open AccessPolarized X‐ray absorption spectroscopy of the low‐temperature photoproduct of carbonmonoxy‐myoglobin
Author(s) -
Della Longa Stefano,
Arcovito Alessandro,
Vallone Beatrice,
Congiu Castellano Agostina,
Kahn Richard,
Vicat Jean,
Soldo Yvonne,
Hazemann Jean Louis
Publication year - 1999
Publication title -
journal of synchrotron radiation
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.172
H-Index - 99
ISSN - 1600-5775
DOI - 10.1107/s0909049599010845
Subject(s) - chemistry , myoglobin , photodissociation , absorption spectroscopy , x ray absorption spectroscopy , crystallography , heme , photochemistry , x ray absorption fine structure , spectroscopy , optics , physics , organic chemistry , quantum mechanics , biochemistry , enzyme
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 P 2 1 ) 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 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.