Mössbauer studies on O2 and CO binding to the heme iron in myoglobin

The EFG tensors at the Fe atoms in myoglobin–O2 and –CO compounds are determined by Mössbauer measurements on single crystals and frozen solutions. In the O2 compound the quadrupole interaction (eQVẑẑ) is negative, the asymmetry parameter (η) is limited to the range of 0.3–0.4, and the direction of the largest EFG component (‖Vẑẑ‖) is most likely in the heme plane. In the CO compound eQVẑẑ is positive, η is limited to the range of 0.2–0.4, and the direction of ‖Vẑẑ‖ is probably perpendicular to the heme plane. These results are discused on the basis of structural analyses and MO calculations of these compounds.