Mössbauer spectroscopy on oxygenated sperm whale myoglobin: Evidence for an Fe³⁺O₂ ^-coupling at the active center

⁵⁷Fe Mössbauer spectra of oxygenated sperm whale myoglobin (MbO₂) show a well resolved quadrupole doublet with a temperature dependent splitting. The temperature dependence of the corresponding electric field gradient tensor (EFG) can be calculated from a Fe³⁺ term scheme for the iron at the active center. The Mössbauer spectra as well as the diamagnetic character of the MbO₂-complex are then understood by an exchange coupling of the Fe³⁺-ion with the O₂ ^- oxygen molecule ion. The resulting groundstate is a diamagnetic singlet. In order to keep the whole complex diamagnetic at room temperature, an exchange coupling with |Ј|⪴300cm^-1is necessary. As the whole model is in fair agreement with many other spectroscopic data, it is believed to be a good starting point for further detailed calculations.