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 diamagnetc character of the MbO2-complex are then understood by an exchange coupling of the Fe3+-ion with 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 [J] greater than or equal to 300cm-1 is 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.