Dynamic structural disorder of the FeO₂ moiety in oxymyoglobin studied by nuclear resonant forward scattering of synchrotron radiation

Oxy- as well as deoxymyoglobin exhibit a pronounced temperature dependence of the quadrupole splitting of the heme iron as detected by conventional Mössbauer spectroscopy. With nuclear resonant forward scattering (NFS) of synchrotron radiation, which can be viewed as Mössbauer spectroscopy in the time domain, it is shown that this spectroscopic behavior, although it is phenomenologically similar in the two cases, is based on completely different physical mechanisms. It is demonstrated that stochastic fluctuations of the iron electric field gradient in MbO₂, which are due to the dynamic structural disorder of the FeO₂ moiety, are the reason for the temperature-dependent alterations of the coherent quantum beat pattern in the NFS spectra of MbO₂, in contrast to deoxyMb where transitions between orbital states of iron take place. This subtle spectroscopic difference cannot be inferred from conventional Mössbauer spectroscopy.