Freeze-quench ⁵⁷Fe-Mössbauer spectroscopy: Trapping reactive intermediates

⁵⁷Fe-Mössbauer spectroscopy is a method that probes transitions between the nuclear ground state (I = 1/2) and the first nuclear excited state (I = 3/2). This technique provides detailed information about the chemical environment and electronic structure of iron. Therefore, it has played an important role in studies of the numerous iron-containing proteins and enzymes. In conjunction with the freeze-quench method, ⁵⁷Fe-Mössbauer spectroscopy allows for monitoring changes of the iron site(s) during a biochemical reaction. This approach is particularly powerful for detection and characterization of reactive intermediates. Comparison of experimentally determined Mössbauer parameters to those predicted by density functional theory for hypothetical model structures can then provide detailed insight into the structures of reactive intermediates. We have recently used this methodology to study the reactions of various mononuclear non-heme-iron enzymes by trapping and characterizing several Fe(IV)-oxo reaction intermediates. In this article, we summarize these findings and demonstrate the potential of the method.