Rapid freeze-quench 57Fe Mössbauer spectroscopy: monitoring changes of an iron-containing active site during a biochemical reaction.

Nuclear gamma resonance spectroscopy, also known as Mössbauer spectroscopy, is a technique that probes transitions between the nuclear ground state and a low-lying nuclear excited state. The nucleus most amenable to Mössbauer spectroscopy is 57Fe, and 57Fe Mössbauer spectroscopy provides detailed information about the chemical environment and electronic structure of iron. Iron is by far the most structurally and functionally diverse metal ion in biology, and 57Fe Mössbauer spectroscopy has played an important role in the elucidation of its biochemistry. In this article, we give a brief introduction to the technique and then focus on two recent exciting developments pertaining to the application of 57Fe Mössbauer spectroscopy in biochemistry. The first is the use of the rapid freeze-quench method in conjunction with Mössbauer spectroscopy to monitor changes at the Fe site during a biochemical reaction. This method has allowed for trapping and subsequent detailed spectroscopic characterization of reactive intermediates and thus has provided unique insight into the reaction mechanisms of Fe-containing enzymes. We outline the methodology using two examples: (1) oxygen activation by the non-heme diiron enzymes and (2) oxygen activation by taurine:alpha-ketoglutarate dioxygenase (TauD). The second development concerns the calculation of Mössbauer parameters using density functional theory (DFT) methods. By using the example of TauD, we show that comparison of experimental Mössbauer parameters with those obtained from calculations on model systems can be used to provide insight into the structure of a reaction intermediate.