M�ssbauer spectroscopy is an excellent tool for investigating the active sites of oxygen activating iron proteins. Our collaboration with John D. Lipscomb started in 1970 and it includes the 1976 report of the first M�ssbauer spectrum of a non-heme dioxygenase,3,4 protocatechuate dioxygenase from P. aeruginosa. Presently we are focusing on 2,3 homoprotocatechuate (HPCA) dioxygenase. Together with the Lipscomb group we have studied the wild-type enzyme as well as three mutant proteins, using  the natural substrate as well as alternate substrates that slow down certain steps in the catalytic cycle, thereby allowing us to access a variety of intermediates. Most recently we studied the variant H200C, characterizing the resting Fe(II) state, the Fe(II)-HPCA complex as well as an exchange coupled  Fe(III)-radical reaction intermediate; His200 is a second sphere active site residue implicated in interactions with substrate as well as oxygen. Using crystallographic information as well as density functional theory calculations allows to access the M�ssbauer data to gain insight into the orbital interactions in various intermediate states. In this lecture we focus on some Fe(III)-radical states that have recently been trapped, with the radical either residing on a superoxo ligand or being located on the catechol substrate.  To complement the dioxygenase studies, we are presently studying with the group of Lawrence Que an Fe(III)-superoxo model complex that not only offers some unexpected spectroscopic challenges but also gives insight into superoxo coordination, in an environment more under control than that found in the protein (oh, those waters). This superoxo complex is a rewarding target for quantum chemical studies.

Dear John, - it's not over yet: Using M�ssbauer spectroscopy for studying oxygen activation

248th National Meeting (2014)

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