Video research highlight: Unexpected behaviour of an iron-containing enzyme

An important part of the research at UniSysCat is the investigation of biological catalysts, namely enzymes, and especially their active centers. Many of these catalytically active centers contain metal ions. Understanding their function and the role of the metal ion in detail is necessary to transfer the chemistry of the enzymes to laboratory-generated catalysts - one of the main objectives of UniSysCat.

In this context, a research team involving the UniSysCat groups of Prof. Kallol Ray and Prof. Peter Hildebrandt studied the reaction cycle of an iron-containing enzyme. By a combination of different spectroscopy techniques (EPR, Mößbauer, UV-Vis, magnetic circular dichroism and resonance-Raman spectroscopy) and computational studies, they shed light on its function, that is on how the enzyme activates oxygen. They find an unexpected intermediate structure in the activation reaction.

The first author of the study, Dustin Kass, PhD student in the group of Kallol Ray, explains how this unexpected intermediate was found and in which labyrinth he moved for this. He reports on his research in a video (YouTube link) - the first of a series, in which young researchers talk about their research highlights. In addition, he describes his work as follows:

Almost 8 years ago my predecessors on this project performed an experiment, where they added oxygen, sodiumtetraphenylborate and acid to an iron complex. The iron complex was the so-called iron-TMCO, that was synthesized in our group before. The initial idea for this fateful experiment was to investigate the formation of a high valent iron(IV) oxido species using dioxygen as the universally available oxidant and to thereby mediate the oxidation capacity of O₂ to perform challenging substrate oxidations. But instead of an iron(IV) oxido species they saw the formation of a mysterious blue intermediate. The characterization of this intermediate turned out to become a very long scientific journey, that held many challenges and surprises but that could be finally brought to an interesting and rewarding outcome.

With the full range of spectroscopic methods, careful reactivity studies and with the valuable contribution of thorough reviewers, who help us to exclude a crazy idea that we had in the meantime (by rejecting the first draft of the paper ;) ) we could eventually describe the mysterious blue species as a novel iron(II) phenoxyl radical complex.

The unexpected formation of the phenoxyl radical is explained by an acid induced decay of the sodiumtetraphenylborate in presence of O₂. Due to the unique properties of the iron TMCO complex, the highly reactive phenoxyl radical cannot oxidize the iron(II) center and we end up with the unusual combination of iron in the oxidation state of +II that allows for the stabilization of the phenoxyl radical. As a fortunate side effect, the iron phenoxyl interaction also helps to mediate the oxidation potential of the phenoxyl so that we a can perform catalytic substrate oxidations, just as initially intended for this project.

The novel structural feature of an iron(II) phenoxyl radical species and the observed reactivity, both compare nicely to important enzymatic examples. In the galactose oxidase or the ribonucleotide reductases one could also identify copper or iron stabilized phenoxyl radicals, that are formed following the activation of O₂. These intermediates are responsible for challenging metabolic steps like the selective alcohol oxidation or the DNA synthesis.

Though this biomimetic correlation was neither intended nor foreseen when starting this project, we are now one step further on the way of identifying and improving catalytical relevant species for the adaptation of enzymatic examples into synthetic catalyst.

 Text and Video by Dustin Kass

The study has been publishes in Nature Chemistry: "Trapping of a phenoxyl radical at a non-haem high-spin iron(II) centre", D. Kass, V. Larson, T. Corona, U. Kuhlmann, P. Hildebrandt, T. Lohmiller, E. Bill, N. Lehnert, Nat. Chem. 2024. DOI: 10.1038/s41557-023-01405-9